Scholarly article on topic 'Replacement of the Saccharomyces cerevisiae acetyl-CoA synthetases by alternative pathways for cytosolic acetyl-CoA synthesis'

Replacement of the Saccharomyces cerevisiae acetyl-CoA synthetases by alternative pathways for cytosolic acetyl-CoA synthesis Academic research paper on "Industrial Biotechnology"

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{Yeast / "Acetylating acetaldehyde dehydrogenase" / "Pyruvate-formate lyase" / Transcriptome / "Precursor supply" / "Metabolic compartments"}

Abstract of research paper on Industrial Biotechnology, author of scientific article — Barbara U. Kozak, Harmen M. van Rossum, Kirsten R. Benjamin, Liang Wu, Jean-Marc G. Daran, et al.

Abstract Cytosolic acetyl-coenzyme A is a precursor for many biotechnologically relevant compounds produced by Saccharomyces cerevisiae. In this yeast, cytosolic acetyl-CoA synthesis and growth strictly depend on expression of either the Acs1 or Acs2 isoenzyme of acetyl-CoA synthetase (ACS). Since hydrolysis of ATP to AMP and pyrophosphate in the ACS reaction constrains maximum yields of acetyl-CoA-derived products, this study explores replacement of ACS by two ATP-independent pathways for acetyl-CoA synthesis. After evaluating expression of different bacterial genes encoding acetylating acetaldehyde dehydrogenase (A-ALD) and pyruvate-formate lyase (PFL), acs1Δ acs2Δ S. cerevisiae strains were constructed in which A-ALD or PFL successfully replaced ACS. In A-ALD-dependent strains, aerobic growth rates of up to 0.27h−1 were observed, while anaerobic growth rates of PFL-dependent S. cerevisiae (0.20h−1) were stoichiometrically coupled to formate production. In glucose-limited chemostat cultures, intracellular metabolite analysis did not reveal major differences between A-ALD-dependent and reference strains. However, biomass yields on glucose of A-ALD- and PFL-dependent strains were lower than those of the reference strain. Transcriptome analysis suggested that reduced biomass yields were caused by acetaldehyde and formate in A-ALD- and PFL-dependent strains, respectively. Transcript profiles also indicated that a previously proposed role of Acs2 in histone acetylation is probably linked to cytosolic acetyl-CoA levels rather than to direct involvement of Acs2 in histone acetylation. While demonstrating that yeast ACS can be fully replaced, this study demonstrates that further modifications are needed to achieve optimal in vivo performance of the alternative reactions for supply of cytosolic acetyl-CoA as a product precursor.

Academic research paper on topic "Replacement of the Saccharomyces cerevisiae acetyl-CoA synthetases by alternative pathways for cytosolic acetyl-CoA synthesis"

ELSEVIER

Replacement of the Saccharomyces cerevisiae acetyl-CoA synthetases by alternative pathways for cytosolic acetyl-CoA synthesis $

Barbara U. Kozak3,1, Harmen M. van Rossum3,1, Kirsten R. Benjamin b, Liang Wuc, Jean-Marc G. Darana, Jack T. Pronka, Antonius J.A. van Maris a,n

a Department of Biotechnology, Delft University of Technology, Kluyver Centre for Genomics of Industrial Fermentation, Julianalaan 67, 2628 BC Delft, The Netherlands

b Amyris Inc., 5885 Hollis Street, Ste. 100, Emeryville, CA 94608, USA c DSM Biotechnology Center, Alexander Fleminglaan 1, 2613 AX Delft, The Netherlands

ARTICLE INFO ABSTRACT

Cytosolic acetyl-coenzyme A is a precursor for many biotechnologically relevant compounds produced by Saccharomyces cerevisiae. In this yeast, cytosolic acetyl-CoA synthesis and growth strictly depend on expression of either the Acsl or Acs2 isoenzyme of acetyl-CoA synthetase (ACS). Since hydrolysis of ATP to AMP and pyrophosphate in the ACS reaction constrains maximum yields of acetyl-CoA-derived products, this study explores replacement of ACS by two ATP-independent pathways for acetyl-CoA synthesis. After evaluating expression of different bacterial genes encoding acetylating acetaldehyde dehydrogenase (A-ALD) and pyruvate-formate lyase (PFL), acslA acs2A S. cerevisiae strains were constructed in which A-ALD or PFL successfully replaced ACS. In A-ALD-dependent strains, aerobic growth rates of up to 0.27 h_1 were observed, while anaerobic growth rates of PFL-dependent S. cerevisiae (0.20 h-1) were stoichiometrically coupled to formate production. In glucose-limited chemostat cultures, intracellular metabolite analysis did not reveal major differences between A-ALD-dependent and reference strains. However, biomass yields on glucose of A-ALD- and PFL-dependent strains were lower than those of the reference strain. Transcriptome analysis suggested that reduced biomass yields were caused by acetaldehyde and formate in A-ALD- and PFL-dependent strains, respectively. Transcript profiles also indicated that a previously proposed role of Acs2 in histone acetylation is probably linked to cytosolic acetyl-CoA levels rather than to direct involvement of Acs2 in histone acetylation. While demonstrating that yeast ACS can be fully replaced, this study demonstrates that further modifications are needed to achieve optimal in vivo performance of the alternative reactions for supply of cytosolic acetyl-CoA as a product precursor.

© 2013 The Authors. Published by Elsevier Inc. on behalf of International Metabolic Engineering Society.

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Article history:

Received 11 June 2013

Received in revised form

3 October 2013

Accepted 11 November 2013

Available online 19 November 2013

Keywords: Yeast

Acetylating acetaldehyde dehydrogenase Pyruvate-formate lyase Transcriptome Precursor supply Metabolic compartments

1. Introduction

The robustness of Saccharomyces cerevisiae in industrial fermentation processes, combined with fast developments in yeast synthetic biology and systems biology, have made this microorganism a popular platform for metabolic engineering (Hong and Nielsen, 2012). Many natural and heterologous compounds whose

☆This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Corresponding author. Fax: +31 15 278 2355.

E-mail address: A.J.A.vanMaris@TUDelft.nl (A.J.A. van Maris).

1 These authors contributed equally to this publication and should be considered co-first authors.

production from sugars is under investigation or already implemented in industry require acetyl-coenzyme A (acetyl-CoA) as a key precursor. Examples of such products include n-butanol, isoprenoids, lipids and flavonoids (Dyer et al., 2002; Koopman et al., 2012; Shiba et al., 2007; Steen et al., 2008; Veen and Lang, 2004).

Acetyl-CoA metabolism in S. cerevisiae is compartmented (Pronk et al., 1996; van Roermund et al., 1995). During respiratory growth on sugars, a substantial flux through acetyl-CoA occurs via the mitochondrial pyruvate dehydrogenase complex (Pronk et al., 1994). However, mutant analysis has shown that mitochondrial acetyl-CoA cannot meet the extramitochondrial requirement for acetyl-CoA in the yeast cytosol, which includes, for example, its use as a precursor for synthesis of lipids and lysine (Flikweert et al., 1999; van den Berg and Steensma, 1995). In this respect, it is relevant to note that S. cerevisiae does not contain ATP-citrate

1096-7176/$-see front matter © 2013 The Authors. Published by Elsevier Inc. on behalf of International Metabolic Engineering Society. All rights reserved. http://dx.doi.org/10.1016/j.ymben.2013.11.005

lyase, an enzyme that plays a major role in translocation of acetyl-CoA across the mitochondrial membrane in mammalian cells and in several non-Saccharomyces yeasts (Boulton and Ratledge, 1981). When sugars are used as the carbon source, cytosolic acetyl-CoA synthesis in S. cerevisiae occurs via the concerted action of pyruvate decarboxylase (Pdc1, 5 and 6), acetaldehyde dehydrogenase (Ald2, 3, 4, 5 and 6) and acetyl-CoA synthetase (Acs1 and 2) (Pronk et al., 1996). Heterologous, acetyl-CoA-dependent product pathways expressed in the S. cerevisiae cytosol exclusively depend on this 'pyruvate dehydrogenase bypass' for provision of acetyl-CoA. Indeed, overexpression of acetyl-CoA synthetase (ACS) from Salmonella enterica has been shown to lead to increased productivities of the isoprenoid amorphadiene by engineered S. cerevisiae strains (Shiba et al., 2007).

The ACS reaction involves the hydrolysis of ATP to AMP and pyrophosphate (PPJ:

Acetate+ATP+CoA - Acetyl - CoA+AMP+PPi

Together with the subsequent hydrolysis of PPi to inorganic phosphate (Pi), this ATP consumption is equivalent to the hydrolysis of 2 ATP to 2 ADP and 2Pi. The resulting ATP expenditure for acetate activation can have a huge impact on the maximum theoretical yields of acetyl-CoA derived products. For example, the production of a C16 lipid from sugars requires 8 acetyl-CoA, whose synthesis via ACS requires 16 ATP. At an effective P/O ratio of respiration in S. cerevisiae of 1 (Verduyn, 1991), this ATP requirement for acetyl-CoA synthesis corresponds to 1 mol of glucose that needs to be respired for the synthesis of 1 mol of product.

In addition to the pyruvate-dehydrogenase complex, other reactions have been described in nature that enable the ATP-independent conversion of pyruvate into acetyl-CoA (Powlowski et al., 1993; Rudolph et al., 1968; Smith and Kaplan, 1980). Many prokaryotes contain an acetylating acetaldehyde dehydrogenase (A-ALD; EC 1.2.1.10) which catalyses the reversible reaction:

Acetaldehyde+NAD + + CoA 2 Acetyl - CoA+NADH + H +.

Although functional expression of bacterial genes encoding A-ALD in S. cerevisiae has been described in the literature, these studies focused on reductive conversion of acetyl-CoA to ethanol as part of a phosphoketolase pathway for pentose fermentation (Sonderegger et al., 2004) or as part of a metabolic engineering strategy to convert acetic acid to ethanol (Guadalupe Medina et al.,

2010). Complete replacement of the native acetaldehyde dehydro-genases and/or ACS of S. cerevisiae by A-ALD, thereby bypassing ATP hydrolysis in the ACS reaction, has not been demonstrated.

In many anaerobic bacteria and some eukaryotes (Stairs et al.,

2011), pyruvate can be converted into acetyl-CoA and formate in the non-oxidative, ATP-independent reaction catalysed by pyruvate-formate lyase (PFL; EC 2.3.1.54):

Pyruvate+CoA 2 Acetyl - CoA+Formate.

PFL and PFL-activating enzyme (PFL-AE; EC 1.97.1.4) from Escherichia coli have previously been expressed in S. cerevisiae (Waks and Silver, 2009). Although formate production by this oxygen-sensitive enzyme system was demonstrated in anaerobic yeast cultures, its impact on cytosolic acetyl-CoA metabolism has not been investigated.

To gain the full potential benefit of ATP-independent cytosolic acetyl-CoA synthesis, the implemented heterologous pathways expressed in S. cerevisiae should, ideally, completely replace the ACS reaction. In wild-type strain backgrounds, deletion of both ACS1 and ACS2 is lethal (van den Berg and Steensma, 1995) and, during batch cultivation on glucose, presence of a functional ACS2 gene is essential (van den Berg and Steensma, 1995) because ACS1 is subject to glucose repression (van den Berg et al., 1996) and its

product is inactivated in the presence of glucose (de Jong-Gubbels et al., 1997). Moreover, it has been proposed that Acs2, which was demonstrated to be partially localized in the yeast nucleus, is involved in histone acetylation (Takahashi et al., 2006). Involvement of Acs isoenzymes in the acetylation of histones and/or other proteins might present an additional challenge in replacing them with heterologous reactions, if this includes another mechanism than merely the provision of extramitochondrial acetyl-CoA.

The goal of this study is to investigate whether the hetero-logous ATP-independent A-ALD and PFL pathways can support the growth of acsl acs2 mutants of S. cerevisiae by providing extra-mitochondrial acetyl-CoA and to study the impact of such an intervention on growth, energetics and cellular regulation. To this end, several heterologous genes encoding A-ALD and PFL were screened by expression in appropriate yeast genetic backgrounds, followed by detailed analysis of S. cerevisiae strains in which both ACS1 and ACS2 were replaced by either of the alternative reactions. The resulting strains were studied in batch and chemostat cultures. Furthermore, genome-wide transcriptional responses to these modifications were studied by chemostat-based transcrip-tome analysis of engineered and reference strains.

2. Methods

2.1. Strains and maintenance

The S. cerevisiae strains used in this study (Table 1) share the CEN.PK genetic background (Entian and Kotter, 2007; Nijkamp et al., 2012). Stock cultures were grown aerobically in synthetic medium (Verduyn et al., 1992). When required, auxotrophic requirements were complemented with synthetic yeast drop-out medium supplements (Sigma-Aldrich, St. Louis, MO, USA) or by growth in YP medium (demineralized water, 10 g L- 1 Bacto yeast extract, 20 g L-1 Bacto peptone). Carbon sources were either 20 gL-1 glucose, 2% (v/v) ethanol and/or 11.3 g L- 1 sodium acetate trihydrate. Stock cultures of S. cerevisiae strains IMZ383 and IMZ384 were grown anaerobically and supplemented with Tween-80 (420 mg L- 1) and ergosterol (10 mg L- 1) added to the medium. Frozen stocks of S. cerevisiae and E. coli were prepared by the addition of glycerol (30% v/v) to the growing shake-flask cultures and aseptically storing 1 mL aliquots at - 80 °C.

2.2. Plasmid construction

Coding sequences of Staphylococcus aureus adhE (NP_370672.1), Escherichia coli eutE (YP_001459232.1) and Listeria innocua lin1129 (NP_470466) were codon-optimized for S. cerevisiae with the JCat algorithm (Grote et al., 2005). Custom-synthesized coding sequences cloned in the pMA vector (Table 2) were provided by GeneArt GmbH (Regensburg, Germany). Gateway Cloning technology (Invitrogen, Carlsbad, A) was used to insert these coding sequences in the intermediate vector pDONR221 (BP reaction) and subsequently into pAG426-pGPD (Alberti et al., 2007) (LR reaction). The resulting plasmids pUDE150 to pUDE152 were transformed into E. coli and their sequences checked by Sanger sequencing (BaseClear BV, Leiden, The Netherlands).

PFL- and PFL-AE-encoding sequences from Thalassiosira pseudonana (Genbank accession no. XM_002296598.1 and XM_ 002296597.1), Chlamydomonas reinhardtii (AJ620191.1 and AY831434.1), E. coli (X08035.1), Lactobacillus plantarum (YP_ 003064242.1 and YP_003064243.1) and Neocallimastix frontalis (AY500825.1 and AY500826.1) were codon-optimized for expression in S. cerevisiae and signal sequences, predicted by TargetP, were removed (Emanuelsson et al., 2007). Expression cassettes in which these coding sequences were flanked by the TPI1 promoter

Table 1

Saccharomyces cerevisiae strains used in this study.

Name Relevant genotype Origin

CEN.PK113-7D MATa MAL2-8C SUC2 P. Kötter

CEN.PK113-5D MATa MAL2-8C SUC2 ura3-53 P. Kötter

CEN.PK102-12A MATa MAL2-8C SUC2 ura3-53 leu2-3,112 his3-A1 P. Kötter

IMK337 CEN.PK102-12 ald2-ald3::loxP-LEU2-loxP This study

IMK342 IMK337 ald4::loxP-HIS3-loxP This study

IMK346 IMK342 ald5::loxP-KanMX4-loxP This study

IMK354 IMK346 ald6::loxP-hphNT1-loxP This study

IMZ282 IMK354 p426GPD This study

IMZ284 IMK354 pUDE047 (URA3, dmpF Pseudomonas sp.) This study

IMZ286 IMK354 pUD043 (URA3, mhpF E. coli (not codon-optimized)) This study

IMZ289 IMK354 pUDE150 (URA3, adhE S. aureus) This study

IMZ290 IMK354 pUDE151(URA3, eutE E. coli) This study

IMZ291 IMK354 pUDE152 (URA3, lin1129 L. innocua) This study

IME140 CEN.PK113-5D p426GPD This study

IMZ304 IMZ290 acs2::loxP-natNT2-loxP This study

IMZ305 IMZ304 acs1 ::AmdS This study

IMK427 CEN.PK102-12A acs2::loxP-HIS3-loxP This study

IMZ374 IMK427 pRS426 (URA3) This study

IMZ369 IMK427 pUDE201 (URA3, PFL, PFL-AE Thalassiosira pseudonana) This study

IMZ370 IMK427 pUDE202 (URA3, pfl, pflA Chlamydomonas reinhardtii) This study

IMZ371 IMK427 pUDE204 (URA3, pflB, pflA Escherichia coli) This study

IMZ372 IMK427 pUDE214 (URA3, pflB, pflA Lactobacillus plantarum) This study

IMZ373 IMK427 pUDE215 (uRA3, PFL, PFL-AE Neocallimastix frontalis) This study

IMZ383 IMZ371 acs1 ::loxP-LEU2-loxP This study

IMZ384 IMZ372 acs1::loxP-LEU2-loxP This study

Table 2

Plasmids used in this study.

Name Characteristics Origin

pMA Delivery vectors GeneArt, Germany

pENTR221 Gateway entry clone Invitrogen, USA

pUG6 Template for loxP-KanMX-loxP cassette Gueldener et al. (2002)

pUG27 Template for loxP-HIS5 (Schizosaccharomyces pombe)-loxP cassette Gueldener et al. (2002)

pUG72 Template for loxP-URA3 (Kluyveromyces lactis)-loxP cassette Gueldener et al. (2002)

pUG73 Template for loxP-LEU2 (K lactis)-loxP cassette Gueldener et al. (2002)

pUG-hphNT1 Template for loxP-hphNT1-loxP cassette de Kok et al. 2011)

pUG-natNT2 Template for loxP-natNT2-loxP cassette de Kok et al. (2012)

pUDE158 Template for AmdS cassette Solis-Escalante et al. (2013)

p426GPD 2 ^m ori, URA3, PTDH3-TCYC1 Mumberg et al. (1995)

pAG426GPD 2 ^m ori, URA3, PTDH3-ccdB-TCYC1 Alberti et al. (2007)

pRS424 2 ^m ori, TRP1 Christianson et al. (1992

pRS426 2 ^m ori, URA3 Christianson et al. (1992

pUDE043 2 ^m ori, URA3, PTDH3-mhpF (E. coli) (not codon-optimized)-TCYCI Guadalupe Medina et al. (2010)

pUDE047 2 ^m ori, URA3, PTDH3-dmpF (Pseudomonas sp.)-TCYC1 Pronk et al. (2011)

pUDE150 2 ^m ori, URA3, PTDH3-adhE (S. aureus)-TCYC1 This study

pUDE151 2 ^m ori, URA3, PTDH3-eutE (E. coli)-TCYC1 This study

pUDE152 2 ^m ori, URA3, PTDH3-lin1129 (L. innocua)-TCYC1 This study

pUDE201 2 ^m ori, URA3, PTPI1-PFL (T. pseudonana)-TGND2, PFBA1-PFL-AE (T. pseudonana)-TPMA1 This study

pUDE202 2 ^m ori, URA3, PTPI1-pfl (C. reinhardtii)-TGND2, PFBM-pflA (C. reinhardtii)-TPMA1 This study

pUDE204 2 ^m ori, URA3, PTpn-pflB (E. coli)-TGND2, pfbai-pAA (E. coli)-TpMAi This study

pUDE214 2 ^m ori, URA3, PTPI1-pflB (L. plantarum)-TGND2, PFBAi-pflA (L. plantarum)-TPMA1 This study

pUDE215 2 ^m ori, URA3, PTPI1-PFL (N. frontalis)-TGND2, PFBA1-PFL-AE (N. frontalis)-TPMA1 This study

and GND2 terminator (PFL) or FBA1 promoter and PMA1 terminator (PFL-AE), were synthesized by GeneArt GmbH. PFL and PFL-AE expression cassettes were PCR amplified with primer combinations 3384-3385 and 3386-3387, respectively (Supplementary Table S1). Overlaps between the fragments were obtained using the primers, enabling homologous recombination. After linearization with Spel and Xhol, the pRS426 backbone was assembled with the PFL and PFL-AE fragments via in vivo homologous recombination in S. cerevisiae CEN.PK113-5D (Kuijpers et al., 2013). Assembled plasmids were purified from uracil prototrophic transformants and transformed into E. coli. Sequences of the resulting plasmids pUDE201 to pUDE215 (Table 2) were checked by sequencing (Illumina HiSeq2000, BaseClear BV, Leiden,

The Netherlands), assembled as described previously (Kuijpers et al., 2013). All plasmid sequences were deposited at GenBank under accession numbers KF170375 (pUDE215), KF170376 (pUDE150), KF170377 (pUDE151), KF170378 (pUDE152), KF170379 (pUDE201), KF170380 (pUDE202), KF170381 (pUDE204) and KF170382 (pUDE214).

2.3. Strain construction

S. cerevisiae strains were transformed according to Gietz and Woods (2002). Knockout cassettes were obtained by PCR using primers listed in Supplementary Table S1 with the templates pUG6, pUG27, pUG72, pUG73 (Gueldener et al., 2002), pUG-

amdS (Solis-Escalante et al., 2013), pUG-hphNT1 (de Kok et al., 2011) and pUG-natNT2. Mutants were selected on solid medium (2% (w/v) agar) with 200 mg L"1 G418, 200 mg L"1 hygromycin B or 100 mg L"1 nourseothricin (for dominant markers) or on dropout (Sigma-Aldrich) or synthetic medium from which the appropriate auxotrophic requirements had been omitted. The Ald" strain IMK354 was obtained by the consecutive deletion of ALD2-ALD3, ALD4, ALD5 and ALD6 in strain CEN.PK102-12A. During strain construction, the Ald" strains were grown on acetate as a carbon source. IMK354 was transformed with the plasmids p426GPD (Mumberg et al., 1995), pUDE043, pUDE047, pUDE150, pUDE151 and pUDE152. In one of the resulting strains, IMZ290, ACS2 and ACS1 were subsequently deleted, yielding IMZ305. IMK427 was constructed by deletion of ACS2 in strain CEN. PK102-12A. The acs2A strain was grown on ethanol as a carbon source. Transformation of strain IMK427 with plasmids pUDE201, pUDE202, pUDE204, pUDE214 and pUDE215 yielded strains IMZ369-IMZ373, respectively (Table 1). In two strains, IMZ371 and IMZ372, ACS1 was deleted. In all cases gene deletions and plasmid presence were confirmed by PCR using the diagnostic primers listed in Supplementary Table S1.

2.4. Molecular biology techniques

PCR amplification with Phusion® Hot Start II High Fidelity Polymerase (Thermo Scientific, Waltham, MA) was performed according to the manufacturer's manual using HPLC- or PAGE-purified, custom-synthesized oligonucleotide primers (Sigma-Aldrich). Diagnostic PCR was done with DreamTaq (Thermo Scientific) and desalted primers (Sigma-Aldrich). DNA fragments obtained by PCR were loaded on gels containing 1% or 2% (w/v) agarose (Thermo Scientific) and 1xTAE buffer (Thermo Scientific), excised and purified (Zymoclean™, D2004, Zymo Research, Irvine, CA). Alternatively, fragments were purified using the GenElute™ PCR Clean-Up Kit (Sigma-Aldrich). Plasmids were isolated from E. coli with Sigma GenElute Plasmid kit (Sigma-Aldrich) according to the supplier's manual. Yeast plasmids were isolated according to (Kuijpers et al., 2013). Yeast genomic DNA was isolated using a YeaStar Genomic DNA kit (Zymo Research). E. coli DH5a (18258012, Invitrogen) was transformed chemically (T3001, Zymo Research) or by electroporation. Chemical transformation was done according to supplier's instructions. Electroporation was done in a 2 mm cuvette (165-2086, BioRad, Hercules, CA) using a Gene PulserXcell Electroporation System (BioRad), following the manufacturer's protocol.

2.5. Media and cultivation

Shake flask cultures were grown at 30 °C in 500 mL flasks containing 100 mL synthetic medium (Verduyn et al., 1992) with 20 g L"1 glucose in an Innova incubator shaker (New Brunswick Scientific, Edison, NJ) set at 200 rpm. Anaerobic cultures were grown in 100 mL shake flasks on the same medium, supplemented with the anaerobic growth factors ergosterol (10mgL"1) and Tween 80 (420 mg L"1) according to (Verduyn et al., 1990b) and incubated on a Unimax1010 shaker (Heidolph, Kelheim, Germany) set at 200 rpm, placed in a Bactron anaerobic chamber (Sheldon MFG Inc., Cornelius, OR, USA) with a gas mixture of 5% H2, 6% CO2, and 89% N2. Optical density at 660 nm was measured in regular time intervals with a Libra S11 spectrophotometer (Biochrom, Cambrige, UK). Controlled batch and chemostat cultivation was carried out at 30 °C in 2-L laboratory bioreactors (Applikon, Schiedam, The Netherlands) with working volumes of 1 L. Chemo-stat cultivation was preceded by a batch phase under the same conditions. When a rapid decrease in CO2 production indicated glucose depletion in the batch cultures, continuous cultivation at a

dilution rate of 0.10 h-1 was initiated. Synthetic medium (Verduyn et al., 1992) supplemented with 7.5 g L-1 or 20 g L-1 glucose was used in aerobic and anaerobic chemostat cultures, respectively. Culture pH was maintained at 5.0 by automatic addition of 2 M KOH. In aerobic cultures, antifoam Pluronic PE 6100 (BASF, Ludwigshafen, Germany) was used. The antifoam was autoclaved (110 °C) separately from the other medium ingredients and added as a 15% (w/v) stock solution to a final concentration of 0.15 g L-1. Aerobic bioreactors were sparged with 500mLmin-1 air and stirred at 800 rpm to ensure fully aerobic conditions. Anaerobic bioreactors were stirred at 800 rpm and sparged with 500mLmin-1 nitrogen ( < 10 ppm oxygen). Anaerobic growth media were supplemented with the anaerobic growth factors ergosterol (10mgL-1) and Tween 80 (420 mg L-1). Antifoam C Emulsion (Sigma-Aldrich) was autoclaved separately (120 °C) as a 20% (w/v) solution and added to a final concentration of 0.2 g L-1. In anaerobic bioreactor batch cultures, the medium contained 25 g L-1 glucose. To minimize diffusion of oxygen, the bioreactors were equipped with Nonprene tubing and Viton O-rings and medium reservoirs were continuously flushed with nitrogen. Specific growth rates were calculated from biomass dry weight determinations.

2.6. Analytical methods

Chemostat cultures were assumed to be in steady state when, after at least five volume changes, the carbon dioxide production rates changed by less than by 4% over 2 volume changes. Steady state samples were taken between 10 and 15 volume changes after inoculation. Dry weight measurements, HPLC analysis of the supernatant and off-gas analysis were performed as described previously (de Kok et al., 2011). Ethanol concentrations were corrected for evaporation as described by Guadalupe Medina et al. (2010). Samples for formate, glycerol and residual glucose determination were taken with the stainless steel bead method for rapid quenching of metabolites (Mashego et al., 2003).

2.7. Enzymatic determination of metabolites

Extracellular formate, glycerol and residual glucose were measured using the Formic Acid Determination Kit (10979732035, R-Biopharm AG, Zaandijk, The Netherlands), Glycerol Enzymatic Determination Kit (10148270035, R-Biopharm AG) and EnzyPlus D-Glucose kit (EZS781, BioControl Systems Inc., Bellevue, WA, USA), respectively. Measurements were done according to manufacturers' instructions, except that the volume of the assays was proportionally downscaled. Absorbance was measured using cuvettes (final volume 1 mL, with at least two replicates) on a LibraS11 spectrophotometer or using 96-well plates (final volume 0.3 mL, with at least three replicates) on a Tecan GENios Pro (Tecan, Giessen, Netherlands).

2.8. Intracellular metabolites determination

Culture samples for intracellular metabolite analysis were taken from bioreactors with a cold methanol rapid quenching method using a specially designed rapid sampling setup (Lange et al., 2001). 1.2 mL of broth was withdrawn into 5 mL of 80% aqueous methanol (v/v) solution pre-cooled to - 40 °C. Samples subsequently were washed with cold methanol and extracted with boiling ethanol as described in Canelas et al. (2008a). The concentrations of glucose, glucose-6-phosphate, fructose-6-phosphate, 2-phosphoglycerate, 3-phosphogly-cerate, glyceraldehyde phosphate, dihydroxyacetone phosphate, 6-phosphogluconate, erythrose-4-phosphate, ribose-5-phosphate, ribulose-5-phosphate, xylulose-5-phosphate, sedoheptulose-7-phos-phate, citrate, fumarate, malate, a-ketoglutarate and trehalose were determined as methoxime-trimethylsilyl derivatives by GC-MS

(Cipollina et al., 2009). The concentrations of succinate, fructose-1, 6-bisphosphate, threhalose-6-phosphate, glucose-1-phosphate, gly-cerol-3-phosphate, phosphoenolpyruvate, UDP-glucose and mannitol-1-phosphate were determined by anion-exchange LC-MS/MS (Van Dam et al., 2002). Concentrations of 20 amino acids and of pyruvate were determined by GC-MS (deJonge et al., 2011). Concentrations of AMP, ADP, ATP, NAD, NADH, NADP, NADPH, CoA, acetyl-CoA and FAD were determined by ultra high performance liquid chromato-graphy hyphenated with tandem mass spectrometry, UPLC-MS/ MS. All measurements were carried out on an AcQuity™ UPLC system (Waters, Milford, MA, USA) coupled to a Waters Quattro Premier XE mass spectrometer, (Micromass MS Technologies-Waters) equipped with an electrospray ion source. The MS was operated in negative ion mode. Metabolite detection was performed in multiple reaction monitoring mode (MRM). The MRM transitions and corresponding instrument settings yielding the highest S/N, were separately found for each individual metabolite by direct infusion into the MS. The chromatographic separation of coenzymes in cell extracts was based on ion pair liquid chromato-graphy on a reverse phase column AcQuity™ UPLC® BEH C18 (1.7 mm, 100 x 2.1 mm2 i.d., waters, Ireland). A linear gradient of mobile phase A (2 mM dibutylammonium acetate, DBAA, and 5% (v/ v) acetonitrile) and mobile phase B (2 mM DBAA and 84% (v/v) acetonitrile) was used to separate the coenzymes. For both analytical platforms, uniformly 13C-labelled cell extracts were used as internal standards (Wu et al., 2005).

2.9. Acetaldehyde determination

Culture samples for acetaldehyde analysis were obtained with the rapid sampling setup used for intracellular metabolites analysis (Lange et al., 2001). The acetaldehyde determination was performed as described by Bekers et al. (2013). Approximately 1.2 mL of broth was withdrawn into 6 mL of pre-cooled (- 40 °C) quenching and derivatization solution containing 0.9 g L-1 2,4-dinitrophenylhy-drazine and 1% (v/v) phosphoric acid in acetonitrile. After mixing and incubation for 2 h on a Nutating Mixer (VWR International, Leuven, Belgium) at 4 °C, samples were stored at - 40 °C. Before analysis 1 mL of defrosted and well mixed sample was centrifuged (15,000g, 3 min). The supernatant was analysed via HPLC using a WATERS WAT086344 silica-based, reverse phase C18 column operated at room temperature with a gradient of acetonitrile as a mobile phase. A linear gradient was generated from eluent A - 30% (v/v) aqueous acetonitrile solution and eluent B - 80% (v/v) aqueous acetonitrile solution. The mobile phase composition was changing from 0% to 100% of eluent B in 20 min, at a flow rate of 1 mL min-1. A calibration curve was prepared with standard solution of 50.9 g L-1 acetaldehyde-2,4-dinitrophenylhydrazine in acetonitrile.

2.10. Microarrays assay and analysis

Sampling for transcriptome analysis from chemostat cultures and total RNA extraction was performed as described previously (Piper et al., 2002). Processing of total RNA was performed according to Affymetrix instructions. RNA target preparation for microarray expression analysis was done with the Gene 3' IVT Express Kit (Affymetrix, Santa Clara, CA) using 200 ng of total RNA. The manufacturer's protocol was carried out with minor modifications, i.e the Affymetrix polyA RNA controls were excluded from the aRNA amplification protocol and the IVT reactions were incubated for 16 h at 40 °C. Quality of total RNA, cDNA, aRNA and fragmented aRNA was checked with an Agilent Bioanalyzer 2100 (Agilent Technologies, Amstelveen, The Netherlands). Hybridization, washing and scanning of Affymetrix chips were done according to manufacturers' instructions. For each strain analysed, microarrays were run on three independent cultures. Processing of

expression data (normalization, expression cut-off, etc.) was performed as described previously (Boer et al., 2003). The Significance Analysis of Microarrays (SAM, version 3.0) (Tusher et al., 2001) add-in to Microsoft Excel was used for comparison of replicate array experiments using a minimal fold change of two and false discovery rate of 1%. Overrepresentation of functional categories in sets of differentially expressed genes was analysed according to Knijnenburg et al. (2007). Transcript data are available at the Genome Expression Omnibus database (http://www.ncbi.nlm.nih. gov/geo) under accession number GSE47983.

A set of acetaldehyde-responsive genes (Aranda and del Olmo, 2004) used as a reference is accessible from www.uv.es/~ arandaa. Statistical significance of the over-representation of these genes in subsets of yeast genes was computed as described in Knijnenburg et al. (2007), replacing functional categories by reference set of acetaldehyde-responsive genes.

2.11. A-ALD activity assay

For preparation of cell extracts, culture samples (corresponding to ca. 62.5 mg dry weight) were harvested from exponentially growing shake flasks cultures on 20 g L" 1 glucose, washed, stored and prepared for sonication as described previously (Postma et al., 1989). Cell extracts were prepared by sonication (4 bursts of 30 s with 30 s intervals and at 0 °C) with an amplitude setting of 7-8 ^m on a Soniprep 150 sonicator (Beun de Ronde BV, Abcoude, The Netherlands). After removal of cells and debris by centrifugation (4 °C, 20 min at 47,000g), the supernatant was used for enzyme assays. Protein concentrations in cell extracts were estimated with the Lowry method (Lowry et al., 1951). A-ALD activity was measured at 30 °C on a Hitachi model 100-60 spectrophotometer by monitoring the reduction of NAD+ at 340 nm in a 1 mL reaction mixture containing 0.1 mM Coenzyme A, 50 mM CHES buffer pH 9.5, 0.8 mM NAD+, 0.2 mM DTT and 20-100 ^L of cell extract. The reaction was started by adding 100 ^L of freshly prepared 100 mM acetaldehyde solution. A-ALD activity in the reverse reaction was assayed as described previously (Guadalupe Medina et al., 2010). Enzyme activities are expressed as ^mol substrate converted per min per mg protein (U mg protein"1). Reaction rates were proportional to the amounts of cell extract added.

2.12. Viability staining

The staining procedure was performed as described previously (Boender et al., 2011), using the FungaLight™ CFDA, AM/Propidium Iodide Yeast Vitality Kit (Invitrogen, Carlsbrad, CA). When membrane integrity is compromised, propidium iodide can diffuse into the cell and intercalate with DNA, yielding a red fluorescence. The acetoxymethyl ester of 5-carboxyfluorescein diacetate (CFDA, AM in DMSO) can permeate through intact membranes. In metaboli-cally active cells, diacetate- and lipophilic blocking-groups are cleaved off by cytosolic non-specific esterases, yielding a charged, green fluorescent product. The pictures were taken with a fluorescent microscope (Imager-D1, Carl-Zeiss, Oberkochen, Germany) equipped with Filter Set 09 (FITC LP Ex. BP 450-490 Beamso. FT 510Em. LP 515, Carl-Zeiss).

2.13. Chitinase digestion

100 ^L of chemostat culture was spun down and resuspended in 100 ^L potassium phosphate buffer (50 mM, pH 6.10) or potassium phosphate buffer (50 mM, pH 6.10) with 1 mg mL"1 of chitinase (Chitinase from Trichoderma viride, > 600 units mg"1, Sigma-Aldrich). The reaction mixture was incubated for 3 h at 30 °C.

3. Results

3.1. Heterologous genes encoding A-ALD and PFL restore fast growth on glucose of Ald ~ and acs2A mutants

To enable analysis of the functional expression of heterologous A-ALD genes, the five genes encoding acetaldehyde dehydrogenases (ALD2, ALD3, ALD4, ALD5 and ALD6; Navarro-Avino et al., 1999) in S. cerevisiae were deleted. In cell extracts of the resulting strain 1MK354, NAD- and NADP-dependent acetaldehyde dehydrogenase activities measured according to Postma et al. (1989) were below the detection limit of the assay of 2 nmol min-1 mg protein-1. Subsequently, four different genes encoding A-ALD, (dmpF from Pseudomonas sp., adhE from Staphylococcus aureus, eutE from E. coli and lin1129 from Listeria innocua) were codon-optimised for expression in S. cerevisiae and individually introduced in strain 1MK354 under the control of the constitutive TDH3 promoter (Table 2). A non-codon-optimised version of E. coli mhpF was also tested. Expression of all tested A-ALD variants enabled fast growth of Ald- S. cerevisiae on synthetic medium agar plates containing 20 g L-1 glucose (Fig. 1, left panel). In shake-flask cultures grown on glucose as sole carbon source, the specific growth rate of the reference strain 1MZ282 (Ald-, empty expression vector) was 0.03 h- 1, which is less than 10% of the growth rate of the Ald + strain 1ME140 (empty vector reference, Table 1) under the same conditions. Of the Ald- strains expressing heterologous A-ALD genes, strains 1MZ290 (expressing E. coli eutE) and 1MZ291 (expressing L. innocua lin1129) showed the highest maximum specific growth rates (0.27 h-1 and 0.25 h-1, respectively). These growth rates were over 75% of that of the Ald+ reference strain 1ME140. The high growth rates of these strains coincided with high activities of A-ALD in cell extracts (Table 3). Based on these results, strain 1MZ290, which expresses E. coli eutE, was selected for further studies.

To investigate functional expression of the PFL and PFL-AE genes (those two genes are further referred to as PFL, unless otherwise

stated) in S. cerevisiae, the acs2A strain IMK427 was used as a screening platform. As reported previously, deletion of ACS2 completely abolished growth on glucose plates due to repression and glucose catabolite inactivation of ACS1 and its gene product (de Jong-Gubbels et al., 1997; van den Berg et al., 1996) (Fig. 1). Genes encoding PFL and PFL-AE from five different organisms (Thalassiosira pseudonana, Chlamydomonas reinhardtii, E. coli, Lactobacillus plantarum and Neo-callimastix frontalis) were codon-optimized and expressed in strain IMK427 under the control of the constitutive TPI1 and FBA1 promoters, respectively. To prevent oxygen inactivation of PFL (Knappe et al., 1969), growth was compared on anaerobic plates with glucose as sole carbon source. Under these conditions, growth was only observed for strains IMZ371 and IMZ372, which expressed PFL from E. coli and L. plantarum, respectively (Fig. 1). These strains were therefore used for further physiological analysis.

3.2. Complementation of acs1 acs2 double mutants by A-ALD or PFL

Viable S. cerevisiae strains in which both ACS1 and ACS2 have been inactivated have not been described in the literature. We therefore investigated whether replacement of the role of acetyl-CoA synthetase in cytosolic acetyl-CoA synthesis by A-ALD or PFL is sufficient to enable growth of acslA acs2A mutants.

Deletion of both ACS genes in strain IMZ290 (Ald- expressing E. coli eutE) yielded strain IMZ305, which, in glucose-grown shake-flask cultures, showed a specific growth rate of 0.26 + 0.01 h-1 (79% of the Ald + Acs+ reference strain IME140). Similarly, deletion of ACS1 in strains IMZ371 and IMZ372 (acs2A expressing PFL and PFL-AE from E. coli and L. plantarum, respectively) resulted in two strains (IMZ383 and IMZ384) that were able to grow anaerobically on glucose. Consistent with an essential role of oxygen-sensitive PFL in acetyl-CoA synthesis, these strains did not grow on glucose aerobically. Specific growth rates on glucose of the Acs- PFL-expressing strains IMZ383 and IMZ384 in anaerobic shake-flask

# cells plated 103 102 101 103 102 101

aerobic anaerobic

Fig. 1. Growth of the Ald- strains expressing A-ALDs (IMZ284-IMZ291), the acs2A PFL and PFL-AE expressing strains (IMZ369-IMZ373), the Ald- reference strain IMZ282, the acs2A reference strain IMZ374 and Ald+ ACS2 reference strain IME140 on synthetic medium agar plates with 20 gL-1 glucose. Plates containing A-ALD strains were incubated aerobically for 48 h. Plates containing PFL strains were incubated anaerobically for 90 h.

Aerobic maximum specific growth rates on glucose and acetylating acetaldehyde dehydrogenases activities with acetaldehyde or acetyl-CoA as a substrate of Saccharomyces cerevisiae strains carrying different acetylating acetaldehyde dehydrogenases. Averages and mean deviations were obtained from duplicate experiments. The detection limit of the enzyme assays was 2 nmol min"1 mg protein"1.

Strain Relevant genotype Growth (h1) Enzyme activity (^mol mg protein 1 min 1)

Acetaldehyde Acetyl-CoA

IME140 ALD2 ALD3 ALD4 ALD5 ALD6 0.33 7 0.004 N. D. N. D.

IMZ282 aldA 0.03 7 0.001 N. D. N. D.

IMZ284 aldA dmpF 0.21 7 0.001 0.31 7 0.06 0.047 0.01

IMZ286 aldA mhpF 0.237 0.004 0.197 0.01 0.02 7 0.001

IMZ289 aldA adhE 0.227 0.001 0.187 0.04 0.067 0.02

IMZ290 aldA eutE 0.277 0.001 7.957 0.33 2.01 7 0.04

IMZ291 aldA lin1129 0.257 0.002 6.577 0.61 1.157 0.13

N. D.=Not detected.

cultures were 0.20 + 0.00 h 1 (73% of the Acs+ reference strain IME140) and 0.14 + 0.00 h-1 (53%), respectively (average values and mean deviations are from at least two independent experiments). Growth and product formation of strain IMZ383 on glucose was further studied in anaerobic bioreactors (Fig. 2), in which its specific growth rate (0.20 + 0.00 h-1) was the same as observed in anaerobic shake flasks. In contrast to the Acs+ reference strain CEN.PK113-7D (data not shown), strain IMZ383 produced formate throughout exponential growth, with a stoi-chiometry of 2.5 + 0.1 mmol formate per gram biomass dry weight.

Because, in an Acs- PFL-expressing yeast strain, production of acetyl-CoA and formate via PFL is stoichiometrically coupled, the turnover of cytosolic acetyl-CoA should at least equal the specific rate of formate production. In S. cerevisiae, cytosolic acetyl-CoA is required for synthesis of lipids, lysine, methionine, sterols and N-acetylglucosamine as well as for protein acetylation. Since the synthesis of unsaturated fatty acids and sterols requires molecular oxygen, these compounds are routinely included in anaerobic growth media. The cytosolic acetyl-CoA requirement for lipid and lysine synthesis in aerobic cultures has earlier been estimated at 1.04 mmol acetyl-CoA per gram dry biomass (Flikweert et al., 1999). Based on published pathways and biomass compositions of S. cerevisiae (Oura, 1972), cytosolic acetyl-CoA requirements of methionine and cysteine for protein biosynthesis can be estimated at a combined 0.05 mmol per gram dry biomass. Additional 0.05 mmol acetyl-CoA per gram dry biomass is required for the synthesis of glutathione (Dhaoui et al., 2011) and chitin synthesis requires another 0.02 mmol N-acetylglucosamine per gram dry biomass (Lesage et al., 2005). Approximately one sixth of the proteins in the yeast proteome is estimated to be lysine acetylated (Henriksen et al., 2012). Even if all those proteins are simultaneously acetylated this would only correspond to an additional acetyl-CoA requirement of circa 2.5 x 10- 3 mmol per gram dry biomass (including also poly-acetyla-tion), assuming a protein content of 39% (Oura, 1972) and an average protein molecular weight of 56 kDa. The combined cytosolic acetyl-CoA requirement is therefore estimated at 1.16 mmol per gram dry biomass. Hence, the observed formate production in the anaerobic cultures of acslA acs2A PFL-expressing S. cerevisiae is sufficient to account for the major 'sinks' of cytosolic acetyl-CoA in biosynthetic pathways.

3.3. Chemostat-based characterization of an Acs- PFL-expressing S. cerevisiae strain

To analyze the physiological impact of replacing ACS1 and ACS2 by PFL, the reference strain CEN.PK113-7D (Acs+) and strain IMZ383 (Acs- expressing E. coli PFL) were grown in anaerobic, glucose-limited chemostats at a dilution rate of 0.10 h -1 (Table 4). Strain IMZ383 showed a 15% lower biomass yield on glucose as

6 9 12 15 18 Time after inoculation (h)

Fig. 2. Growth of IMZ383 (Acs- expressing E. coli PFL) in an anaerobic bioreactor on synthetic medium with an initial glucose concentrations of 25 gL-1. The indicated averages and mean deviations are from a single batch experiment that is qualitatively representative of duplicate batch experiments. Symbols: extracellular formate (°) and dryweight (▼).

well as a proportionally increased ethanol production rate and reduced glycerol production rate relative to the reference strain. Conversely, acetate production by the PFL-expressing strain was five-fold higher than in the reference strain (Table 4). Formate, which was not detectable in cultures of the reference strain, was produced by the PFL-expressing strain at a rate of 0.18 mmol g biomass h-1. Acetate and formate have previously been shown to cause a reduction of biomass yields in cultures of S. cerevisiae by uncoupling the plasma membrane pH gradient (Abbott et al., 2007; Overkamp et al., 2002; Verduyn et al., 1990a). In anaerobic chemostat cultures, this uncoupling causes a concomitant increase of the ethanol production rate (Abbott et al., 2007; Overkamp et al., 2002; Verduyn et al., 1990a). Production of acetate and formate therefore offers a plausible explanation for the reduced biomass yield of the PFL-expressing strain.

Transcriptome analysis on anaerobic glucose-limited chemostat cultures yielded 71 genes whose transcript levels were higher, and 29 genes whose transcript levels were lower in IMZ383 (excluding URA3, ACS1 and ACS2) than in the reference strain. The set of up-regulated genes showed an overrepresentation of the GO categories ion transport, cellular iron ion homeostasis and cofactor metabolic process. The transcriptional responses of genes involved in iron homeostasis may be related to the assembly in yeast of PFL-AE, which contains an oxygen-sensitive [4Fe-4S] cluster required

Table 4

Physiology of the wild-type strain CEN.PK113-7D and the E. coli PFL expressing strain IMZ383 in anaerobic chemostat cultures with 25 g L-1 glucose, pH 5.0 and a dilution rate of 0.1 h-1. Values and standard deviations shown are from three replicates.

Units CEN.PK113-7D IMZ383

Relevant genotype Ald+Acs+ Ald+Acs- PFL

Dilution rate h-1 0.099 7 0.004 0.1027 0.004

Biomass yield g biomass g glucose 1 0.096 7 0.002 0.0827 0.002

qglucose mmol g biomass- h-1 - 5.76 7 0.15 - 6.94 7 0.31

qethanol mmol g biomass- h-1 9.51 7 0.43 11.387 0.49

qCo2 mmol g biomass- h-1 9.937 0.12 12.01 7 0.37

qglycerol mmol g biomass- h-1 0.82 7 0.06 0.707 0.03

qlactate mmol g biomass- h-1 0.067 0.004 0.047 0.002

qpyruvate mmol g biomass- h-1 0.01 7 0.004 0.037 0.001

qacetate mmol g biomass- h-1 0.027 0.003 0.11 7 0.02

qformate mmol g biomass- h-1 N. D. 0.187 0.01

Residual glucose gL-1 0.057 0.002 0.157 0.01

Carbon recovery % 104 7 2 100 7 1

N. D.— Not detected.

for activation of PFL (Kulzer et al., 1998). Additionally, transcript levels of the formate dehydrogenase genes FDH1 and FDH2 were over 25-fold higher in the PFL-expressing strain. Among the genes that showed lower transcript levels in IMZ383 (Table 5), the GO category 'glycine metabolic process' was overrepresented. The four down-regulated genes in this category, GCV1, GCV2, GCV3 and SHM2 encode proteins that, together with Lpd1, form the glycine cleavage system, which contributes to the synthesis of the C1-donor 5,10-methylene tetrahydrofolate (5,10-MTHF) in S. cerevisiae (Nagarajan and Storms, 1997). The observed down-regulation suggests that conversion of formate, produced by PFL, to 5,10-MTHF via Mis1 or Ade3 (Shannon and Rabinowitz, 1988; Staben and Rabinowitz, 1986) reduces the requirement for synthesis of C1-donor compounds via the glycine cleavage pathway.

3.4. Characterization of an Ald- Acs - A-ALD expressing S. cerevisiae strain in chemostats

The physiology of strain IMZ305 (Ald- Acs- expressing E. coli eutE) was studied in aerobic glucose-limited chemostats at a dilution rate of 0.10 h-1 and compared to that of the reference strain CEN.PK113-7D (Ald+Acs+). Under fully aerobic conditions, the low residual glucose concentration in glucose-limited chemo-stat cultures at low dilution rates ( < ca. 0.25 h-1), enables a fully respiratory sugar metabolism in wild-type S. cerevisiae strains (Postma et al., 1989; Verduyn et al., 1984). Although the sugar metabolism of both strains was indeed completely respiratory (Table 6), the biomass yield on glucose of strain IMZ305 was 14% lower than that of the reference strain. The lower biomass yield was in agreement with higher rates of oxygen consumption and CO2 production (Table 6). This lower biomass yield of strain IMZ305 was unexpected in view of the improved ATP-stoichiometry for producing cytosolic acetyl-CoA via A-ALD.

Chemostat-based transcriptome analysis yielded 362 genes whose expression levels were different in strains CEN.PK113-7D (Ald+Acs+) and IMZ305 (Ald-Acs- expressing E. coli eutE) based on the statistical criteria applied in this study. URA3, ALD2, ALD3, ALD4, ALD5, ALD6, ACS1 and ACS2 were not included in this comparison. Fisher's exact test analysis indicated the overrepre-sentation of several functional categories, involved in energy, metabolism (isoprenoids, triterpenes, fatty acids, sterol, lipids, monocarboxylic acids, carbohydrates and energy reserves), response to stress and cell rescue, defence and virulence (Table 5).

To investigate the impact of possible changes in the levels of acetaldehyde, which is a substrate of both the A-ALD and ALD

reactions, the transcriptome data were compared to those from a previous study on the transcriptional response of S. cerevisiae to acetaldehyde (Aranda and del Olmo, 2004). Of a set of 1196 genes that showed an over 2-fold increase in transcript level upon exposure to acetaldehyde in the study of Aranda and del Olmo (2004), 56 were also up-regulated in strain IMZ305 relative to the reference strain. This overlap of the two gene sets was statistically highly significant (p-value 4.54 x 10- 8) and showed an overrepre-sentation of the GO category 'response to stress' (p-value 1 x 10-5). Other genes that were previously reported to be up-regulated in response to acetaldehyde (e.g. MET8, TPO2 and MUP1 (Aranda and del Olmo, 2004) similarly showed elevated expression levels in IMZ305. Analysis of combined intracellular and extracellular concentrations of acetaldehyde in chemostat cultures, using a fast sampling and derivatization protocol, showed that acetaldehyde concentrations in cultures of strain IMZ305 were 12-fold higher than in cultures of the reference strain CEN. PK113-7D (Table 7).

Microscopic analysis of cultures of strain IMZ305 (Ald-Acs-expressing E. coli eutE) showed that this strain formed multi-cellular aggregates. The same atypical morphology was observed in shake-flask and chemostat cultures of IMZ290 (Ald- expressing E. coli eutE) and IMZ291 (Ald - expressing L. innocua lin1129) which indicates that it is not caused by deletion of ACS1 and ACS2 and is not dependent on the type of acetylating acetaldehyde dehydro-genase. The multicellular aggregates could not be disrupted by sonication, but were resolved into single cells by incubation with chitinase (data not shown). Multicellular aggregates were not observed in cultures of strain IMZ383 (Acs- expressing E. coli PFL and PFL-AE), further indicating that they were specifically linked to the expression of A-ALD and/or ALD deletion. Viability staining of chemostat cultures of strain IMZ305 (Ald - Acs- expressing E. coli eutE) indicated that integrity of the plasma membrane was compromised in a significant fraction of the cells in the multicellular aggregates (Fig. 3).

Cytosolic acetyl-CoA is a key precursor in yeast metabolism as well as a regulator of several important metabolic enzymes. To investigate whether replacing Acs1 and Acs2 by A-ALD affected concentrations of key compounds in central carbon metabolism, intracellular metabolite analysis was performed on aerobic, glucose-limited chemostat cultures of strain IMZ305 (Ald-Acs-expressing E. coli eutE) and on the reference strain CEN.PK113-7D (Ald+Acs+). The compounds analysed included acetyl-CoA, intermediates of glycolysis, pentose phosphate pathway (PPP) and tricarboxylic acid cycle (TCA), amino acids, nucleotides, coenzymes and trehalose. A full list of analysed compounds and measured levels is presented in supplementary materials (Table S2). Intra-cellular concentrations of most of the compounds, including acetyl-CoA and CoA, were not significantly different in the two strains, suggesting that replacing ACS by A-ALD as main source of cytosolic acetyl-CoA did not cause major changes in the central metabolic pathways. However, for a few metabolites a fold-change of at least two was observed (Table 7). Lysine, whose synthesis requires cytosolic acetyl-CoA, was among six amino acids whose intracellular concentrations were higher in strain IMZ305. Intra-cellular levels of glycerol-3-phosphate, which forms the backbone of glycerolipids and is thereby linked to lipid synthesis, another important biosynthetic sink of cytosolic acetyl-CoA, were also higher in this strain. Further changes in IMZ305 included higher concentrations of four intermediates of the non-oxidative part of the pentose phosphate pathway and changes in the levels of several glycolytic intermediates (Table 7). The levels of 3-phosphoglycerate and 2-phosphoglycerate were lower in IMZ305, while that of dihydroxyacetone phosphate was higher. In view of the reversibility of the triose phosphate isomerase, glyceraldehyde-3-phosphate dehydrogenase and phosphoglycerate mutase reactions, the resulting

MIPS and GO categories overrepresented in the genes that were significantly differentially expressed (0.5 > fold change (FC) or FC > 2, false discovery rate r 1%) in glucose-limited chemostat cultures of IMZ305 (Ald-Acs-, expressing E. coli eutE, aerobic) and IMZ383 (Acs- expressing E. coli PFL, anaerobic) compared to the reference strain CEN. PK113-7D (Ald+Acs+) grown under the same conditions.

Term id Description ka nb p-Value

IMZ305 up (148 genes)

MIPS 01.06.06 Isoprenoid metabolism 7 41 3.96 x 10 -5

01.06.06.11 Tetracyclic and pentacyclic triterpenes (cholesterin, steroids and hopanoids) metabolism 7 36 1.62 x 10 -5

32 Cell rescue, defense and virulence 28 558 8.28 x 10-5

GO lc G0:0006950 Response to stress 13 161 9.46 x 10 -5

cd G0:0016125 Sterol metabolic process 7 44 6.36 x 10 -5

IMZ305 down (212 genes)

MIPS 01 Metabolism 80 1530 3.75 x 10-6

01.05 C-compound and carbohydrate metabolism 37 510 3.76 x 10-6

01.06.05 Fatty acid metabolism 8 25 8.62 x 10 -7

02 Energy 34 360 2.00 x 10 - 8

02.19 Metabolism of energy reserves (e.g. glycogen, trehalose) 11 53 9.36 x 10 -7

GO l G0:0008152 Metabolic process 36 389 1.23 x 10 -8

GO:0006635 Fatty acid beta-oxidation 5 9 4.36 x 10 -6

G0:0055114 0xidation reduction 23 270 2.67 x 10-5

c G0:0032787 Monocarboxylic acid metabolic process 18 155 3.13 x 10-6

G0:0006635 Fatty acid beta-oxidation 5 9 4.36 x 10 -6

G0:0009062 Fatty acid catabolic process 5 10 8.48 x 10 -6

G0:0019395 Fatty acid oxidation 5 10 8.48 x 10 -6

G0:0034440 Lipid oxidation 5 10 8.48 x 10 -6

G0:0016042 Lipid catabolic process 9 43 8.76 x 10-6

G0:0005975 Carbohydrate metabolic process 28 361 1.97 x 10-5

G0:0044242 Cellular lipid catabolic process 7 27 2.03 x 10 -5

G0:0044262 Cellular carbohydrate metabolic process 26 323 2.11 x 10-5

G0:0055114 0xidation reduction 25 305 2.26 x 10-5

G0:0044281 Small molecule metabolic process 52 916 4.47 x 10 -5

G0:0009056 Catabolic process 42 683 4.77 x 10-5

G0:0006631 Fatty acid metabolic process 10 65 4.78 x 10-5

G0:0016054 0rganic acid catabolic process 9 53 5.16 x 10-5

G0:0046395 Carboxylic acid catabolic process 9 53 5.16 x 10-5

G0:0015980 Energy derivation by oxidation of organic compounds 16 161 7.88 x 10-5

IMZ383 up (71 genes)

MIPS 20.01 Transported compounds (substrates) 19 585 1.30 x 10 -5

32 Cell rescue, defense and virulence 20 558 1.56 x 10 -6

GO l G0:0006811 Ion transport 12 176 4.12 x 10 - 7

c G0:0051186 Cofactor metabolic process 10 192 4.49 x 10 -5

G0:0006879 Cellular iron ion homeostasis 5 41 8.11 x 10-5

G0:0055072 Iron ion homeostasis 5 41 8.11 x 10-5

G0:0006811 Ion transport 8 108 2.28 x 10-5

IMZ383 down (29 genes)

MIPS 01 Metabolism 20 1530 3.58 x 10-7

01.01 Amino acid metabolism 7 243 8.09 x 10 -5

01.01.09.01 Metabolism of glycine 4 8 2.38 x 10-8

01.01.09.01.02 Degradation of glycine 4 6 5.12 x 10 - 9

01.05 C-compound and carbohydrate metabolism 13 510 9.58 x 10 -8

01.05.05 C-1 compound metabolism 4 8 2.38 x 10-8

01.05.05.07 C-1 compound catabolism 3 5 8.38 x 10-7

02.16 Fermentation 4 48 5.83 x 10-5

GO l G0:0006730 0ne-carbon metabolic process 6 16 3.94 x 10 -11

G0:0055114 0xidation reduction 8 270 1.83 x 10 -5

c G0:0006082 0rganic acid metabolic process 13 397 4.68 x 10 -9

G0:0019752 Carboxylic acid metabolic process 13 397 4.68 x 10 -9

G0:0043436 0xoacid metabolic process 13 397 4.68 x 10 -9

G0:0042180 Cellular ketone metabolic process 13 410 6.93 x 10 -9

G0:0044281 Small molecule metabolic process 17 916 3.82 x 10-8

G0:0006544 Glycine metabolic process 4 9 4.26 x 10 -8

G0:0009071 Serine family amino acid catabolic process 3 5 8.38 x 10-7

G0:0006730 0ne-carbon metabolic process 6 76 8.92 x 10 -7

G0:0032787 Monocarboxylic acid metabolic process 7 155 4.33 x 10 -6

G0:0009069 Serine family amino acid metabolic process 4 34 1.45 x 10 -5

G0:0055114 0xidation reduction 8 305 4.41 x 10-5

G0:0016054 0rganic acid catabolic process 4 53 8.63 x 10 -5

G0:0046395 Carboxylic acid catabolic process 4 53 8.63 x 10 -5

IMZ305 up n IMZ383 up (8 genes)

MIPS 20.01.01.01.01.01 Siderophore-iron transport 2 12 9.02 x 10 -5

GO c G0:0006879 Cellular iron ion homeostasis 3 41 1.35 x 10 -5

G0:0055072 Iron ion homeostasis 3 41 1.35 x 10 -5

G0:0034755 Iron ion transmembrane transport 2 5 1.37 x 10 - 5

G0:0034220 Ion transmembrane transport 3 48 2.18 x 10-5

G0:0033212 Iron assimilation 2 8 3.83 x 10-5

G0:0030005 Cellular di-, tri-valent inorganic cation homeostasis 3 72 7.40 x 10 -5

G0:0055066 di-, tri-valent inorganic cation homeostasis 3 72 7.40 x 10 -5

Table 5 (continued )

Term id Description ka nb p-Value

IMZ305 downn IMZ383 down (8 genes)

MIPS 02.25 Oxidation of fatty acids 2 9 4.93 x 10 -5

IMZ305 down n IMZ383 up (10 genes)

No enrichment

IMZ305 up n IMZ383 down (0 genes)

Empty set

a The number of genes differentially expressed present in the set b The number of genes of the set present in the whole genome. c GO categories are divided between the GO leaf categories d GO categories are divided between the GO complete categories.

Table 6

Physiology of S. cerevisiae strains CEN.PK113-7D (Ald+Acs+) and IMZ305 (Ald-Acs- expressing E. coli eutE) in aerobic glucose-limited chemostat cultures at a dilution rate of 0.1 h-1. Averages and standard deviations were obtained from three replicates.

Units CEN.PK113-7D IMZ305

Relevant genotype Ald+Acs+ Ald-Acs- eutE

Dilution rate h-1 0.100 7 0.001 0.0987 0.001

Biomass yield g biomass g glucose 1 0.501 7 0.002 0.4297 0.009

qglucose mmol g biomass- h-1 -1.10 7 0.01 -1.27 7 0.02

qethanol mmol g biomass- h-1 0.007 0.00 0.007 0.00

qco2 mmol g biomass- h-1 2.78 7 0.09 3.527 0.04

qoxygen mmol g biomass- h-1 - 2.61 7 0.14 - 3.36 7 0.02

qpyruvate mmol g biomass- h-1 N. D. N. D.

qglycerol mmol g biomass- h-1 N. D. N. D.

qacetate mmol g biomass- h-1 N. D. N. D.

Residual glucose gL-1 0.03 7 0.01 0.047 0.02

Carbon recovery % 102 7 1 98 7 2

N. D.— Not detected.

6-fold higher ratio of dihydroxyacetone phosphate and 3-phosphoglycerate is likely to reflect a lower cytosolic NADH/NAD+ ratio in IMZ305. This interpretation was supported by direct measurements of total intracellular NADH and NAD+ concentrations in the two strains (Table 7).

4. Discussion

Previous studies have demonstrated that increasing the availability of cytosolic acetyl-CoA improved rates of product formation in S. cerevisiae strains engineered for production of isoprenoids, fatty acids and polyhydroxybutyrate. Some of these studies were based on the overexpression of an acetyl-CoA synthetase that was insensitive to inhibition by acetylation, either alone or in combination with acetaldehyde dehydrogenase (Chen et al., 2013; Shiba et al., 2007). In other studies, availability of acetyl-CoA was boosted by expression of murine ATP-citrate lyase (Tang et al., 2013) or of a fungal phosphoketolase pathway that generates acetate (Kocharin et al., 2013). These previously studied hetero-logous pathways still require ATP for synthesis of cytosolic acetyl-CoA and, moreover, the native pathway via acetyl-CoA synthetase was still active in the engineered strains. The present study demonstrates, for the first time, that the native pathway for cytosolic acetyl-CoA biosynthesis in S. cerevisiae can be entirely replaced by heterologous pathways that, at least in terms of pathway stoichiometry, do not involve a net investment of ATP.

In wild-type S. cerevisiae genetic backgrounds, deletion of the two genes encoding isoenzymes of acetyl-CoA synthetase (ACS1 and ACS2) resulted in a complete loss of viability, which was originally entirely attributed to a key role of acetyl-CoA synthetase

Table 7

Steady-state intracellular meatabolites concentrations (^moUg dry weight-1) and acetaldehyde concentration in the broth (mM) in S. cerevisiae cultures of CEN. PK113-7D (Ald+Acs+) and IMZ305 (Ald-Acs- expressing E. coli eutE) in aerobic glucose-limited chemostat cultures at a dilution rate of 0.1 h-1. Averages and mean deviations were obtained from two replicates.

Metabolite CEN.PK113-7D IMZ305

Glycolysis

Fructose-1,6-bisphosphate 0.307 0.02 1.967 0.12

Dihydroxyacetone phosphate 0.197 0.02 0.537 0.05

2-phosphoglycerate 0.427 0.05 0.167 0.02

3-phosphoglycerate 4.41 7 0.62 2.067 0.27

Phosphoenolpyruvate 1.71 7 0.37 0.297 0.06

Pentose phosphate pathway

Ribulose-5-phosphate 0.11 7 0.03 0.237 0.03

Xylulose-5-phosphate 0.267 0.06 0.547 0.08

Sedoheptulose-7-phosphate 2.607 0.46 5.937 0.68

Erythrose-4-phosphate 0.007 0.00 0.01 7 0.00

Amino acids

Alanine 2.047 0.05 5.247 1.29

Isoleucine 0.27 7 0.10 0.667 0.11

Histidine 1.797 0.20 4.657 0.63

Lysine 3.307 0.50 9.607 1.72

Proline 0.25 7 0.10 0.647 0.16

Threonine 0.55 7 0.10 1.287 0.24

Tyrosine 0.37 7 0.10 0.937 0.13

Coenzymes and cofactors

AMP 0.327 0.02 0.727 0.16

NAD+ 2.657 0.18 3.047 0.32

NADH 0.157 0.04 0.057 0.01

NADP+ 0.57 7 0.08 0.787 0.07

NADPH 2.707 0.62 2.107 0.72

Glycerol-3-phosphate 0.037 0.02 0.107 0.01

Intra-extracellular

Acetaldehyde 0.0027 0.000 0.0287 0.003

in cytosolic acetyl-CoA synthesis (Pronk et al., 1994; van den Berg and Steensma, 1995). Later studies investigated the role of Acs2, which has a dual cytosolic and nuclear localization, in histone acetylation (Falcon et al., 2010; Takahashi et al., 2006). Using a temperature-sensitive allele of ACS2, Takahashi et al. showed that, in glucose-grown cultures, inactivation of ACS2 caused global histone deacetylation and massive changes in gene expression, affecting over half of the yeast transcriptome. Using a similar approach Galdieri and Vancura (2012) proposed that reduced nucleocytosolic acetyl-CoA concentrations primarily affect cell physiology via histone deacetylation rather than via biosynthetic constraints. The limited and different transcriptome changes in strains 1MZ305 (Ald-Acs- expressing E. coli eutE) and 1MZ383 (Acs- expressing E. coli PFL and PFL-AE) relative to an Acs+

Fig. 3. Fluorescent micrographs of double stained cells aggregates formed in chemostat cultures of 1MZ305 strain (Ald-Acs- expressing E. coli eutE). Cells were stained with acetoxymethyl ester of 5-carboxyfluorescein diacetate (CFDA, AM in DMSO, green) and propidium iodide (PI, red) to indicate metabolically active cells and cells with compromised integrity of the membrane, respectively. The bar corresponds to 20 mm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

reference strain indicated that, in these strains, activities of A-ALD and PFL, respectively, were sufficient to cover acetyl-CoA requirements for histone acetylation. These results also support the conclusion of Takahashi et al. (2006) that Acs2 affects histone acetylation via provision of acetyl-CoA rather than via a direct catalytic or regulatory function in the acetylation process.

1n addition to a possible effect on histone acetylation, changes in cytosolic acetyl-CoA biosynthesis might affect central metabolism via acetylation of non-histone proteins (Guan and Xiong, 2011; Spange et al., 2009; Starheim et al., 2012) and via its direct participation in key reactions. The fast growth of the engineered strains 1MZ305 and 1MZ383 suggest that there were no major kinetic limitations in acetyl-CoA provision. This conclusion was further substantiated by the minor differences in intracellular metabolite levels of strain 1MZ305 relative to a reference strain. 1ntracellular acetyl-CoA concentrations in the two strains, which reflect the combination of mitochondrial and nucleocytosolic pools, were not significantly different. This observation is consistent with the conclusion of Cai et al. (2011) that intracellular acetyl-CoA concentrations are subject to strong homeostatic regulation. However, the higher intracellular lysine concentrations in strain 1MZ305 might be indicative for increased availability of cytosolic acetyl-CoA in this A-ALD-expressing strain.

Despite the stoichiometric advantage of the A-ALD pathway with respect to ATP costs for acetyl-CoA synthesis, the biomass yield on glucose of strain 1MZ305 was lower than that of the Ald+ Acs+ reference strain. Moreover, strain 1MZ305 exhibited a reduced viability and formation of multicellular aggregates. Tran-scriptome analysis (Table 5) and direct measurements of acetal-dehyde (Table 7) strongly suggested that these phenomena were due to acetaldehyde toxicity. Similar multicellular aggregates observed in S. cerevisiae cultures exposed to boric acid stress were attributed to activation of cell wall repair and overproduction of chitin, thereby disturbing cell division (Schmidt et al., 2010). Although whole-broth concentrations of acetaldehyde in cultures of strain 1MZ305 were previously not reported to be toxic to S. cerevisiae (Stanley et al., 1993), accumulation of acetaldehyde inside cells (Stanley and Pamment, 1993) may have led to underestimation of intracellular concentrations in our experiments. Matsufuji et al. (2013) recently showed that reaction with glu-tathione contributes to acetaldehyde tolerance. Although the

metabolic fate of the resulting acetaldehyde - glutathione adducts is unclear, regeneration of free glutathione from these adducts may well require metabolic energy and/or reducing equivalents.

The increased acetaldehyde level in strain 1MZ305 could be the result of the lower affinity for acetaldehyde of A-ALD in comparison to the native non-acetylating acetaldehyde dehydrogenases (Table 8) (Wang et al., 1998). Literature data on acetylating acetaldehyde dehydrogenases from other organisms suggest that a high KM for acetaldehyde is a common characteristic of these enzymes (Sanchez, 1998; Smith and Kaplan, 1980). The standard free energy change (AG°', pH 7, ionic strength of 0.2 M) of the A-ALD reaction is -13.7 kJ mol-1 (Flamholz et al., 2012). 1f measured intracellular metabolite concentrations (this study) and a cytosolic NAD+/NADH ratio of 100 (Canelas et al., 2008b) are assumed, AG' would be positive at the acetaldehyde concentrations measured in the reference strain (+ 7.2 kJ mol-1), while it would be close to zero (+0.7 kJ mol-1) at the higher acetaldehyde concentration measured in strain 1MZ305. 1ncreased intracellular levels of acetaldehyde may therefore not only be a consequence of a low affinity of A-ALD for acetaldehyde, but also be thermodyna-mically required for acetyl-CoA synthesis via A-ALD. A lower NADH/NAD+ ratio, as indicated by the observed changes in intracellular levels of dihydroxyacetone phosphate and 3-phos-phoglycerate, could similarly reflect a need to decrease AG' in order to allow a sufficiently high rate of acetaldehyde oxidation via A-ALD. 1f this interpretation is correct, applicability of this reaction for product formation in engineered yeast strains will either require improved tolerance to acetaldehyde or further changes in the cytosolic concentrations of acetyl-CoA, NADH and/or NAD+.

Also the biomass yield on glucose of strain 1MZ383 (Acs-expressing E. coli PFL) in anaerobic cultures was lower than that of an Acs+ reference strain. Weak-acid uncoupling by formate, the formation of which was stoichiometrically coupled to growth of this strain, offers a plausible explanation for this reduced biomass yield (Overkamp et al., 2002). To prevent accumulation and possible toxicity, the formate formed in the PFL reaction should preferably be oxidized to CO2 via formate dehydrogenase. Depending on the product of interest, this conversion by formate dehydrogenase may also be required for redox balancing of product formation in engineered pathways. Although FDH1 and FDH2, encoding the two NAD+ -dependent formate dehydrogenase isoenzymes in S. cerevisiae, were transcriptionally strongly up-regulated in strain 1MZ383, in vivo formate dehydrogenase activity was apparently not sufficient to oxidize all formate produced. Under standard conditions, formate oxidation (formate+NAD + 2CO2+NADH+H+) is thermodynami-cally feasible (AG'° = -13.9 kJ mol-1, pH 7, ionic strength of 0.2 M; Flamholz et al., 2012) and calculations showed that AG' is also

Table 8

Kinetic parameters of the acetylating acetaldehyde dehydrogenases EutE from E. coli and Lin1129 from L. innocua and of three out of five acetaldehyde dehydro-genases from S. cerevisiae (Ald2, Ald5 and Ald6). Activities were assayed as the oxidation of acetaldehyde to acetyl-CoA or acetate, respectively Data for EutE and Lin1129 are the average of triplicate measurements on cell extracts from two independent shake-flask cultures.

Enzyme KM for VMax (iimol mg VMax/KM References

acetaldehyde protein1 (^molL "*) min1)

EutE 1.5 x 103 9.4 7 0.4 0.006 This study

Lin1129 3.9 x 103 9.2 7 1.3 0.002 This study

Ald2 10 5.2 0.52 Wang et al.

(1998)

Ald5 58 1.1 0.019 Wang et al.

(1998)

Ald6 24 24 1 Wang et al.

(1998)

negative under industrially relevant conditions (data not shown). Inefficient anaerobic oxidation of formate upon overexpression of FDH1 was previously attributed to the sequential bi-bi two-substrate kinetics of formate dehydrogenase, which leads to a strongly decreased affinity for formate at low NAD+ concentrations (Geertman et al., 2006).

In S. cerevisiae, several metabolic reactions as well as protein deacetylation, yield free acetate. Nevertheless, no increased acetate concentrations were observed in cultures of strain 1MZ305 (Ald~Acs~ E. coli eutE). This observation suggests that small amounts of acetate can be activated to acetyl-CoA via an Acs-independent pathway. A mitochondrial CoA transferase encoded by ACH1 may, in theory, be responsible for acetate activation, using succinyl-CoA as CoA donor (Fleck and Brock, 2009). However, ACH1 was not up-regulated, neither in strain 1MZ305 nor in strain 1MZ383 (Acs_ expressing E. coli PFL). The latter strain did show increased production of acetate in comparison with Acs+ reference strain, probably via pyruvate decarboxylase and acetaldehyde dehydrogenase, which were still present in this strain. This observation suggests that a possible Acs-independent pathway for acetate activation in anaerobic S. cerevisiae cultures only has a very low capacity.

5. Outlook

This study demonstrates that it is possible to entirely replace the ATP-intensive native pathway of cytosolic acetyl-CoA formation in S. cerevisiae by two different ATP-independent routes. Although these metabolic engineering strategies are stoichiome-trically sound, our results show how their application can be kinetically and/or thermodynamically challenging. Before the full potential of the A-ALD and PFL strategies for generating cytosolic acetyl-CoA for industrial product formation can be realized, problems related to acetaldehyde toxicity and formate reoxidation, respectively, will need to be addressed. Availability of strains in which A-ALD and/or PFL are the only source of cytosolic acetyl-CoA will be of great value for their further optimization, either by targeted genetic modification or by evolutionary approaches. Alternatively, the native yeast pathway might be replaced by other mechanisms for cytosolic acetyl-CoA synthesis with an improved ATP stoichiometry, such as ATP-citrate lyase (which requires an input of one ATP for each pyruvate converted to cytosolic acetyl-CoA) or the combination of phosphoketolase and phosphotransa-cetylase. Even before the remaining challenges are solved, A-ALD and PFL can provide increased fluxes towards products that require cytosolic acetyl-CoA. For some products, this may ultimately even contribute to the replacement of expensive aerobic processes by cost-effective anaerobic conversions.

Acknowledgments

This work was carried out within the BE-Basic R&D Program, which was granted a FES subsidy from the Dutch Ministry of Economic Affairs, Agriculture and Innovation (EL&I). We thank our colleagues Reza Maleki Seifar, Feibai Zhu, Marinka Almering, Marijke Luttik, Erik de Hulster, Marcel van den Broek and Nakul Barfa for technical support.

Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ymben.2013.11.005.

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