Saccharomyces cerevisiae has recently been engineered to use acetate, a primary inhibitor in lignocellulosic hydrolysates, as a cosubstrate during anaerobic ethanolic fermentation. However, the original metabolic pathway devised to convert acetate to ethanol uses NADH-specific acetylating acetaldehyde dehydrogenase and alcohol dehydrogenase and quickly becomes constrained by limited NADH availability, even when glycerol formation is abolished. We present alcohol dehydrogenase as a novel target for anaerobic redox engineering of S. cerevisiae. Introduction of an NADPH-specific alcohol dehydrogenase (NADPH-ADH) not only reduces the NADH demand of the acetate-to-ethanol pathway but also allows the cell to effectively exchange NADPH for NADH during sugar fermentation. Unlike NADH, NADPH can be freely generated under anoxic conditions, via the oxidative pentose phosphate pathway. We show that an industrial bioethanol strain engineered with the original pathway (expressing acetylating acetaldehyde dehydrogenase from Bifidobacterium adolescentis and with deletions of glycerol-3-phosphate dehydrogenase genes GPD1 and GPD2) consumed 1.9 g liter ؊1 acetate during fermentation of 114 g liter ؊1 glucose. Combined with a decrease in glycerol production from 4.0 to 0.1 g liter ؊1 , this increased the ethanol yield by 4% over that for the wild type. We provide evidence that acetate consumption in this strain is indeed limited by NADH availability. By introducing an NADPH-ADH from Entamoeba histolytica and with overexpression of ACS2 and ZWF1, we increased acetate consumption to 5.3 g liter ؊1and raised the ethanol yield to 7% above the wild-type level. Saccharomyces cerevisiae is the principal microorganism used to produce ethanol, of which 93 billion liters were globally produced in 2014 (1). However, new strains are needed for the production of second-generation biofuels. Pretreatment and monomerization of lignocellulosic substrates produce complex sugar mixtures, including the C 5 sugars xylose and arabinose that are poorly fermented by most wild-type S. cerevisiae strains (2, 3). In addition, a variety of inhibitory compounds are released, such as furfural, hydroxymethylfurfural, methylglyoxal, and acetate (4-6). It is therefore desirable not only to expand the substrate range of S. cerevisiae (3, 7-9) but also to increase its inhibitor tolerance (10-13).Acetate is released during deacylation of hemicellulose and lignin and can be present in concentrations of Ͼ10 g liter Ϫ1 in cellulosic hydrolysates (4,14). At low pH particularly, this weak organic acid is a potent inhibitor of S. cerevisiae metabolism (15-17). Unfortunately, wild-type S. cerevisiae strains are poorly equipped to eliminate acetate from the medium under anoxic conditions. S. cerevisiae can grow on acetate as the sole carbon source under oxic conditions by converting acetate to acetyl coenzyme A (acetyl-CoA) with acetyl-CoA synthetase (ACS) (EC 6.2.1.1) and by either respiring acetyl-CoA in the tricarboxylic acid (TCA) cycle or upgrading acetyl-CoA to four-...

BackgroundPseudo-nitzschia multiseries Hasle (Hasle) (Ps-n) is distinctive among the ecologically important marine diatoms because it produces the neurotoxin domoic acid. Although the biology of Ps-n has been investigated intensely, the characterization of the genes and biochemical pathways leading to domoic acid biosynthesis has been limited. To identify transcripts whose levels correlate with domoic acid production, we analyzed Ps-n under conditions of high and low domoic acid production by cDNA microarray technology and reverse-transcription quantitative PCR (RT-qPCR) methods. Our goals included identifying and validating robust reference genes for Ps-n RNA expression analysis under these conditions.ResultsThrough microarray analysis of exponential- and stationary-phase cultures with low and high domoic acid production, respectively, we identified candidate reference genes whose transcripts did not vary across conditions. We tested eleven potential reference genes for stability using RT-qPCR and GeNorm analyses. Our results indicated that transcripts encoding JmjC, dynein, and histone H3 proteins were the most suitable for normalization of expression data under conditions of silicon-limitation, in late-exponential through stationary phase. The microarray studies identified a number of genes that were up- and down-regulated under toxin-producing conditions. RT-qPCR analysis, using the validated controls, confirmed the up-regulation of transcripts predicted to encode a cycloisomerase, an SLC6 transporter, phosphoenolpyruvate carboxykinase, glutamate dehydrogenase, a small heat shock protein, and an aldo-keto reductase, as well as the down-regulation of a transcript encoding a fucoxanthin-chlorophyll a-c binding protein, under these conditions.ConclusionOur results provide a strong basis for further studies of RNA expression levels in Ps-n, which will contribute to our understanding of genes involved in the production and release of domoic acid, an important neurotoxin that affects human health as well as ecosystem function.