| Summary: | Sluggish or stuck wine fermentations occasionally occur in wineries and wines from these problem fermentations are susceptible to microbial spoilage and oxidation. Many factors, including the presence of acetic acid are correlated with these fermentations. The effect of acetic acid inhibition on the growth and fermentation rate of Saccharomyces cerevisiae has been well studied but mainly under laboratory conditions in media containing 1 % to at most 5 % glucose while shaking. These conditions are vastly different from wine-making conditions and we have a limited knowledge as to why stuck or sluggish fermentations occur. We used DNA microarray technology to investigate how 0.5, 1.5 or 3.0 g/L acetic acid affected the transcriptome of fermenting wine yeast after 20, 50 or 70 % of the sugars had been fermented. Global gene expression analyses revealed that when S. cerevisiae is under acetic acid stress, several metabolic processes are affected. The data collected after addition of 1.5 g/L acetic acid added at 20 % of the sugars were fermented show an up-regulation of PDR12 and PMA1 that encode ATP-dependent protein pumps to expel the weak acid anion and proton to prevent internal acidification. GLK1, TPS3, TSL1, TPS1, TPS2, NTH1, and NTH2 involved in trehalose metabolism were up-regulated as were GPD1, HOR2, and DAK1 involved in glycerol metabolism. Genes in sphingolipid metabolism including SUR2, LAC1, YPC1, SCS7 and DPL1 were up-regulated. The up-regulation of the genes in the TCA and electron transport chain even with 17.6 % (w/v) sugars remaining in the fermentation was surprising and might well be in response to an increased demand for ATP by the proton pumps. Genes involved in the general stress response including the transcription factors MSN2/4 were up-regulated as were 66 of 181 genes thought to be dependent on Msn2p/Msn4p. Electron microscopic studies of yeast cells exposed to acetic acid revealed a decrease in the number of autophagic bodies and autophagosomes as the length of exposure to acetic acid increased beyond 30 minutes. Data from remaining time points and acetic acid concentrations could not be grouped into known metabolic pathways. Our data have provided new insights into how yeast cells respond to acetic acid stress and also has potential to further our understanding of sluggish and stuck alcoholic fermentations that could prevent or minimize these problem fermentations. === Medicine, Faculty of === Medical Genetics, Department of === Graduate
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