Mohammed, Idris (2007) Gene expression profile of ethanol-stressed yeast in the presence of acetaldehyde. PhD thesis, Victoria University.

Abstract

One of the major yeast stressors during fermentation is ethanol accumulation. Ethanol stress is associated with reduced cell growth and viability, consequently lowering yeast productivity. Although the underlying causes of ethanol inhibition of cells are yet to be identified, it has been discovered that yeast acclimatise more quickly to ethanol stress in the presence of low acetaldehyde concentrations; however, the biochemical processes underpinning this effect are unknown. The objective of this project was to identify the mechanisms associated with the acetaldehyde-mediated adaptation of yeast to ethanol stress, which may facilitate the development of yeast strains with improved ethanol tolerance and/or strategies for improving ethanol tolerance in yeast. Gene array analysis was used to study gene expression in Saccharomyces cerevisiae during acclimatisation to non-lethal ethanol stress, in the presence and absence of acetaldehyde. Acetaldehyde caused significant changes in gene expression in ethanol-stressed yeast. For example, many genes associated with protein biosynthesis were more highly expressed, as were pyruvate decarboxylase genes. Interestingly, however, there was no significant increase in the expression of trehalose synthesis genes or genes encoding HSPs; genes which, in previous studies, appeared to be associated with acclimatisation to ethanol-stress. In addition, acetaldehyde did not have a major impact on gene expression in non-stressed cultures. The results of this project are consistent with the speculation that the addition of acetaldehyde to ethanol-stressed S. cerevisiae primes glycolytic flux in ethanol-stressed cells by regenerating NAD+ from accumulated NADH. This, in turn, stimulates glyceraldehyde-3- phosphate dehydrogenase activity and might account for the acetaldehyde-mediated increased expression levels of pyruvate decarboxylase genes; elevated levels of pyruvate would potentially increase the need for PDC activity. Overall, these speculated effects of acetaldehyde on ethanol-stressed yeast would increase glycolytic rate and energy production.

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