Transcriptome of S. cerevisiae during production of D-xylonate

Production of D-xylonate in the yeast S. cerevisiae represents an example of bioprocess development for more sustainable production of value-added chemicals from cheap raw material or waste. Previously it was shown that the production of D-xylonate led to its significant intracellular accumulation and to dramatic loss of viability during the production process. In order to identify the physiological or pathological responses associated with D-xylonate production, we performed a time-course transcriptome analysis of D-xylonate production in yeast cultivated in a bioreactor. Comparison of the transcriptomes of D-xylonate producing strain with control strain showed considerably higher expression in the xylonate producing strain of the genes controlled by the cell wall integrity pathway (CWI) and of some genes previously identified as upregulated in response to the organic acid stress. Surprisingly, also genes encoding proteins involved in translation, ribosome structure and RNA metabolism – the processes commonly found to be down-regulated under virtually every condition causing cellular stress – were upregulated during the D-xylonate production. The overall transcriptional responses were, therefore, very dissimilar to those previously reported as being associated with diverse stresses including the organic acid treatment and production. In addition, it was observed that the consumption of ethanol was slower and the level of trehalose was lower in the D-xylonate producing strain. Validation experiments including Slt2 kinase phosphorylation profiles and the quantitative PCR analyses of selected gene showed remarkably good match with our findings and confirmed the observations made in the transcriptome analysis. The production of organic acids has a major impact on the physiology of yeast cells. There is, however, very limited overlap at the transcriptional level in responses to treatment or production of different acids. The loss of viability, observed during production and accumulation of D-xylonate, seems to be caused by erroneous interpretation of environmental signals causing a failure in entering the stationary phase and eventually leading to depletion of scarce resources by the affected cells. This, together with intracellular acidification, inevitably results in cell death. The study compares two yeast strains, a D-xylonate producing strain, expressing XylB gene from C. crescentus, and the isogenic parental strain, an industrial hydrolysate tolerant S. cerevisiae strain B67002. The strains were grown in bioreactors and samples for microarrays were taken from 6 time points during the cultivation (0h, 7h, 23h, 47h, 49h and 71h). Three replicate cultures were performed for each strain. The total number of microarrays is 36. Each arrays measures 5,777 genes from Saccharomyces cerevisiae with eight 60-mer probes per gene, with three-fold technical redundancy.