P1: Comparative expression analysis of unconventional xylose fermenting yeasts

Unconventional yeasts such as Spathaspora passalidarum and Scheffersomyces stipitis are capable of fermenting a wider range of carbon sources than Saccharomyces cerevisiae. We examined transcript data from S. passalidarum and S. stipitis cells cultured in bioreactors under defined oxygenation conditions, and concluded that their regulatory mechanisms differ significantly from conventional yeast. For example, fermentation is induced by oxygen limitation rather than by the presence of glucose. This enables the induction of fermentative activity in response to xylose and cellobiose rather than requiring glucose. When fermenting xylose S. stipitis and S. passalidarum both induce transcripts for xylose assimilation and the pentose phosphate pathway. S. cerevisiae does not exhibit a native fermentative response to xylose. Rather, S. cerevisiae engineered for xylose metabolism increases its respiratory activity. Some of the responses however are similar. As seen in S. cerevisiae, glycolysis is induced along with pyruvate decarboxylase. With S. stipitis, changes in metabolic enzymes are accompanied by changes in transcripts for protein kinases, some of which are clearly higher under oxygen limitation, while others are higher under aerobic conditions. Such changes are not as pronounced or abundant with S. passalidarum. This yeast, however, shows a much stronger induction of growth-related transcripts than does S. stipitis under oxygen limitation. These differences account in part for its greater capacity for sustained fermentative activity on hemicellulosic and cellulosic sugars. Our proprietary tools for transformation, mating and selection of these yeasts have proven useful in the development of new strains based on rational metabolic engineering and evolutionary adaptation.