Proteome and transcription analysis of recombinant xylose-utilising Saccharomyces cerevisiae

Economically feasible production of fuel ethanol from lignocellulosic material relies on quantitative conversion of the carbon present in the biomass that may contain 30-40% hemicellulose. Xylose is the most abundant pentose sugar in the hemicellulose and it is the second only to glucose in natural abundance. Hexose sugars are readily utilised by most industrial micro-organisms but efficient utilization of pentoses present in the hemicellulose fraction is still a challenge. Xylose fermentation by Saccharomyces cerevisiae has been enabled by introducing the genes encoding xylose reductase (XYL1, XR) and xylitol dehydrogenase (XYL2, XDH) from the yeast Pichia stipitis naturally utilising xylose. Over-expression of the endogenous xylulokinase-encoding gene (XKS1) of S. cerevisiae further enhances xylose consumption (1). Introduction of the xylose-utilisation pathway into S. cerevisiae not naturally fermenting pentose sugars has a major impact on the overall cellular metabolism as the carbon introduced will now flow through the pentose phosphate pathway. In addition, the introduction of redox enzymes into S. cerevisiae affects the redox balance of the cell as xylose reductase has a preference for NADPH, while xylitol dehydrogenase is specific for NAD+. This has been attributed to be one of the major reasons for inefficient incorporation of xylose-derived carbon into the central carbon pathways leading to ethanol by the oxido-reductive pathway. Genome wide approaches offer an attractive and global strategy to study the overall cellular metabolism under different physiological conditions. We have studied both on proteomic and genomic level and under different culture conditions the recombinant S. cerevisiae to reveal novel changes in the metabolism of xylose fermenting yeast (2, 3, 4).