Systems biology of yeast: Genome-wide analysis of pentose metabolism in Saccharomyces cerevisiae

We are interested in broadening the substrate utilisation range of the yeast Saccharomyces cerevisiae and in connection to this to study nutritional responses and signalling at a global level. S. cerevisiae does not naturally utilise pentose sugars unlike most other fungi, and recombinant S. cerevisiae strains have been constructed that contain the xylose utilisation pathway from other yeasts. The xylose reductase enzyme (XR) prefers NADPH as a cofactor while the next step in the pathway xylitol dehydrogenase (XDH), NAD+. This is believed to create a redox cofactor imbalance in the cells and limit xylose fermentation. Other suggested rate-limiting steps in xylose utilisation and the use of xylose as a fermentative carbon source include xylose uptake and limitations in the pentose phosphate pathway (PPP) reactions. In order to understand redox and nutrient regulation in general, and the physiology of xylose utilising S. cerevisiae, we carried out chemostat cultures on xylose as a carbon source, and compared those with glucose cultures. Transcriptional profiling and proteomics were carried out. The results are in good agreement. In addition to the expected responses in cellular redox metabolism and PPP, also new responses towards xylose as a carbon source were discovered such as the up-regulation of pathways for alternative carbon source utilisation and responses for nutritional control and starvation. The physiology of the recombinant yeast appears to be neither fully repressed (fermentative) nor derepressed (gluconeogenic).