Engineering Saccharomyces cerevisiae strains for xylose metabolism

Diminishing fossil fuels and environmental concerns are driving research into biofuels. Currently, the major commercial biofuel is bioethanol from yeast fermentations and is mostly produced from starch or sugar based feedstocks causing potential conflict with food supplies. Introducing xylose metabolising pathways into yeast for biofuel production could make lignocellulose a viable alternative feedstock, as lignocellulosic hydrolysates contain high levels of xylose. Two prominent xylose metabolism pathways have been described; the first metabolises xylose via xylose reductase, xylose dehydrogenase and xylulokinase enzymes (the T1 pathway), whereas the other consists of xylose isomerase and xylulokinase enzymes (the T2 pathway). In this project, genes encoding the enzymes from both pathways were successfully introduced into yeast strains.  While minimal growth on xylose containing plates was observed for strains bearing the T1 pathway, strains with the T2 pathway did grow somewhat better than controls. Given this minimal growth, it is surprising that during extended growth on liquid media, both strains deplete xylose from the media and lead to ethanol production. The lag in xylose metabolism and ethanol production may be associated with a slow uptake of xylose from the media. To overcome this potential problem a pentose transporter was introduced into the yeast containing the two pathways. For yeast bearing the T1 pathway, little difference in xylose growth was observed, whereas expression of the xylose transporter appears to inhibit the growth of strains bearing the T2 pathway. These results highlight the complex relationship that likely exists between the transport and metabolism of five carbon sugars.