S87: Identification and Expression of a Novel Bacterial Xylose Isomerase in Saccharomyces cerevisiae

S. cerevisiae is the ideal host for ethanol production from corn starch and sugar cane but it is unable to ferment xylose, the most abundant pentose sugar found in lignocellulosic feedstocks. Yeast expressing a xylose assimilation pathway based on the sequential activities of NADP⁺-dependent xylose reductase and NADH-dependent xylitol dehydrogenase suffer a redox imbalance that results in the production of xylitol, reducing ethanol yield, and a microaerophilic oxygen requirement difficult to maintain at commercial scale.  Strains expressing a redox-neutral xylose isomerase lack sufficient activity and are strongly inhibited by xylitol, a common metabolic intermediate. We have identified a novel xylose isomerase that is phylogenetically-distinct from other known xylose isomerases, actively expressed in yeast and relatively uninhibited by xylitol. We have introduced the gene into metabolically engineered laboratory and commercial yeast strains and demonstrated their ability to grow on xylose as the sole carbon source and to ferment it to ethanol under completely anaerobic conditions. We have further improved anaerobic xylose fermentation by directed and evolutionary adaptation. This presentation will focus on biochemical characterization of this novel xylose isomerase along with genetic modifications that have greatly improved its activity.