XYLOSE ISOMERASE AND XYLITOL DEHYDROGENASE COMBINATION FOR XYLOSE FERMENTATION TO ETHANOL AND B. FRAGILIS XYLOSE ISOMERASE

摘要

Disclosed herein is a newly discovered problem and solution for engineering S. cerevisiae to ferment xylose to make ethanol utilizing xylose isomerase to convert xylose to xylulose for entry, via xylulokinase, into the pentose phosphate pathway. When grown on a media containing xylose xylitol tends to accumulate in the cell despite the absence of xylose reductase activity in S. cerevisiae. Xylitol inhibits the activity of xylose isomerases. One solution described is to simultaneously express an exogenous xylitol dehydrogenase along with the exogenous xylose isomerase while optionally also overexpressing xylulokinase in the absence of expression of a xylose reductase. Another solution is a xylose isomerase from Bacteroides fragilis which is less inhibited by xylitol than other xylose isomerases, exemplified by E.coli xylose isomerase. Expression ofthe Bacteroidesfragilis xylose isomerase may be used alone, or in combination with expression ofaxylitol dehydrogenase and optionally over expression of xylulokinase to improve ethanol production from xylose.