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Abstract

Disclosed herein is a newly discovered problem and solution for engineeringto ferment xylose to make ethanol utilizing xylose isomerase to convert xylose to xylulose for entry, via xylulokinase, into the pentose phosphate pathway. The problem is that when grown on a media containing xylose xylitol tends to accumulate in the cell despite the absence of xylose reductase activity in. 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 fromwhich is less inhibited by xylitol than other xylose isomerases, exemplified byxylose isomerase. Expression of thexylose isomerase may be used alone, or in combination with expression of a xylitol dehydrogenase and optionally over expression of xylulokinase to improve ethanol production from xylose.