Abstract
Xylose-assimilating S. cerevisiae was constructed by introducing the xylose reductase and xylitol dehydrogenase genes originating from P. stipitis. Good growth of the transformant in the xylose medium was observed under aerobic conditions. Under the limited oxygen condition the transformant produced a lesser amount of ethanol than P. stipitis, and a remarkable amount of xylitol was accumulated. A mutant, IM2, in which the ratio of xylose reductase to xylitol dehydrogenase activities was lower than the parental strain, indicated an improved fermentation with less accumulation of xylitol and a higher yield. The limited feeding of xylose could also improve the fermentation with reduced xylitol accumulation as well as increased ethanol yield. The facts suggest strongly that the path of the conversion from xylitol to xylulose is the ‘bottleneck’ due to a poor regeneration of NAD essential for the conversion. An appropriate oxygen supply also improved the ethanol production and the production rate, suggesting it may contribute to the NAD recycle from NADH.
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