Monday, April 19, 2010
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Xylose fermentation by recombinant Saccharomyces cerevisiae through functional expression of xylose isomerase from Bacteroides stercoris

Suk Jin HA1, Jin-Ho Choi1, Soo Rin Kim2, and Yong-Su Jin2. (1) Energy Biosciences Institute, University of Illinois at Urbana-Champaign, 1206 W. Gregory Dr, Urbana, IL 61801, (2) Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, 1206 W. Gregory Dr, Urbana, IL 61801

Efficient fermentation of pentose, which is abundant in hydrolyzates of lignocellulosic biomass, is essential for implementing economic biofuel production processes. In spite of many beneficial traits, Saccharomyces cerevisiae is not amenable for cellulosic ethanol production because this yeast cannot ferment xylose. Heterologous expression of genes (XYL1, XYL2, and XYL3) coding for xylose reductase (XR), xylitol dehydrogenase (XDH), and xyulokinase (XK) from Pichia stipitis enabled S. cerevisiae to ferment xylose. However, yields and productivities of the engineered S. cerevisiae strains were significantly lower than Pichia stipitis. Among the many putative limiting factors which hamper xylose fermentation in S. cerevisiae, redox imbalance caused by cofactor difference between XR and XDH is a major problem which results in xylitol accumulation. In order to bypass this problem, we cloned a bacterial xylose isomerase (xylA) from Bacteroides stercoris HJ-15. Both the isolated xylA and a codon-optimized xylA were functionally expressed in S. cerevisiae. The resulting recombinant S. cerevisiae was able to grow and ferment xylose as a sole carbon source without any adaptation. Interestingly, significant amounts of xylitol were still accumulated during xylose fermentation by the xylose isomerase (XI) expressing S. cerevisiae even though the introduced XI does not cause any redox imbalance. As a result, the observed ethanol yields (~0.35 g ethanol/g xylose) from xylose were lower than the theoretical yield (0.51 g ethanol/g xylose). This result suggests that xylitol accumulation in the engineered strains expressing XYL1, XYL2, and XYL3 might not have been solely caused by the redox imbalance. This newly isolated xylose isomerase is expected to serve as a genetic toolbox since it displayed different kinetic parameters as compared to the other bacterial and fungal xylose isomerases which have been functionally expressed in S. cerevisiae.

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