Comparative Genomics as a Tool for Enhanced Ethanol Fermentation from Xylose

Xylose is considered as the second most abundant 5-carbon sugar (a pentose) from pretreated/hydrolyzed lignocellulosic biomass, which can potentially be converted to ethanol by fermentation. The most abundant sugar in pretreated/hydrolyzed lignocellulosic biomass is usually glucose (a 6-carbon sugar, or hexose). However, in many ethanol fermentations from pretreated/saccharified lignocellulosic substrates, xylose is not as efficiently utilized to ethanol (compared to glucose), due to the absence of pentose-metabolizing capabilities of the fermenting yeast (Saccharomyces cerevisiae). While molecular biology techniques have attempted to engineer Saccharomyces cerevisiae strains which harbor genes for pentose metabolism, the fermentative capacity of these strains "pales in comparison to glucose". Thus, industrial fermentation of pentoses (such as xylose) by Saccharomyces cerevisiae still has limited economic feasibility.

Recently, scientists from the University of Wisconsin, the Great Lakes Bioenergy Research Center, the US Department of Energy Joint Genome Institute, and Michigan State University report a new approach for understanding pentose metabolism in yeasts. While previous studies used approaches involving "metabolic modeling, single-species genome/expression analysis and directed evolution", they used "comparative genomics". They sequenced the genomes of xylose-fermenting yeast species, and applied a cross-species comparative genomics approach. In so doing, they identified several genes, that when expressed in Saccharomyces cerevisiae, can "significantly improve xylose-dependent growth and xylose assimilation". The approach can be a strategy for improving the economic feasibility of industrial ethanol fermentation from pentoses. The full paper is published in the Proceedings of the National Academy of Sciences (PNAS) (URL above).