Construction of a xylan-fermenting yeast strain through codisplay of xylanolytic enzymes on the surface of xylose-utilizing Saccharomyces cerevisiae cells

Hemicellulose is one of the major forms of biomass in lignocellulose, and its essential component is xylan. We used a cell surface engineering system based on alpha-agglutinin to construct a Saccharomyces cerevisiae yeast strain codisplaying two types of xylan-degrading enzymes, namely, xylanase 11 (XYNII) from Trichoderma reesei QM9414 and beta-xylosidase (XyIA) from Aspergillus oryzae NiaD300, on the cell surface. In a high-performance liquid chromatography analysis, xylose was detected as the main product of the yeast strain codisplaying XYNII and XyIA, while xylobiose and xylotriose were detected as the main products of a yeast strain displaying XYNII on the cell surface. These results indicate that xylan is sequentially hydrolyzed to xylose by the codisplayed XYNII and XylA. In a further step toward achieving the simultaneous saccharification and fermentation of xylan, a xylan-utilizing S. cerevisiae strain was constructed by codisplaying XYNII and XyIA and introducing genes for xylose utilization, namely, those encoding xylose reductase and xylitol dehydrogenase from Pichia stipitis and xylulokinase from S. cerevisiae. After 62 h of fermentation, 7.1 g of ethanol per liter was directly produced from birchwood xylan, and the yield in terms of grams of ethanol per gram of carbohydrate consumed was 0.30 g/g. These results demonstrate that the direct conversion of xylan to ethanol is accomplished by the xylan-utilizing S. cerevisiae strain.