Coexpression of the Bacillus pumilus β-xylosidase (xynB) gene with the Trichoderma reesei β-xylanase 2 (xyn2) gene in the yeast Saccharomyces cerevisiae

Abstract

The xynB gene encoding the Bacillus pumilus β-xylosidase was expressed separately and jointly with the Trichoderma reesei β-xylanase (xyn2) gene in the yeast Saccharomyces cerevisiae. Both genes were placed under the transcriptional control of the glucose-derepressible alcohol dehydrogenase 2 promoter (ADH2(P)) and terminator (ADH2(T)) sequences. The xynB gene was fused in frame to the yeast mating factor α1 secretion sequence (MFα1(S)) to effect secretion in S. cerevisiae. The fusion protein was designated Xlo1. Xlo1 produced in S. cerevisiae exhibited low affinity for xylobiose, but eventually hydrolyzed xylobiose and xylotriose to the monomeric constituent, D-xylose. Coproduction of Xyn2 and Xlo1 by S. cerevisiae led to a 25% increase in the amount of reducing sugars released from birchwood xylan compared to S. cerevisiae producing only the Xyn2 β-xylanase. However, no D-xylose was produced from birchwood xylan, presumably due to very low Xlo1 β-xylosidase activity and its low affinity for xylobiose.