Degradation of Xylan to d -Xylose by Recombinant Saccharomyces cerevisiae Coexpressing the Aspergillus niger β-Xylosidase ( xlnD ) and the Trichoderma reesei Xylanase II ( xyn2 ) Genes

Abstract

ABSTRACT The β-xylosidase-encoding xlnD gene of Aspergillus niger 90196 was amplified by the PCR technique from first-strand cDNA synthesized on mRNA isolated from the fungus. The nucleotide sequence of the cDNA fragment was verified to contain a 2,412-bp open reading frame that encodes a 804-amino-acid propeptide. The 778-amino-acid mature protein, with a putative molecular mass of 85.1 kDa, was fused in frame with the Saccharomyces cerevisiae mating factor α1 signal peptide (MFα1 s ) to ensure correct posttranslational processing in yeast. The fusion protein was designated Xlo2. The recombinant β-xylosidase showed optimum activity at 60°C and pH 3.2 and optimum stability at 50°C. The K i (app) value for d -xylose and xylobiose for the recombinant β-xylosidase was determined to be 8.33 and 6.41 mM, respectively. The XLO2 fusion gene and the XYN2 β-xylanase gene from Trichoderma reesei , located on URA3 -based multicopy shuttle vectors, were successfully expressed and coexpressed in the yeast Saccharomyces cerevisiae under the control of the alcohol dehydrogenase II gene ( ADH2 ) promoter and terminator. These recombinant S. cerevisiae strains produced 1,577 nkat/ml of β-xylanase activity when expressing only the β-xylanase and 860 nkat/ml when coexpressing the β-xylanase with the β-xylosidase. The maximum β-xylosidase activity was 5.3 nkat/ml when expressed on its own and 3.5 nkat/ml when coexpressed with the β-xylanase. Coproduction of the β-xylanase and β-xylosidase enabled S. cerevisiae to degrade birchwood xylan to d -xylose.