Molecular characterisation and expression analysis of the first hemicellulase gene (bxl1) encoding β-xylosidase from the thermophilic fungus Talaromyces emersonii

The gene coding for β-xylosidase, bxl1, has been cloned from the thermophilic filamentous fungus, Talaromyces emersonii. This is the first report of a hemicellulase gene from this novel source. At the genomic level, bxl1 consists of an open reading frame of 2388 nucleotides with no introns that encodes a putative protein of 796 amino acids. The bxl1 translation product contains a signal peptide of 21 amino acids that yields a mature protein of 775 amino acids, with a predicted molecular mass of 86.8kDa. The deduced amino acid sequence of bxl1 exhibits considerable homology with the primary structures of the Aspergillus niger, Aspergillus nidulans, Aspergillus oryzae, and Trichoderma reesei β-xylosidase gene products, and with some β-glucosidases, all of which have been classified as Family 3 glycosyl hydrolases. Northern blot analysis of the bxl1 gene indicates that it is induced by xylan and methyl-β-D-xylopyranoside. D-Xylose induced expression of bxl1 but was shown to repress induction of the gene at high concentrations. The presence of six CreA binding sites in the upstream regulatory sequence (URS) of the bxl1 gene indicates that the observed repression by D-glucose may be mediated, at least partly, by this catabolite repressor.