Molecular cloning, gene expression analysis and structural modelling of the cellobiohydrolase I from Penicillium occitanis

The filamentous fungus Penicillium occitanis produces a complete set of cellulolytic enzymes needed for efficient solubilization of native cellulose. Cellobiohydrolase I (CBHI), the most abundant cellulolytic enzyme produced by this micro-organism, has been purified and characterized. In this report, the cDNA encoding this enzyme, isolated from a cDNA bank of P. occitanis, and the equivalent genomic sequence have been cloned. DNA sequencing revealed that the cbh1 gene is intronless and has an open reading frame of 1587 bp encoding a putative polypeptide of 529 amino acids. This polypeptide has a predicted molecular mass of 52.5 kDa and consists of a fungal cellulose binding module (CBM) and a catalytic module, linked together by a serine–threonine-rich region. Northern blot analysis showed that cbh1 mRNA expression is partially constitutive since, besides being highly induced by cellulose, it is slightly repressed by glucose. Comparative investigation of different cellobiohydrolases I 3D structures by molecular modelling showed that poor hydrogen bonding, together with a more open configuration of the active site account for the weak binding and the relative insensitivity of P. occitanis CBHI to product inhibition.