Cloning of the cbhI and cbhII genes involved in cellulose utilisation by the straw mushroom Volvariella volvacea.

The straw mushroom Volvariella volvacea is cultivated on substrates rich in cellulose and has been shown to produce a family of cellulolytic enzymes. A PCR-based strategy was adopted to clone genes involved in cellulose utilisation, using degenerate primers designed to amplify conserved catalytic domain sequences of cellobiohydrolases (CBHs). PCR with these primers produced two DNA fragments with sequence similarity to the cbhI and cbhII gene families detected in Trichoderma, Phanerochaete and Agaricus species. Full-length clones of these genes were obtained from an EMBL3 genomic library, and RACE-PCR was used to verify the presence of introns. The cbhI homologue has a coding region of 1722 bp, containing two introns, generating a 536 amino acid polypeptide product. The cbhII gene has a coding region of 1693 bp, containing five introns, and gives rise to a 470-amino acid polypeptide product. Northern and PCR analyses were used to study the expression of the genes. These revealed that transcripts of both genes were induced on medium containing cellulose with cbhI being expressed more strongly than cbhII - but were repressed on medium containing glucose.