Genetic modification of carbon catabolite repression in the filamentous fungus Trichoderma reesei for improved protein production

The aim of this work was to genetically modify the carbon catabolite repression of Trichoderma reesei to obtain mutant strains derepressed in cellulase production. Several genes coding for regulators of cellulase expression have been isolated and characterized from T. reesei including the cre1 gene mediating the carbon catabolite repression in the presence of glucose. Glucose repression has been shown to occur upon binding of CREI protein to specific sequences in the promoter of the major cellulase gene cbh1. CREI target sequences have also been identified in the promoter regions of other cellulase and hemicellulase genes such as e.g. cbh2 and xyn1. The CREI protein of T. reesei is similar to many other fungal proteins mediating glucose repression. It was recently shown, however, that a truncated form of CREI (cre1-1) present in the hypercellulolytic T. reesei strain Rut-C30 is responsible for derepression of (hemi)cellulase gene expression on glucose-containing media. Therefore, to study the effect of cre1 on cellulase and hemicellulase expression, the wild type cre1 gene present in the T. reesei strain QM6a was either replaced by the "Rut-C30 -type" truncated cre1 -1 gene or completely removed. Bioreactor cultivations on lactose and glucose media were carried out with these cre1-1 and delta-cre1 mutant strains and analysed for cellulase and hemicellulase production. Northern analysis indicated remarkably higher expression levels of several (hemi)cellulase genes in both mutant strains on lactose when compared to the non-modified parent strain. In accordance, 20-fold higher cellobiohydrolase and endoglucanase activities and almost 10-fold higher xylanase activities were detected in the lactose culture medium of mutant strains.