Induction and catabolite repression mechanisms of cellulase in fungi.

Cellulases are induced in most of fungi only when cellulose or an inducer exists. In Hypocrea jecorina and Penicillium purpurogenum, the respective inducers are sophorose and gentiobiose, which do not have beta-1,4 linkages though cellobiose, which has this linkage, is an inducer in other fungi. beta-Glucosidase, which catalyzes transglucosylation, is the key enzyme in converting cello-oligosaccharides to the inducers for cellulase induction in H. jecorina and P. purpurogenum. There are three states in the regulation of cellulase at the transcriptional level in fungi: expression at a basal level, mass secretion of cellulases induced by inducers, and glucose or catabolite repression. Expression at a basal level allows a small amount of cellulase to hydrolyze cellulose to soluble oligosaccharides or to an inducer if cellulose exists near the mycelia. Once the inducer enters the cell, it triggers full-scale transcription of the cellulase gene mediated by activator proteins and activating elements. After cellulose is degraded a large amount of glucose is liberated, which causes catabolite repression.