The bgl1 gene of Trichoderma reesei QM 9414 encodes an extracellular, cellulose‐inducible β‐glucosidase involved in cellulase induction by sophorose

Summary We have investigated the effect of disruption of the bgl1 ‐(β‐glucosidase l‐encoding) gene of Trichoderma reesei on the formation of other β‐glucosidase activities and on the induction of cellulases. To this end the bgl1 locus was disrupted by insertion of the Aspergillus nidulans amdS (acetamidase‐encoding) gene. The bgl1 ‐disrupted strain did not produce the 75kDa extracellular β‐glucosidase on cellulose or lactose, but still formed β‐glucosidase activity on glucose, cellobiose, xylan or β‐1,3‐glucan, suggesting that the enzyme(s) exhibiting this β‐glucosidase activity is (are) not encoded by bgl1. The cellulose‐inducer sophorose induced the bgl1 ‐encoded β‐glucosidase, whereas the remaining β‐glucosidase activity was induced by methyl‐β‐D‐glucoside. The bgl1 ‐gene product was mainly secreted into the medium, whereas the other β‐glucosidase activity was mainly associated with the cells. A bgl1 ‐multicopy strain formed higher amounts of cellulases than the parent strain. Nonsaturating concentrations of sophorose efficiently induced cellobiohydrolase I formation in the bgl1 ‐multicopy strain, but less efficiently in the bgl1 ‐disrupted strain. The multicopy strain and the parent strain were comparably efficient at saturating sophorose concentrations. The β‐glucosidase inhibitor nojirimycin strongly inhibited induction in all strains. These data suggest that the bgl1 ‐encoded β‐glucosidase is not identical to the plasma‐membrane‐bound, constitutive, methyl‐β‐glucoside inducible β‐glucosidase, but represents an extracellular cellulose‐induced enzyme. Both enzymes contribute to rapid induction of cellulases by modifying the inducer sophorose.