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

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Abstract
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.