The bgI1 gene encoding extracellular β‐glucosidase from Trichoderma reesei is required for rapid induction of the cellulase complex

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Abstract
We have used a targeted gene deletion event to remove the coding region for the bgli gene encoding an extracellular β-glucosidase from the genome of the cellulolytic fungus Trichoderma reesei. The bgli null mutants were used to investigate the role of [1-glucosidase in the hydrolysis of cellulose and induction of the other cellulolytic enzyme components. In the absence of extracellular βglucosidase, growth of bgl1 null strains on several carbon sources was the same as that of the parent (as measured by mycelial dry weight). However, levels of extracellular protein and total endoglucanase production were seen to lag relative to those levels observed in the control strain. The mRNA levels of the CBHI, CBHII, EGI, and EGII cellulase genes (cbh1, cbh2, egli and egl3) showed a corresponding lag in induction, suggesting that the absence of extracellular β-glucosidase has an effect on the co-ordinate regulation of the other cellulase genes at the level of transcription. The addition of a potent inducer of the cellulase complex (sophorose) resulted in normal rates of cellulase gene mRNA production and extracellular protein release. This indicates that the absence of β-glucosidase is not affecting some intrinsic cellular ability to produce mRNA or secrete protein. These data suggest that a functional β-glucosidase is at feast partially responsible for the efficient induction of the depolymerase enzymes of the cellulase complex. The observation that the cellulase complex is induced, albeit after a lag, suggests that other enzymes are present that can substitute for the function of β-glucosidase during induction.

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