Role of the hga1-encoded extracellular beta-galactosidase of Hypocrea jecorina in lactose metabolism and cellulase induction

Lactose is the only economic carbon source for protein production under the control of cellulase promoters by the ascomycete H. jecorina (anamorph: Trichoderma reesei). However, the mechanism by which lactose triggers cellulase formation is not understood. We have investigated the role of beta-galactosidase in lactose metabolism and cellulase induction in H. jecorina. A genomic copy of the bga1 gene predicting a protein with a MW of 111 kDa (incl. a signal sequence) was cloned using degenerate primers. The Bga1 belongs to the Glycosyl hydrolases family 35. Transcriptional analysis of the bga1 expression shows that it is highly expressed on L-arabinose, D-galactose and lower on lactose. Deletion of the gene showed that it is not essential for growth on lactose but that knock-out strains grew slower and produced about half the biomass of the wt strain, whereas amplification of the bga1 gene under the pki1 promoter resulted in faster growth with a reduced lag phase. Bga1 is not necessary for lactose-dependent induction of cbh1 gene expression in H. jecorina but its overexpression impairs lactose-induction of cbh1 expression. Analysis of the deletion strain further showed that bga1 encodes the major extracellular beta-galactosidase which is also partially cell-wall bound. An additional cell wall-bound beta-galactosidase activity could be shown but no evidence for an intracellular beta-galactosidase was obtained.