A unique protease regulatory gene from Trichoderma reesei

The reduction of unwanted endogenous proteases has already been an important target for strain improvement of fungal host strains used in protein production for many years. Targeted deletion of specific proteases has been used extensively for this purpose. Surprisingly, only very little is known about regulatory circuits that specifically control fungal protease production. The only protease-specific regulator gene discovered to date is the prtT gene from Aspergillus niger, which encodes a canonical Zn2-Cys6 activator protein involved in the expression of a wide range of protease genes (Punt et al., 2008). Interestingly, homologues of prtT are only found in Aspergillus species, whereas no prtT homologue is present in Trichoderma reesei. We aimed to discover a similar protease master switch in our research. T. reesei mutants with strongly reduced overall protease levels and strongly reduced expression of a number of protease genes were obtained using a biological screen for the selection of protease-deficient mutants (Braaksma et al., 2008). Genome sequencing of a number of these strains followed by SNP analysis revealed that several of these mutants carried mutant alleles from a single gene, which we termed pea1 for protease-expression-affected. Disruption of this gene in both T. reesei and Fusarium sp. confirmed the role of pea1 in protease gene expression. Intriguingly, the encoded protein does not show any similarity to known regulatory proteins, indicating that a completely new regulatory circuit may be governing protease gene expression in T. reesei, which opens the way to further research in this area.