The protein disulphide isomerase gene of the fungus Trichoderma reesei is induced by endoplasmic reticulum stress and regulated by the carbon source

The gene pdi1 encoding protein disulphide isomerase was isolated from the filamentous fungus Trichoderma reesei by degenerate PCR based on a consensus PDI active-site sequence. It was shown that the Trichoderma pdi1 cDNA is able to complement a yeast mutant with a disrupted PDI1 gene. The putative T. reesei PD1I protein has a predicted 20-amino acid N-terminal signal sequence and the C-terminal fungal consensus ER retention signal HDEL. The mature protein shows strong conservation relative to other fungal protein disulphide isomerases. The T. reesei pdi1 promoter has two possible unfolded protein response (UPR) elements and it was shown by treatments with dithiothreitol and tunicamycin that the gene is under the control of the UPR pathway. Expression of a heterologous protein, an IgG antibody Fab fragment, in Trichoderma increases pdi1 expression, probably by inducing the UPR. The level of T. reesei pdi1 mRNA is also regulated by the carbon source, being lowest in glucose-containing media and highest on carbon sources that induce the genes encoding extracellular enzymes. The mechanism of this regulation was studied by examining psi1 mRNA levels under conditions where the extracellular enzymes are induced by sophorose, as well as in the strain RutC-30, which is mutant for the glucose repressor gene cre1. The results suggest that neither sophorose induction nor glucose repression by the CREI protein affect the pdi1 promoter directly.