We have cloned a gene, prs12, from the filamentous fungus Trichoderma reesei which encodes a fungal homologue of the mouse and Drosophila regulatory subunit 12 of the 26S proteasome (mov34). Sequencing of both a genomic and a cDNA-clone predicts a 342-aa protein with high overall identity (56-68 %) to the homologous counterparts from human, mammals, Drosophila and Saccharomyces cerevisiae. The predicted protein contains several consensus sequences for phosphorylation, three of which are conserved in all published Prs12p homologues. Its C-terminus is rich in alternating K and E/D, and resembles a potential KEKE-motif. Prs12 exhibits a basal level of transcription during normal growth, but its expression is significantly increased over 60-120 min under conditions of stress evoked by the addition of cadmium ions and hygromycin B. It is also stimulated by the addition of tunicamycin and 2-mercaptoethanol, suggesting its regulation by the presence of unfolded proteins in the endoplasmic reticulum and by hygromycin B. Consistent with this behaviour, motifs in the prs12 5'-upstream sequences show sequence homology with the consensus sequences for general stress response, and for an ER traffic-response element.