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Background The ascomycete  Trichoderma reesei  is one of the most efficient industrial producers of cellulase. Gene targeting by homologous recombination is a key technique for improving strains and constructing mutants. In  T. reesei ,  tku70  (homologous to human KU70) was deleted to block non-homologous end-joining, which led to 95% of transformants exhibiting homologous recombination. Results Two genes located in close proximity to  tku70  were identified: the ferrochelatase gene  hem8  ( tre78582 , homologous to  Aspergillus niger hemH  and  Cryptococcus neoformans HEM15 ) and a putative DNA methylation modulator-2 gene  dmm2  ( tre108087 , homologous to  Neurospora crassa dmm-2 ). Genome-wide surveys of 324 sequenced fungal genomes revealed that the homologues of the three genes of interest are encoded in tandem in most  Sordariomycetes . The expression of this three-gene cluster is regulated by blue light. The roles of these three genes were analyzed via deletion and complementation tests. The gene  hem8  was originally described as a novel and highly distinct auxotrophic marker in  T. reesei  and we found that the product protein, HEM8, catalyzes the final step in heme biosynthesis from highly photoreactive porphyrins. The lethal phenotype of the  hem8  deletion could be overcome by hematin supplementation. We also studied the functions of  tku70  and  dmm2  in DNA repair using mutagen sensitivity experiments. We found that the Δ tku70  strain showed increased sensitivity to bleomycin, which induces DNA double-strand breaks, and that the Δ dmm2  strain was sensitive to bleomycin, camptothecin (an inhibitor of type I topoisomerases), and hydroxyurea (a deoxynucleotide synthesis inhibitor). The double-mutant Δ tku70 &amp; dmm2  showed higher sensitivity to hydroxyurea, camptothecin, and bleomycin than either of the single mutants. Knockout of  dmm2  significantly increased cellulase production. Conclusions Our data show, for the first time, that ferrochelatase encoded by  hem8  catalyzes the final step in heme biosynthesis from highly photoreactive porphyrins and that  dmm2  encodes a putative DNA methylation modulator-2 protein related to DNA repair and cellulase expression in  T. reesei . Our data provide important insights into the roles of this three-gene cluster in  T. reesei  and other  Sordariomycetes  and show that the DNA methylation modulator DMM2 affects cellulase gene expression in  T. reesei .

A three-gene cluster in Trichoderma reesei reveals a potential role of dmm2 in DNA repair and cellulase production