Neurospora MUS-30 is an LSH/DDM1 homolog required for normal genome maintenance

LSH/DDM1 enzymes are required for DNA methylation in higher eukaryotes and have poorly defined roles in genome maintenance in yeast, plants, and animals. The filamentous fungus Neurospora crassa is a tractable system that encodes a single LSH/DDM1 homolog (NCU06306). We report that the Neurospora LSH/DDM1 enzyme is encoded by mutagen sensitive-30 (mus-30), a locus identified in a genetic screen over 25 years ago. We show that MUS-30-deficient cells have normal DNA methylation, but are hypersensitive to the DNA damaging agent MMS (methyl methanesulfonate). MUS-30 is a nuclear protein, consistent with its predicted role as a chromatin remodeling enzyme, and levels of MUS-30 are increased following DNA damage. MUS-30 co-purifies with Neurospora WDR76, a homolog of yeast Changed Mutation Rate-1 and mammalian WD40 repeat domain 76. Deletion of wdr76 rescued MMS-hypersensitivity of Dmus-30 strains, demonstrating that the MUS-30-WDR76 interaction is functionally important. DNA damage-sensitivity of Dmus-30 is also partially suppressed by deletion of methyl adenine glycosylase-1, a component of the base excision repair machinery (BER); however, the rate of BER is not affected in Dmus-30 strains. It was reported that mammalian LSH is required for efficient double strand break (DSB) repair. We found that MUS-30-deficient cells were not defective for DSB repair, and we observed a negative genetic interaction between Dmus-30 and Dmei-3, the Neurospora RAD51 homolog required for homologous recombination. These data are consistent with a role for MUS-30 that is independent of DSB repair. Our findings demonstrate that LSH/DDM1 enzymes are key regulators of genome stability in eukaryotes.