Mutations in the dim-1 gene of Neurospora crassa reduce the level of DNA methylation

Mutants that show reduced DNA methylation were identified in a mutant screen based on the assumptions that (i) the nucleoside analog 5-azacytidine (5-azaC) promotes the formation or potentially lethal DNA-methyltransferase adducts; (ii) reduction in DNA methyltransferase will decrease the sensitivity of cells to 5-azaC, and (iii) this potential selective advantage will be enhanced in mutants that are deficient in the repair of 5-azaC-induced DNA damage. Of fifteen potential repair mutants screened for sensitivity to 5-azaC, five (mus-9, mus-10, mus-11, mus-18, and uvs-3) showed moderately increased sensitivity and two (mus-20, mei-3) showed highly increased sensitivity. A mus-20 mutation was used to isolate three non-complementing methylation mutants. The mutations, named dim-1 (defective in methylation), reduced female fertility, reduced methylation by 40-50%, and altered patterns of methylation. In wild-type strains hypomethylation per se fails to alter methylation specificity. We demonstrate a growth-phase-dependent change in methylation patterns, detectable only in hypomethylated DNA from dim(+) cultures. This map represent a growth-phase-dependent change in the relative amounts of distinct species of methyltransferase, one of which may be encoded by the dim-1 gene.