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-azacy tidine (5-azaC) promotes the formation or potentially lethal DNA-methy ltransferase adducts; (ii) reduction in DNA methyltransferase will dec rease the sensitivity of cells to 5-azaC, and (iii) this potential sel ective advantage will be enhanced in mutants that are deficient in the repair of 5-azaC-induced DNA damage. Of fifteen potential repair muta nts screened for sensitivity to 5-azaC, five (mus-9, mus-10, mus-11, m us-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 muta tions, named dim-1 (defective in methylation), reduced female fertilit y, reduced methylation by 40-50%, and altered patterns of methylation. In wild-type strains hypomethylation per se fails to alter methylatio n specificity. We demonstrate a growth-phase-dependent change in methy lation patterns, detectable only in hypomethylated DNA from dim(+) cul tures. This map represent a growth-phase-dependent change in the relat ive amounts of distinct species of methyltransferase, one of which may be encoded by the dim-1 gene.