Control and Function of DNA Methylation in Neurospora crassa

Abstract

Genomic DNA of the filamentous fungus Neurospora crassa is largely devoid of methylation. Methylation at the 5 position of cytosine is heavy in certain areas of the genome, however. Overall, approximately 1.5% of the cytosines in N. crassa DNA are methylated (Russell et al. 1987; Foss et al. 1993; Selker et al. 1993b), but in certain regions, nearly 100% of the cytosines are methylated (Selker and Stevens 1985; Selker et al. 1993a). The total number of such methylated patches in the N. crassa genome is probably in the hundreds, based on the total amount of methylation (1.5%); the size and density of the two best-characterized, natural, methylated patches (1.6 kb for ζ–η, Selker et al. 1993b; and ~2 kb for ψ–63, B. Margolin and E. Selker, unpubl.); and the size of the genome (~4 × 10⁷ bp, Orbach et al. 1988b). Genetic analyses have revealed that mutations in a number of genes can reduce the level of DNA methylation in Neurospora (Foss et al. 1993; Roberts and Selker 1995; H. Foss and E. Selker, unpubl.), and one mutant apparently lacks all DNA methylation in vegetative tissue (dim-2, for defective in methylation; Foss et al. 1993).

The cytosine residues that are methylated in Neurospora are not present exclusively, or even predominantly, in CpG dinucleotides (Bull and Wootton 1984; Selker and Stevens 1985; Selker et al. 1993a). This is unlike the situation in higher eukaryotes and some other fungi that have been studied (Selker 1993). Non-CpG methylation cannot be propagated...