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Control and Function of DNA Methylation in Neurospora crassa

Ann T. Hagemann, Eric U. Selker


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...

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