A GENE ESSENTIAL FOR DE-NOVO METHYLATION AND DEVELOPMENT IN ASCOBOLUSREVEALS A NOVEL TYPE OF EUKARYOTIC DNA METHYLTRANSFERASE STRUCTURE

Molecular mechanisms determining methylation patterns in eukaryotic ge nomes still remain unresolved. We have characterized, in Ascobolus, a gene for de novo methylation. This novel eukaryotic gene, masc1, encod es a protein that has all motifs of the catalytic domain of eukaryotic C5-DNA-methyltransferases but is unique in that it lacks a regulatory N-terminal domain. The disruption of masc1 has no effect on viability or methylation maintenance but prevents the de novo methylation of DN A repeats, which takes place after fertilization, through the methylat ion induced premeiotically (MIP) process. Crosses between parents harb oring the masc1 disruption are arrested at an early stage of sexual re production, indicating that the activity of Masc1, the product of the gene, is crucial in this developmental process.