Methylation of the Deoxyribonucleic Acid of Physarum polycephalum at Various Periods during the Mitotic Cycle

The methylation of major nuclear DNA-cytosine was found to occur throughout the mitotic cycle of Physarum polycephalum, during the period of DNA synthesis (S), as well as during the remainder of interphase (G2) when essentially no synthesis de novo of the major nuclear DNA takes place. After incubation of the mold with methionine methyl-3H, 5-methylcytosine was the only radioactive methylated minor base detected in major nuclear DNA, although 3H was also found to be incorporated during the S period into DNA-adenine, -guanine, and -thymine, presumably by conversion of the methionine methyl group to 1-carbon intermediates incorporated into DNA nucleotides during synthesis de novo. During the G2 period, methylation of major nuclear DNA occurred at a relatively constant rate which was approximately 50% of that observed during the S period. Incorporation of 3H from methionine methyl-3H into 5-methylcytosine of both mitochondrial DNA and the nuclear heavy satellite DNA was also observed. It was estimated that the three DNA fractions of Physarum contain 1 methylcytosine residue for every 12 to 24 cytosine residues.