5-Methylcytosine in Chlorelle pyrenoidosa DNAs

The presence of 5-methylcytosine in Chlorella pyrenoidosa (strain 211/8b) DNA's has been investigated by means of paper chromatography and thermal chromatography on hydroxyapatite. It has been shown that nuclear DNA contains 3.5 mol% 5-methylcytosine whereas no significant amount of this base can be detected in chloroplast DNA. The thermal chromatography of nuclear DNA labelled from [6-3H]- or [Me-14C] methionine lead us to conclude that the 5-methylcytosine content is directly proportional to the G + C content of the various DNA fractions. The existence of methylated sequences in DNA is postulated and the biological function of the 5-methylcytosine is discussed.