The conversion of 5-methyldeoxycytidine to thymidine in vitro and in vivo.

The pyrimidine, 5-methylcytosine, was demonstrated by Wyatt (1) to be a component of plant and animal deoxyribonucleic acid.’ The deoxyriboside of 5-methylcytosine was isolated from wheat germ DNA by Dekker and Elmore (a), who also characterized this nucleoside. No microorganisms or viruses have yet been shown to contain this base, although the nucleic acids of many of these organisms have been examined for its presence. Its absence in bacteria may be accounted for in several possible ways; for example, (a) the base is not synthesized, or (b) it is metabolized at a rate which does not permit accumulation. Other hypotheses might also be required in some instances since, for example, insect viruses which do contain DNA lacking 5-methylcytosine multiply in nuclei in which 5-methylcytosine is a structural component of DNA. Little is known of the origin or metabolism of this relatively rare base and its derivatives. The formation of a methyl group possibly proceeds via a hydroxymethylpyrimidine, thereby relating 5-methylcytosine to hydroxymethylcytosine, a pyrimidine found uniquely in certain virus nucleic acids (3). However, it is not evident whether the formation of the methyl