METHYLATION OF MAMMALIAN DNA - STUDIES ON NOVIKOFF HEPATOMA CELLS IN TISSUE CULTURE

Tritiated purine and pyrimidine deoxyribonucleoside DNA precursors, [14C-methyl]methionine, and amethopterin have been used to study the methylation of polymeric DNA in Novikoff hepatoma cells grown in tissue culture. The results indicate that approximately one out of every 12 to 14 DNA-cytosine residues is labeled in the 5-position of the pyrimidine ring with a [14C-methyl] group. A dependency of continued DNA methylation upon DNA synthesis was also demonstrated. Other experiments eliminated the possibility that labeled DNA-5-methylcytosine arose from incorporation of 5-methyldeoxycytidine triphosphate into DNA. An apparent deamination of 5-methylcytosine in polymeric DNA resulting in the formation of DNA-thymine was also observed. Although no information from any in vivo or in vitro system exists concerning the functional significance of DNA methylation, a shift of DNA-5-methylcytosine to DNA-thymine might be involved in initiation of DNA replication and/or transcription. © 1969.