[Possible origin and evolution of enzymatic methylation of eukaryotic DNA. Methylation of cytosine residues in 3 palindromic families: RYRY, YYRR, and YYRYRR].

Data on nearest neighbors of 5-methylcytosine residues in eukaryotic DNA were analyzed. It was found that the methylation sites CG and CNG may be located in three palindromic families: RYRY, YYRR and YYRYRR. It was shown that all the methylated sequences in these DNAs can appear as a result of 5-MeC----T substitutions, proceeding by deamination of 5-MeC residues in the "prototype" sites of each of these families: GCGC, CCGG and CCGCGG. The multiplicity of DNA-methyltransferases in eukaryotic cells and their evolutionary origin from prokaryotic type II methylases, recognizing analogous sequences in DNA, are discussed.