[5-Methylcytosine in pyrimidine sequences of plant and animal DNA: specificity of methylation].

A method for two-dimensional DEAE-TLC fractionation and a subsequent quantitative spectrophotometric determination of the 5-methylcytosine containing pyrimidine oligodeoxyribonucleotides isolated from plant and animal DNA is proposed. Using this method, the distribution of 5-methylcytosine among pyrimidine oligodeoxyribonucleotides in various plant and animal DNA was studied. It was found that the methylation patterns of DNA in higher plants and animals are quite different, but they are similar among various plants or among different animal DNAs. More than half of the 5-methylcytosine residues in animal DNAs are localized in the monopyrimidine fraction (Pu-C-Pu); however, in plant DNAs there are two types of heavily methylated sequences (Pu-C-Pu and Pu-(CT)-Pu). It is suggested that methylation of plant DNAs occurs in two different sites possessing the second order symmetry with axis lined across the dinucleotide CpG or complementary AT base pair. Possible involvement of eucaryotic DNA-methylases in replicative and reparative DNA synthesis and in hormonal regulation of DNA methylation at and after DNA synthesis in plants and animals as well as a possible role of DNA methylation in regulation of gene activity in tissues and cells are discussed.