[In vitro methylation of nuclear DNA from various organs of a rat: Tissue and age differences in acceptor ability of DNA].

DNA-methylase activities have been found, isolated and partially purified from the nuclear and cellular extracts of rat liver, kidneys, spleen, brain and lungs. These metylases are capable of methylating in vitro homologous and heterologous DNA from rat tissues in the presence of (H3-methyl)-S-adenosylmethionine. The radioactive methyl groups incorporated in vitro into DNA were found only in 5-methylcytosine. No radioactivity was revealed in either N6-methyladenine or N6-dimethyladenine. Rat spleen DNA-methylase differs in some properties (pH and temperature dependences) from those of DNA methylases from other rat organs. This enzyme methylates various DNA to a higher extent than DNA-methylases from other rat tissues. There may exist a tissue specificity of DNA-methylases from other rat tissues. There may exist a tissue specificity of DNA-methylases and a different mode of their action and recognition of nucleotide sequences. The nuclear DNA from various rat organs methylated in vitro by the same enzyme possesses a different ability to accept methyl groups from S-adenosylmethionine: there exists a tissue specifity of DNA methylation in vivo and in vitro. The degree of DNA methylation changes with age tissue specifically. Aging is accompanied with decrease in the 5-methylcytosine content of DNA of some tissues (brain and others) and increase in methyl-accepting capacities of these DNA in the in vitro methylation by home- and heterologous DNA-methylases. The age changes detected in DNA are correlated with exhaustion of tissue functional activity with age and support our previous suggestion that DNA methylation is a mechanism for regulation of transcription and gene activity.