| Abstract: | DNA-methylase activities which methylate cytosine residues in homo- and heterologous DNA were detected in mitochondria and nuclei from rat liver and beef heart. Adenine modifying DNA-methylases in mitochondria and nuclei were not found. DNA from mitochondria and nuclei differ significantly in the methylation degree and in the pattern of the 5-methyl-cytosine distribution by pyrimidine isostichs as DNA in vivo and in vitro being methylated. Mitochondrial DNA methylase has the maximum activity at 30 degrees and pH 7.8 this enzyme(s) differ(s) from the nuclear one(s) in the pH dependence of its activity. After exhaustive in vitro methylation of various DNA by the nuclear enzyme DNA-methylase from mitochondria additionally introduces CH3 groups from S-adenosylmethionine into these DNA (about 3 times more CH3 groups than nuclear enzyme). Nuclear DNA-methylase also methylates DNA which is previously fully-methylated by the mitochondrial enzyme, but to a lesser degree. In conditions of exhaustive DNA methylation mitochondrial enzyme introduces into E. coli B DNA about four times more methyl groups as compared to the nuclear one. After the methylation of E. coli B DNA by mitochondrial enzyme the label (3H-methyl) was detected predominantly in mono-, and in case of nuclear enzyme--in di- and tripyrimidine fragments. Mitochondrial DNA-methylase differs from the nuclear one in the nature of recognized DNA sequences; these enzymes seems to be represented by different proteins. The mitochondrial enzyme methylates shorter nucleotide sequences in DNA as compared to the nuclear DNA-methylase. All these data suggest there exist organoid specificity of genome methylation in animal cell and the modification-restriction systems in animal nucleus and mitochondria are different in character. |
