Structure of animal mitochondrial DNA: nucleotide composition, pyrimidine clusters, and methylation character

The nucleotide composition, relative concentration of pyrimidine clusters, and the degree of methylation of the mitochondrial and nuclear DNA's of various vertebrates and the protozoan Crithidia oncopelti have been studied. With respect to the relative concentration of GC pairs, the mtDNA of animals (bull, rat) does not differ from the corresponding nDNA. The relative concentration of GC pairs in the mtDNA of certain fish and birds is 1.5-2.5 mole% higher than in the respective nDNA. The kinetoplast DNA of the protozoan C. oncopelti (where the relative concentration of the GC pairs is 42.9 mole %) differs very sharply in composition from the nDNA (where the relative concentration of GC pairs is 51.3 mole %). The mtDNA's and kDNA's studied are distinguished from the respective nDNA'S by a lower degree of clustering of pyrimidine nucleotides. The proportion of mono- and dipyrimidine fragments in the mtDNA and kDNA is 30 mole %, while in the nDNA it does not exceed 23 mole %. The relative concentration of long pyrimidine clusters (hexapyrimidine clusters of larger) in the mtDNA is smaller than in the nDNA by a factor of 2-5. The low degree of clustering of the pyrimidine nucleotides is apparently characteristic of all the known mtDNA's and may support the fact that they have a single type of organization and are of a single origin. All the vertebrate mtDNA's studied contain 5-methylcytosine as a minor base (1.5-3.15 mole %), and their level of methylation is 1.5-2 times greater than that in the respective nDNA's. It has been shown that animals display species specificity with respect to the 5-methylcytosine content in the mtDNA. Its distribution among the pyrimidine clusters in the bovine heart mtDNA differs substantially from that in the nDNA. This suggests that the methylation specificities of nuclear and mitochondrial DNA are different. A DNA methylase, which effects the in vitro methylation of cytosine residues both in the homologous mtDNA and in different heterologous DNA's, has been found in rat liver and bovine heart mitochondria. The specificity of the in vitro methylation of the cytosine residues in the same heterologous Escherichia coli B DNA by the nuclear and mitochondrial enzymes is different: The mitochondrial enzyme methylates predominantly in monopyrimidine fragments, and the nuclear enzyme methylates mostly in di- and tripyrimidine fragments. They, therefore, recognize different nucleotide sequences.