[Decrease in the level of DNA methylation in cattle with age].

The 5-methylcytosine content (m5C = 1.76 mole%) in heart muscle DNA of 6 month old cattle embryos is 1.6 times higher than in 10--12 year old cows. The m5C amount in thymus and spleen DNA in old animals is also significantly lower than in embryos and calves. Besides the similarities in reassociation kinetics and GC-content established previously, DNA from tissues of animals of various age are also rather close in the amounts of the respective pyrimidine clusters differing in length and base composition. Thus, the organisation of the nucleotide sequences in the cow genome does not change significantly with age, and the differences in the m5C content detected are due to decrease in the level of DNA methylation. This decrease is not random but is observed mainly in the sequence Pur-m5C-Pur (about two-fold). After heterologous DNA methylation the ratio of newly formed radioactive m5C and m6A residues in DNA from young animals is higher than in DNA from old ones: m5C residues in the original animal DNA interfere with in vitro formation of m6A residues. This means that in vivo DNA of old animals is actually undermethylated as compared with DNA of young ones. This DNA undermethylation with age is maximal in heart muscle. The tissue specific decrease in DNA methylation level with age is to be regarded as one of the mechanisms for distortion of DNA replication and transcription.