[Changes in the specificity of DNA methylation in cattle blood lymphocytes under chronic lymphoid leukemia].

Under conditions of chronic (spontaneous) lympholeucosis the amount of 5-methylcytosine in cattle blood lymphocyte DNA is decreased approximately by 30%. No other changes in the DNA (e. g. GC-content, Tm, amount of pyrimidine sequences differing in their lengths and composition) were observed. Thus, the decrease in the amount of 5-methylcytosine in lymphocyte DNA is due to a decrease in DNA methylation. This decrease is non-random and involves mainly the Pu-m5C-Pu sequences without affecting the long pyrimidine blocks. In nuclear extracts from lymphocytes of healthy animals the DNA-methylase activity having an optimum at pH 6,0 was found; the DNA-methylase activities found in the nuclear extracts of leukaemic cow lymphocytes had their optima at pH 5,5 and 7,5. In vitro the DNA-methylase activities of leukaemic lymphocytes nuclei methylate the cytosine residues of DNA in other sequences than enzyme(s) of the extracts from normal lymphocyte nuclei. Changes in the pattern of DNA-methylase activities as well as the decrease and distortions in the character of DNA methylation may underlie the disturbances in the regulation of transcription of genome and cause the transformation of cells under conditions of lympholeucosis.