[Effect of hydrocortisone on methylation and molecular population of DNA in rat liver].

The content of 5-methyl cytosine in rat liver DNA increases 1,7-fold 8 hours after intraperitoneal injection of hydrocortisone (5 mg per 100 g animal weight). The content of GC, physicochemical parameters (Tm, delta T, etc.) and DNA renaturation pattern did not show any changes. No changes were observed in the pattern of H3-thymidine incorporation into rat liver DNA: after hydrocortisone injection the radioactivity was found to be equally distributed in all isolated sequences of DNA, differing in the degree of reiteration (specific radioactivities of these DNA, fractions are very similar). Thus, the molecular population of DNA in liver cells remains unchanged, which suggests that the hormone-induced increase in the 5-methyl cytosine content is due to a change in the DNA methylation level. The methyation level of unique sequences (COt greater than 600), i. e. that of structural genes, does not undergo any essential changes. The reversible methylation of DNA regulated by hormones seems to be one of the mechanisms controlling gene activity.