[Replicative methylation of DNA in L-cells: effect of S-isobutyladenosine and cycloheximide and possible existence of two DNA-methylases].

Methylation of DNA in cultured cells of mouse fibroblasts (L-cells) occurs at least in two steps, i. e. methylation of Okazaki fragments (up to 100 . m5C/C + m5C = 2.8-2.9) and methylation of linkage sites of DNA formed by ligation of the fragments (up to 6.0). The synthesized Okazaki fragments are not subjected to further methylation, since about one half of their methylation sites (CG) remains non-modified. The transmethylation inhibitor S-isobutyladenosine (SiBA) inhibits the methylation of the "linkage" sites of the newly synthesized DNA without affecting that of the Okazaki fragments. The repression of protein synthesis (including that of histones) by cycloheximide in the course of replication reveals some additional methylation sites. The level of methylation of the newly synthesized polymeric DNA reaches thereby 6.0, which corresponds to modification of all its CG-dinucleotides. A model for replicative methylation of DNA based on the existence of two DNA-methylases differing in their specificity is proposed. It is assumed that one of the DNA methylases is highly specific and functions within the complex with DNA-polymerase, while the other possesses a restricted specificity and functions in a free form (i. e. apart from the replicative complex).