The level of 5-methyl cytosine residues is higher in the DNA of polyoma-virus-transformed BHK-21/PyY cells than in the DNA of the non-transformed BHK-21/C13 cells. The 5-methyl cytosine residues arise as a result of the transfer of methyl groups from S-adenosyl-L-methionine to DNA cytosine moieties by DNA methylases located in nuclei of these cells. DNA methylases have been isolated and partially purified from nuclei of BHK-21/C13 and BHK-21/PyY cells and most of the properties of these two enzymes appear to be identical. However slight differences exist in the sequences these enzymes methylate in substrate E. coli DNA as shown by studies of the level of methylation of pyrimidine isostiohs fractionated from the in vitro methylated substrate. These differences can neither account for the differences in the m vivo level of DNA methylation in the two hamster cell lines nor can they provide any evidence to suggest that the higher level of methylation in the BHK-21/PyY DNA may be due to the presence of a virus-coded DNA methylase(s) in these cells. It seems that the two BHK-21 cell genomes may be organised differently. Further studies have shown differences in the arrangements of the two genomes with respect to 5-methyl cytosine residues. The highly repetitive region of the BHK-21/PyY cell DNA is twice as methylation as the corresponding region of the BHK-21/C13 cell DNA. The higher level of methylation of the BHK-21/PyY cell DNA may therefore be due to possible reiteration of certain sequences containing 5-methyl cytosine which may simply occur less frequently in the BHK-21/C13 cell genome. The role of eukaryotic DNA methylation is as yet uncertain. It has been shown in this work that nuclei of BHK-21/C13 cells contain a SAM-dependent nuclease which appears to degrade unmethylated DNA but Inactive towards DNA previously methylated by the BHK-21/PyY DNA methylase. The biological significance of this enzyme is unknown and it is not yet certain whether or not this enzyme is analogous to any of the bacterial restriction enzymes
