Complete DNA methylation does not prevent polyoma and simian virus 40 virus early gene expression

The effect of DNA methylation on polyoma virus and simian virus 40 gene expression was investigated. For this purpose, the cytosines of all C-G dinucleotides of the viral DNAs were methylated by the use of rat liver methylase and the completeness of methylation was verified by dinucleotide analysis and restriction endonuclease treatment. The biological activity of unmethylated and fully methylated DNAs was tested by microinjecting them into tissue culture cells. The functions analyzed included early and late viral gene expression, viral DNA replication, oncogenic transformation efficiency, and virus maturation. No difference in any of these biological functions was observed between methylated and unmethylated DNA. Early gene expression of methylated DNA is not the result of demethylation because viral DNA reextracted from the injected cells, under nonpermissive conditions, retained the methylation pattern of the input DNA. In contrast, viral DNA extracted from transformed cells or from intact virus particles was partially or completely demethylated.