Methylation of integrated viral DNA sequences in hamster cells transformed by adenovirus 12

Data have been presented which demonstrate that the Ad12 DNA sequences integrated into the DNA of four lines of Ad12-transformed hamster cells are extensively methylated. Methylation in mammalian-cell DNA presumably occurs at 5'-CCGG-3' sequences mainly at the internal C residue. The majority, though not all, of the 5'-CCGG-3' sequences present in integrated Ad12 DNA are methylated. Further experiments will be required to elucidate the functional significance of the methylation of integrated Ad12 DNA. Ad12 DNA isolated from purified virions is not methylated to any significant extent. Recent results indicate that segments of the integrated viral DNA, which are expressed into mRNA in transformed cells, are undermethylated, whereas segments that are not expressed are extensively methylated.