DNA methylation and transcriptional controls of proviral DNA in avian sarcoma virus-transformed mammalian cells.

Restriction mapping has been used to study the integration state of the single provirus present in the DNA of two subclones, RS2/3 and RS2/6, of hamster cells transformed in vitro by Rous sarcoma virus, but differing markedly in their level of proviral transcription which was higher in RS2/3 cells. It was observed that both proviruses are complete and located at the same integration site in each DNA. However, the RS2/6 provirus and its flanking cellular sequences were found to be hypermethylated, although a very short region was hypomethylated at about 1 kb upstream of the src gene. A low level of methylation was observed in RS2/3 cells, in the proviral region. Northern analysis of viral RNA detected only the src mRNA in RS2/6 cells, whereas the two other viral mRNA were found in RS2/3 cells, however their levels were very low compared to that of the src mRNA. These findings suggest a correlation between the methylation state and the transcriptional control of the proviral genes. Sequences responsible for such a control by methylation should lie within both the provirus and its 5' flanking cellular sequences.