BrdU-induced changes in the provirus of PR-RSV in mammalian cells.

Previously we demonstrated differences in the organization and methylation pattern of integrated Rous sarcoma proviral sequences in helper-dependent virogenic rat TWERC cells. In the present study we attempted to induce changes in the integrated viral genome of TWERC cells using 5-bromodeoxyuridine (BrdU). Four clones (A, B, C, and F) derived from the parental cell line were treated for 10 months with different concentrations of BrdU. Restriction enzyme analysis of the parental cell line and its clones showed that the cells contained in their genomic DNA two copies of deleted provirus. In TWERC cells, the PR-RSV provirus lost the whole env gene and part of the 3' end of the pol gene. The proviral sequences in DNA from the TWERC-derived clones were found to be hypermethylated. A slightly different situation was seen in clone C where demethylation of the provirus in the 3' region and in the 5' end of the genome was found. The level of mRNA expression both in the parental cells and in the clones correlated with the methylation pattern of the PR-RSV provirus. Clone C was less methylated and expressed more virus-specific RNA. The possible role of BrdU in these events is discussed.