Different levels of DNA modification at 5'CCGG in murine erythroleukemia cells and the tissues of normal mouse spleen.

The DNA of a Friend erythroleukemic cell line (clone DS-19) and that of mouse spleen from which it was derived were compared with respect to modification at 5'CCGG. Methylation patterns in clone DS-19 DNA were very different from those in normal spleen DNA. Our results suggest that the mechanism by which the internal cytosine in the 5'CCGG sequence is modified has been partially inactivated in clone DS-19. We observed a general decrease of about two-fold in total modification at this site in DS-19 DNA. This general decrease was shown to extend to 5'CCGG sites in specific classes of repeated sequences with two-dimensional displays of restriction fragments. Interspersed repeated sequences which are concertedly modified (i.e., modified at many different chromosomal locations) in spleen, were found to be unmodified at these locations in the cell line. However, we did not detect a difference in DNA modification at 5'CCGG with these techniques when uninduced and hexamethylene-bis-acetamide (HMBA) induced cells of clone DS-19 were compared. In other experiments, we obtained further evidence that selected classes of repeated sequences in Physarum polycephalum are methylated to the same extent independent of chromosomal location.