Repair of alkylated DNA: Drosophila have DNA methyltransferases but not DNA glycosylases.

DNA methyltransferase activity has been identified in crude extracts of Drosophila melanogaster pupae for the removal of methyl groups from O-6 methylguanine appearing in alkylated DNA. Additionally, N-7 methylguanine and 3 methyladenine appear to be uniquely susceptible to methyltransferase activity that resides in Drosophila pupae. Consistent with this, tests to detect DNA glycosylase activity for the repair of the latter two modified bases was unsuccessful, even though a substantial loss of methyl groups from these bases was observed. Conversely, the repair of methylated purines was not detected in extracts of Drosophila embryos. The removal of methyl groups from methylated purines was dependent upon incubation temperature and was proportional to the amount of protein added to reaction mixtures. Results indicate that the methyl group is attached to protein during the repair of methylated DNA, suggesting that it is similar to the O6-methylguanine-DNA methyltransferase identified in other organisms. Although other explanations are possible, the inability to detect DNA glycosylase activity suggests that Drosophila may not rely on base excision repair for the removal of modified or nonconventional basis in DNA.