Cycle-dependent removal of certain methylated bases from DNA of 10T1/2 cells treated with N-methyl-N'-nitro-N-nitrosoguanidine.

The loss of N-methyl-N-nitro-N-nitrosoguanidine (MNNG)-induced adducts from DNA was quantitated during the G1 and S phases of synchronously proliferating mouse 10T1/2 cells exposed to MNNG (2 microgram/ml) when they were in either confluence-induced arrest of proliferation or at the G2-S border. When treated at either time, N-7-methylguanine and O6-methylguanine were not excised from the template DNA during the subsequent S phase. However, both lesions were efficiently removed during the G1 phase immediately following exposure to MNNG, as well as during the second G2 phase after an S phase during which no loss occurrred. In contrast, N-3-methyladenine was lost rapidly during both the G1 and S phases following MNNG treatment. N-7 Methylguanine and O6-methylguanine were removed from logarithmically growing cell populations more slowly than from cells passing synchronously through the G1 phase. However, when the observed rate of loss in logarithmic cultures was corrected for the fraction of the logarithmic population located in the S phase, the rates of loss of the two methylated bases were the same as those observed in the G1 phase in synchronous cultures.