Repair of alkylation damage: stability of methyl groups in Bacillus subtilis treated with methyl methanesulfonate.

Bacillus subtilis was not inactivated and was able to replicate even though approximately 3 x 10(4) methyl groups added by methyl methanesulfonate (MMS) were bound to the deoxyribonucleic acid (DNA) of each organism. No significant loss of methyl groups from the DNA occurred for several generations upon incubation of methylated wild-type or MMS-sensitive cells. Single-strand breaks were not observed in the DNA from cells treated at this low MMS dose. Higher doses of MMS resulted in significant killing of both wild-type and MMS-sensitive strains, and the DNA extracted from such treated cells sedimented more slowly than control DNA through alkaline sucrose gradients, indicating the presence of breaks or apurinic sites (or both). These breaks were repaired upon incubation of wild-type but not of MMS-sensitive strains. Repair of damage induced by alkylating agents is probably the repair of breaks which occur as a consequence of high levels of alkylation.