Efficiency of the adaptive response of Escherichia coli to alkylating agents

When cultures of Escherichia coli are exposed to a low level of the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) they accumulate mutations for about 20 min and then become resistant to further mutagenesis by that level of MNNG. This 'adaptive response, has been shown to be due, at least in part, to induction of the rapid repair of O6-alkylguanine which appears to be the main mutagenic and carcinogenic lesion produced by simple alkylating agents. A similar kind of repair has been demonstrated in the livers of rats exposed to nitrosamines, and this presumably helps to protect animals against carcinogenesis by the various alkylating agents thjat are widespread in our environment. It seemed important, therefore, to find out just how effectively such adaptive responses can control mutations rates.