Induction of the SOS response in Escherichia coli cells under osmotic stress and in the presence of N-methyl-N′-nitro-N-nitrosoguanidine

We investigated the dynamics of the SOS response induction and the frequency of reversions induced by the monofunctional alkylating compound N-methyl-N'-nitro-N-nitrosoguanidine in Escherichia coli cells exposed to osmotic stress for 1 h. During the stress treatment of the wild-type cultures adapted and not adapted to the alkylating agent, the maximum SOS response values and induced reversion frequencies were recorded twice. The SOS response values and induced reversion frequencies remained unchanged during the whole period after attaining the maximum values in adapted and nonadapted cells carrying a mutation in the excision repair gene. Presumably, the SOS mutagenesis mechanisms are turned on in the cells with an inactivated excision repair system earlier than in wild-type cells.