Adaptive doses of MNNG efficiently induce a recA-trp gene fusion

The recA gene product plays a critical role in the Escherichia coli SOS system. To facilitate studies of the regulation of the recA operon, we constructed a gene fusion between the recA operon and the Salmonella trp operon. This gene fusion closely mimics the behavior of the authentic recA operon in vivo. Using the gene fusion, we looked at the effect of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) on recA expression. In contrast to the expectations from previous work, we found that low (0.5 microgram/ml) adaptive doses of MNNG induce the recA-trp gene fusion as efficiently as other SOS inducers without inducing either lambda or phi 80 prophages. These results are a clear example of the ability of some agents to induce a subset of the SOS-dependent operons. They force a reevaluation of many of the arguments that have led to the conclusion that the SOS and adaptive responses are completely independent.