SOS induction and autoregulation of the himA gene for site-specific recombination in Escherichia coli.

The himA gene of EScherichia coli controls the lysogenization of bacteriophage lambda at the level of catalysis of site-specific recombination and expression of the lambda int and cI genes required for lysogenic development. We have analyzed the regulation of himA by two methods: (i) beta-galactosidase synthesis from a lacZ gene inserted into the himA gene and (ii) detection of radioactive HimA protein after fractionation by two-dimensional gel electrophoresis. We find that himA- mutations produce enhanced expression of the himA gene, indicating that HimA protein controls its own synthesis. The himA gene is also induced by treatment of cells with UV or mitomycin C, suggesting control by the inducible DNA repair (SOS) system regulated by the LexA and RecA proteins. Regulation of himA follows the pattern expected for a typical SOS gene: constitutive high expression in mutants that have inactive LexA or the altered RecA conferred by the recA441 (tif1) mutation and low noninducible expression in a mutant that has a deleted recA gene. We conclude that the himA gene is a component of the inducible SoS response, repressed by LexA and induced by the capacity of activated RecA to cleave LexA. We suggest that HimA may be subject to SOS induction because it functions as an "acquisitionase" for new genetic material and thus is of special utility under conditions of impaired capacity for growth of the bacterial population.