Isolation and characterization of an operator-constitutive mutation in the recA gene of E. coli K-12.

The recA gene of E. coli is regulated by a specific repressor, the lexA protein, which binds to an operator in the recA regulatory region. We describe in this paper the isolation and characterization of a mutant thought to carry an operator-constitutive mutation in the recA gene. This mutation has the following properties: 1) It partially suppresses the UV sensitivity of lexA- strains. 2) It maps near the recA gene. 3) It allows constitutive high-level synthesis of recA protein in both lexA- and lexA+ backgrounds. 4) It allows constitutive synthesis of the recA messenger RNA. 5) It is cis-acting. The mutation does not restore induced cellular mutagenesis in a lexA- background. The expression of induced repair and mutagenesis of UV irradiated phage lambda or the regulation of the lexA gene is not affected by the presence of the mutation in either a lexA+ or lexA- strain. These observations confirm other findings that high levels of recA protein synthesis per se is not sufficient for the expression of UV inducible functions and that the lexA protein represses other genes besides the recA gene.