THE ISOLATION AND CHARACTERIZATION OF AN OPERATOR CONSTITUTIVE MUTATION IN THE RECA GENE OF ESCHERICHIA COLI K-12.

The lexA protein in E. coli is a specific repressor of the recA gene. The lexA protein is cleaved by the recA protein in response to DNA damage. Cleavage derepresses the recA gene resulting in high level synthesis of recA protein and the expression of other DNA damage inducible functions (SOS functions). The lexA3 mutation makes the lexA protein resistant to cleavage and thus inhibits expression of DNA damage inducible functions. A mutant of E. coli has been isolated which exhibits many of the properties expected of a strain carrying an operator-constitutive mutation in the recA gene. The mutation partially suppresses the UV sensitivity of lexA3 strains, maps near the recA structural gene, allows constitutive synthesis of the recA protein and the recA message, and is cis-acting. Strains carrying the recAo('c) mutation were used to study the role of amplified levels of recA protein in the expression of certain SOS functions. The recAo('c) mutation did not suppress the UV inhibitory effect of the lexA3 mutation on the expression of UV induced cellular mutagenesis, and the reactivation and mutagenesis of UV irradiated phage (lamda). The expression of these functions in lexA('+) strains was not enhanced by the recAo('c) mutation. Constitutive recA synthesis did not result in lethal filamentous growth. These results are consistent with those reported elsewhere that the expression of SOS function is not dependent on high levels of recA protein and that the various "SOS genes" are repressed by the lexA protein as is the recA gene. Thus, recA protein is required in SOS expression for the inactivation of lexA protein and recA amplification is a consequence, not a cause of SOS expression. The DNA sequence of the recA operator region from a (lamda)precA transducing phage thought to carry the recAo('c) mutation isolated here, was determined. No difference was detected between the supposed mutant DNA and wild type controls. The significance of these results and the possibility that the recAo('c) mutation was not transferred to the phage are discussed