Identification of the lexA gene product of Escherichia coli K-12

The Escherichia coli lexA gene encodes a product important in induction of the recA gene and the expression of various cellular functions, including mutagenesis and prophage induction. As a start in a biochemical analysis of the lexA function, a family of lambda transducing phages carrying lexA+, lexA3, lexA3 spr-54, and lexA3 spr-55 alleles of the lexA gene was isolated and characterized. Polypeptides synthesized by these phages were examined. lambdalexA+ made a distinctive protein 24 kilodaltons (kd) in size. Lambda lexA3, which encodes an active mutant form of the protein dominant to wild-type function, made a slightly larger protein 25 kd in size. The latter protein was shown to be the mutant lexA3 gene product by the fact that lambda lexA3 spr-55, which carries an amber mutation in lexA3, made the 25-kd protein in hosts with an amber suppressor but not in a suppressor-free host. In hosts carrying a multicopy lexA3 plasmid, neither the 25-kd nor the 24-kd protein was made. This result suggests that lexA is autoregulated and that expression of the 24-kd protein made by lambda lexA+ is subject to the same controls. This and other evidence argues that the 24-kd protein is the product of the wild-type lexA+ gene.