A genetic and biochemical analysis of LexA repressor cleavage in Escherichia coli K-12.

The LexA repressor of Escherichia coli represses a set of genes that are expressed in response to DNA damage. After inducing treatments, the repressor is inactivated in vivo by a specific cleavage reaction which requires RecA protein. Under physiological conditions in vitro, RecA-dependent cleavage also occurs. At alkaline pH, however, the specific cleavage reaction occurs spontaneously without RecA, a reaction which is termed autodigestion. The LexA repressor is, therefore, thought to cleave itself with RecA acting to stimulate autodigestion. A set of lexA (Ind⁻) mutants that are deficient in in vivo RecA-mediated cleavage but retain significant repressor function were isolated. These 20 mutations resulted in amino acid substitutions in 12 positions, most of which are conserved between LexA and four other cleavable proteins. All the mutations were located in the hinge region or C-terminal domain of the protein, portions of LexA previously implicated in the specific cleavage reactions. Furthermore, these mutations were clustered in three regions, around the cleavage site (Ala-84-Gly-85) and in blocks of conserved amino acids around two residues, Ser-119 and Lys-156, which are believed essential for the cleavage reactions. These three regions of the protein thus appear to play important roles in the cleavage reaction. Many of the mutant proteins were purified in order to further characterize their properties in both autodigestion and RecA-mediated cleavage. All of these mutant proteins are found to be deficient in both cleavage reactions. A mutant protein, replacing Lys-156 to Arg, requires a higher pH condition than the wild-type protein does for both cleavage reactions. The results suggest that deprotonation of Arg-156, and by inference Lys-156 in the wild-type protein, is required for both autodigestion and RecA-mediated cleavage; and that in the latter reaction RecA acts to reduce the pKa of Lys-156, allowing efficient cleavage of wild-type repressor under physiological conditions. Finally, several mutant proteins affecting amino acids around the cleavage site and the proposed nucleophile in the cleavage reaction (Ser-119) could not efficiently act as a competitive inhibitor in the RecA-mediated cleavage of wild-type repressor, presumably because they affect RecA binding