Represses a number of genes involved in the response to DNA damage (SOS response), including recA and lexA. In the presence of single-stranded DNA, RecA interacts with LexA causing an autocatalytic cleavage which disrupts the DNA-binding part of LexA, leading to derepression of the SOS regulon and eventually DNA repair. Hydrolysis of Ala-|-Gly bond in repressor LexA. Homodimer. No consensus sequence similar to the SOS-box from E.coli or from B.subtilis was found upstream of the lexA gene, suggesting the presence of another target sequence specific for the Thermotogales. Belongs to the peptidase S24 family.