The LexA repressor or LexA (Locus for X-ray sensitivity A) is a transcriptional repressor () that represses SOS response genes coding primarily for error-prone DNA polymerases, DNA repair enzymes and cell division inhibitors. LexA forms de facto a two-component regulatory system with RecA, which senses DNA damage at stalled replication forks, forming monofilaments and acquiring an active conformation capable of binding to LexA and causing LexA to cleave itself, in a process called autoproteolysis. The dimerization domain binds to other LexA polypeptides to form dumbbell shaped dimers. The DNA-binding domain is a variant form of the helix-turn-helix DNA binding motif, and is usually located at the N-terminus of the protein. and antibiotic resistance determinants, as well as the activation of integron integrases, potentially increasing the likelihood of acquisition and dissemination of antibiotic resistance by bacteria. This offers potential for combination therapy that combines quinolones with strategies aimed at interfering with the action of LexA, either directly or via RecA.

References