The bacterial SOS machinery. The SOS machinery is operated by the interplay of two key regulators, an SOS repressor LexA and an inducer RecA. RecA responds to DNA damage by binding to ssDNA, which triggers LexA autocleavage. The LexA repressor dissociates from the SOS boxes in order to derepress and induce transcription of the SOS regulon. These genes work to either repair or bypass the lesions of DNA damage.