SOS response as an adaptive response to DNA damage in prokaryotes.

Escherichia coli possesses an elaborate adaptive mechanism called the "SOS response" to cope with various types of DNA damage. More than 20 SOS genes, most of which are known to be involved in the functions that promote the survival of DNA-damaged cells, are induced by treatments that damage DNA or inhibit DNA synthesis. All the SOS genes share similar sequences in the regulatory regions called the "SOS box", to which LexA repressor binds to repress the transcription in the absence of DNA damage. The SOS signal appears to be the single-stranded DNA produced in vicinity of DNA damage, to which RecA protein binds to be activated as a coprotease. The activated RecA promotes autocleavage of LexA protein by allosteric interaction, which activates the latent serine protease activity of LexA. The induced products of the SOS genes repair DNA lesions by various mechanisms, including recombination, excision repair and error-prone repair, and as the consequence, the SOS signal in the cell decreases and the repression of the SOS genes is restored.