The global SOS response to DNA damage. (a) In the default state of the E. coli cell, the LexA repressor prevents transcription of the SOS genes. In response to DNA damage, LexA is stimulated to undergo autocleavage, inactivating itself and allowing transcription of the SOS genes. (b) Autocleavage of the LexA repressor requires RecA protein. DNA damage creates sites of single-stranded DNA, which are quickly bound by RecA. DNA-bound RecA becomes a coprotease for LexA, and their association facilitates the destruction of LexA and induction of the SOS response.