The SOS response is a global response to DNA damage in which the cell cycle is arrested and DNA repair and mutagenesis are induced. The SOS repair system in bacteria is normally controlled by LexA repressor proteins.
Under normal conditions, LexA represses expression of the SOS genes. After DNA damage occurs, single-stranded (ssDNA) regions accumulate at replication forks, blocking DNA polymerase.
RecA binds to these ssDNA regions and becomes activated, after which it binds to and induces expression of the LexA repressor, lowering LexA protein concentrations and allowing expression of the SOS genes to repair the DNA damage.