Control of the SOS regulatory system by the level of RecA protease

The SOS regulatory system of E. coli controls the cellular response to DNA damage and other treatments which interfere with normal DNA replication. This system can exist in two states--a repressed state, in which a set of genes (SOS genes) is repressed by the LexA repressor; and an induced state, in which the RecA protein is activated as a specific protease which cleaves LexA repressor, leading to derepression of the SOS genes. This article reviews evidence that the state of the SOS regulatory system is controlled by the level of RecA protease activity. This level is controlled in turn by a reversible activation by one or more cofactors. In vitro studies indicate that ATP or dATP and single-stranded polynucleotide are both required to activate the protease; the identity of the in vivo cofactors ("inducing signals") is not yet certain. New experiments are also described which characterize the in vivo cleavage of LexA repressor. These data support the model that the level of RecA protease controls the state of the regulatory system.