In Escherichia coli, the SOS response is induced upon DNA damage and results in the enhanced expression of a set of genes involved in DNA repair and other functions. The initial step, self-cleavage of the LexA repressor, is promoted by the RecA protein which is activated upon binding to single-stranded DNA. In this work, induction of the SOS response by the addition of mitomycin C was found to be prevented by overexpression of the dinI gene. dinI is an SOS gene which maps at 24.6 min of the E.coli chromosome and encodes a small protein of 81 amino acids. Immunoblotting analysis with anti-LexA antibodies revealed that LexA did not undergo cleavage in dinI-overexpressed cells after UV irradiation. In addition, the RecA-dependent conversion of UmuD to UmuD' (the active form for mutagenesis) was also inhibited in dinI-overexpressed cells. Conversely, a dinI-deficient mutant showed a slightly faster and more extensive processing of UmuD and hence higher mutability than the wild-type. Finally, we demonstrated, by using an in vitro reaction with purified proteins, that DinI directly inhibits the ability of RecA to mediate self-cleavage of UmuD.