Inhibition of Escherichia coli RecA coprotease activities by DinI

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.