A model for the abrogation of the SOS response by an SOS protein: a negatively charged helix in DinI mimics DNA in its interaction with RecA

DinI is a recently described negative regulator of the SOS response in Esch erichia coli. Here we show that it physically interacts with RecA and preve nts the binding of single-stranded DNA to RecA, which is required for the a ctivation of the latter. DinI also displaces ssDNA from a stable RecA-DNA c ofilament, thus eliminating the SOS signal. In addition, DinI inhibits RecA -mediated homologous DNA pairing, but has no effect on actively proceeding strand exchange. Biochemical data, together with the molecular structure, d efine the C-terminal cy-helix in DinI as the active site of the protein. In an unusual example of molecular mimicry, a negatively charged surface on t his a-helix, by imitating single-stranded DNA, interacts with the loop L2 h omologous pairing region of RecA and interferes with the activation of RecA .