Specific amino acid changes enhance the anti-recombination activity of the UmuD'C complex

In addition to being an essential component of trans-lesion synthesis, the UmuD'C complex is an antagonist of RecA-mediated homologous recombination. When constitutively expressed at an elevated concentration, the UmuD'C complex sensitizes recA+ bacteria to DNA damage, whereas it has no effect on bacteria expressing a RecA [UmuR] protein that overcomes recombination inhibition. Using as a genetic screen enhanced cell killing on mitomycin plates, we isolated novel umuD' and umuC mutations that restored mitomycin sensitivity to recA D112G [UmuR] bacteria overproducing the UmuD'C complex. The mutations were named [Rin++] because a characterization in a recA+ as well in a recA D112G background showed that they enhanced UmuD'C-promoted recombination inhibition in two assays, conjugational recombination and recombinational repair of palindrome-containing DNA. The [Rin++] mutations affect five amino acids, G25D, S28T, P29L, E35K, and T95R, in UmuD' and seven, F10L, Y270C, K277E, F287L, F287S, K342Q and F351I, in UmuC. These amino acids might play a key role in the UmuD'C anti-recombination activity. None of the [Rin++] mutations enhanced UmuD'C-promoted mutagenic bypass of UV lesions, in contrast, several lead to a defect in this process. In this study, we discuss a few molecular mechanisms that could account for the recombination and mutagenesis phenotypes of a mutant UmuD'C [Rin++] complex.