The UmuD′C mutagenesis complex accumulates slowly and parsimoniously after a 12 J m−2 UV flash to attain after 45 min a low cell concentration between 15 and 60 complexes. Meanwhile, RecA monomers go up to 72 000 monomers. By contrast, when the UmuD′C complex is constitutively produced at a high concentration, it inhibits recombinational repair and then markedly reduces bacterial survival from DNA damage. We have isolated novel recA mutations that enable RecA to resist UmuD′C recombination inhibition. The mutations, named recA [UmuR], are located on the RecA three-dimensional structure at three sites: (i) the RecA monomer tail domain (four amino acid changes); (ii) the RecA monomer head domain (one amino acid change, which appears to interface with the amino acids in the tail domain); and (iii) in the core of a RecA monomer (one amino acid change). RecA [UmuR] proteins make recombination more efficient in the presence of UmuD′C while SOS mutagenesis is inhibited. The UmuR amino acid changes are located at a head-tail joint between RecA monomers and some are free to possibly interact with UmuD′C at the tip of a RecA polymer. These two RecA structures may constitute possible sites to which the UmuD′C complex might bind, hampering homologous recombination and favouring SOS mutagenesis.