SPECIFIC RECA AMINO-ACID CHANGES AFFECT RECA-UMUD'C INTERACTION

The UmuD'C mutagenesis complex accumulates slowly and parsimoniously a fter a 12 J m(-2) UV flash to attain after 45 min a low cell concentra tion between 15 and 60 complexes. Meanwhile, RecA monomers go up to 72 000 monomers. By contrast, when the UmuD'C complex is constitutively p roduced at a high concentration, it inhibits recombinational repair an d then markedly reduces bacterial survival from DNA damage. We have is olated novel recA mutations that enable RecA to resist UmuD'C recombin ation 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 dom ain (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 (on e amino acid change). RecA [UmuR] proteins make recombination more eff icient in the presence of UmuD'C while SOS mutagenesis is inhibited. T he UmuR amino acid changes are located at a head-tail joint between Re cA monomers and some are free to possibly interact with UmuD'C at the tip of a RecA polymer. These two RecA structures may constitute possib le sites to which the UmuD'C complex might bind, hampering homologous recombination and favouring SOS mutagenesis.