In this study, the authors have determined the crystal structure of full-length RecA from K. pneumoniae, revealing for the first time a β-strand conformation for the flexible Cterminal tail. Using a combination of biochemical techniques, they then show that this tail inhibits the formation of RecA filament, DNA binding, and DNA strand exchange during homologous recombination, but promotes LexA self-cleavage and enhances the SOS response.