Facilitated loading of RecA protein is essential to recombination by RecBCD enzyme

Although the RecB2109CD enzyme retains most of the biochemical functions associated with the wild-type RecBCD enzyme, it is completely defective for genetic recombination. Here, we demonstrate that the mutant enzyme exhibits an aberrant double-stranded DNA exo-nuclease activity, intrinsically producing a 3*-terminal single-stranded DNA overhang that is an ideal substrate for RecA protein-promoted strand invasion. Thus, the mutant enzyme constitutively processes double-stranded DNA in the same manner as the x-modified wild-type RecBCD enzyme. However, we further show that the RecB2109CD enzyme is unable to coordinate the loading of RecA protein onto the single-stranded DNA produced, and we conclude that this inability results in the recombination-defective phenotype of the recB210