Helical interactions in homologous pairing and strand exchange driven by RecA protein.

In a search for mutants that affect genetic recombination, Clark and Margulies (1) discovered the recA gene in Escherichia coli, but subsequently recA mutants were found to be pleiotropic, affecting not only recombination but repair, mutagenesis, phage induction, and cell division (2). Not surprisingly, many years passed before the basis of this complex phenotype was understood. A milestone was the discovery by Roberts e t al. (3) that the product of the recA gene catalyzes the cleavage of the X repressor and thereby inactivates it. There followed the discoveries that by proteolytic cleavage RecA protein also catalyzes the inactivation of the cellular LexA repressor, which derepresses more than 20 genes, and the activation of the UmuD protein, which is required for UV-induced mutagenesis (3-6). Especially after the discovery that RecA protein derepresses other genes, it appeared likely that the role of RecA protein in genetic recombination was indirect, via its effect on the expression of those other genes. Experiments on a temperature-sensitive mutant of recA showed that the derepression of other genes was not a sufficient explanation for the role of recA in recombination (7), but these experiments could not exclude the possible activation of recombination proteins by cleavage, as for example in the case of UmuD protein whose cleavage activates its role in UV mutagenesis. However, the cloning of the recA gene (8) and the further finding that RecA protein has DNA-dependent ATPase activity (9,lO) led to the discovery that RecA protein also has remarkable recombination activities in vitro (11-13). Although mutants exist that are proficient in cleavage of LexA repressor and yet are defective in recombination (14), the observations on the recombination activities of RecA protein in vitro provide the only compelling evidence that it plays a direct role in recombination, for the reason just described (15-18).