Relating biochemistry to biology: how the recombinational repair function of RecA protein is manifested in its molecular properties.

The multiple activities of the RecA protein in DNA metabolism have inspired over a decade of research in dozens of laboratories around the world. This effort has nevertheless failed to yield an understanding of the mechanism of several RecA protein-mediated processes, the DNA strand exchange reactions prominent among them. The major factors impeding progress are the invalid constraints placed upon the problem by attempting to understand RecA protein-mediated DNA strand exchange within the context of an inappropriate biological paradigm-namely, homologous genetic recombination as a mechanism for generating genetic diversity. In this essay I summarize genetic and biochemical data demonstrating that RecA protein evolved as the central component of a recombinational DNA repair system, with the generation of genetic diversity being a sometimes useful byproduct, and review the major in vitro activities of RecA protein from a repair perspective. While models proposed for both recombination and recombinational repair often make use of DNA strand cleavage and transfer steps that appear to be quite similar, the molecular and thermodynamic requirements of the two processes are very different. The recombinational repair function provides a much more logical and informative framework for thinking about the biochemical properties of RecA and the strand exchange reactions it facilitates.