Chi sites, RecBC enzyme, and geneneralized recombination
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With the goal of elucidating the molecular basis of genetic recombination, our laboratory has studied special sites that promote recombination in their neighborhood and enzymes essential for recombination in Escherichia coli. The genetic properties of Chi sites of E. coli and phage [lambda], determined by the laboratory of F. Stahl, suggest that Chi is a recognition sequence for a recombination-promoting enzyme. Our laboratory's finding that Chi is the unique nucleotide sequence 5' G-C-T-G-G-T-C-G 3' (or its complement or both) substantiates this view. Genetic studies suggest that Chi may be recognized by the E. coli RecBC enzyme. Based upon electron micrographic studies of reaction intermediates we have proposed a model by which RecBC enzyme unwinds and rewinds DNA. We discuss a mechanism where by RecBC's hypothesized nicking of DNA at Chi sites during DNA unwinding can promote recombination in a manner consistent with the established genetic properties of Chi.