Old and new concepts for the role of chi in bacterial recombination

The DNA sequence 5′[GCTGGTGG]3′, which is called X, stimulates recombination that is mediated by the RecBCD pathway of Escherichia coll. In 1981, a model was proposed in which the RecBCD enzyme enters DNA at a double-chain end. The enzyme then travels between the chains by unwinding and rewinding the DNA at different rates so that the traveling enzyme becomes encumbered by a region of unwound DNA. Upon meeting X, the enzyme was supposed to cut one of the two unwound chains, generating thereby a recombinagenic single-chain end. The model, based on microscopical observations of RecBCD enzyme interacting with linear duplex DNA, was supported by the subsequent finding that RecBCD acting in vitro under certain conditions did deliver a nick at X. This widely embraced model has been challenged by a model in which the exonuclease activity of RecBCD destroys DNA from the enzyme's entry site to X. The role of X according to the new model is to inhibit this nuclease activity of RecBCD, perhaps by ejecting the RecD subunit from the enzyme, thereby revealing the enzyme's recombinase activity.