OLD AND NEW CONCEPTS FOR THE ROLE OF CHI IN BACTERIAL RECOMBINATION

The DNA sequence 5'[GCTGGTGG]3', which is called chi, stimulates recom bination that is mediated by the RecBCD pathway of Escherichia coli. I n 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 unwi nding and rewinding the DNA at different rates so that the traveling e nzyme becomes encumbered by a region of unwound DNA, Upon meeting chi, the enzyme was supposed to cut one of the two unwound chains, generat ing thereby a recombinagenic single-chain end. The model, based on mic roscopical observations of RecBCD enzyme interacting with linear duple x DNA, was supported by the subsequent finding that RecBCD acting in v itro under certain conditions did deliver a nick at chi, This widely e mbraced model has been challenged by a model in which the exonuclease activity of RecBCD destroys DNA from the enzyme's entry site to chi, T he role of chi according to the new model is to inhibit this nuclease activity of RecBCD, perhaps by ejecting the RecD subunit from the enzy me, thereby revealing the enzyme's recombinase activity.