ON THE ROLE OF ATP HYDROLYSIS IN RECA PROTEIN-MEDIATED DNA STRAND EXCHANGE .3. UNIDIRECTIONAL BRANCH MIGRATION AND EXTENSIVE HYBRID DNA FORMATION

We have identified two functions for RecA-mediated ATP hydrolysis duri ng DNA strand exchange, First, ATP hydrolysis renders RecA protein-med iated DNA strand exchange unidirectional (5' to 3' with respect to the single-stranded DNA). In the presence of a nonhydrolyzable analog ade nosine 5'-O-(3-thiotriphosphate) (ATP gamma S), DNA strand exchange is bidirectional. Second, ATP hydrolysis is required for extensive forma tion of hybrid DNA. In the presence of ATP hydrolysis, the length of t he exchanged region is limited only by the available homology, whereas in the absence of ATP hydrolysis, only 2 kilobase pairs or less of hy brid DNA are formed before branch migration is blocked in the majority of paired intermediates. Both of these functions of RecA protein-medi ated ATP hydrolysis are crucial in ensuring the effectiveness of recom binational DNA repair, especially when the lesion to be repaired is di stant from the initial crossover point.