recA protein promotes homologous-pairing and strand-exchange reactions between duplex DNA molecules

Purified recA protein, product of the recA+ gene, promotes homologous pairing between intact covalent circular duplex DNA and circular single-stranded DNA carrying a short hybridized fragment [West, S. C., Cassuto, E. & Howard-Flanders, P. (1981) Nature (London) 290, 29-33.]. In this paper we investigate the interaction of duplex fragments with circular single-stranded DNA carrying the hybridized fragment and find that recA protein promotes an efficient strand-exchange reaction between interacting DNA molecules. The exchange is dependent upon linear duplex DNA fragments that are homologous to, but extend beyond, the short fragment present on the hybridized DNA substrate. The reactions require stoichiometric amounts of recA protein and the presence of ATP.