Strand exchange between a P full-site and a MeP half-site promoted collaboratively by Flp and Flp(R191A). A, the exchange reactions between the labeled MeP half-site and the unlabeled P full-site are schematically represented, with the thin wavy line representing the ester bond formed between MeP and the 5′-hydroxyl group. B, the predicted lengths of the labeled recombinant strands resulting from the two possible orientations of the half-site with respect to the full-site are 186 and 130 nucleotides, respectively. The reactions were carried out with a fixed amount of Flp and increasing amounts of Flp(R191A). The reactions were analyzed by 6% urea-PAGE gels. S(26-mer) stands for the labeled strand of the MeP half-site, and RP1(186-mer) and RP2(131-mer) stand for the labeled strands in the recombinant products.