FIG. 8.

Evidence that strand invasion occurs while topoisomerase is bound to DNA. (A) Effect of acceptor concentration. Reaction mixtures containing 50 mM Tris-HCl (pH 7.5), 25 nM 5′-labeled 60-mer–12-mer substrate, and 125 nM topoisomerase were incubated at 37°C for 15 min. Strand transfer was initiated by adding 0.125, 0.25, or 1.25 μM acceptor C. (B) Effect of topoisomerase concentration. Reaction mixtures containing 50 mM Tris-HCl (pH 7.5), 25 nM 5′-labeled 60-mer–12-mer substrate, and either 0.125 or 1.25 μM topoisomerase were incubated at 37°C for 15 min. Strand transfer was initiated by adding 1.25 μM acceptor C. The extents of flap recombinant formation are plotted as a function of time.