UvsY promotes presynaptic filament assembly on Gp32-covered ssDNA by a double hand-off mechanism (adapted from ). UvsY protein facilitates the loading of UvsX recombinase onto ssDNA and the concomitant displacement of Gp32 ssDNA-binding protein from ssDNA. The figure shows UvsX loading and Gp32 displacement from the perspective of a single UvsY hexamer, as if looking down the helical axis of a nascent presynaptic filament. The cooperative binding of Gp32 to ssDNA extends the polynucleotide lattice. The first handoff occurs as hexameric UvsY recognizes and binds to the extended ssDNA (Step 1), then converts it into a wrapped conformation(s) (Steps 2-3), destabilizing Gp32-ssDNA interactions in the process. The UvsY-wrapped ssDNA complex is postulated to be in equilibrium between "closed" and "open" conformations (Step 3), the latter of which is recognized by the ATP-bound form of UvsX protein to nucleate presynaptic filament assembly (Step 4) while displacing Gp32. (A) Steps 3-4 constitute a step-wise mechanism for Gp32 displacement and UvsX loading by UvsY, which may occur under low-salt conditions. (B) Under high-salt conditions UvsY does not displace Gp32 from ssDNA directly, so filament assembly likely occurs by a concerted mechanism in which synergistic action of UvsY and ATP-bound UvsX is required to displace Gp32.