Dynamic instability in T4 presynaptic filaments is coupled to the UvsX ATPase cycle and to UvsX/Gp32 competition for binding sites (adapted from ). A. Gp32 covers free ssDNA rapidly to protect it from nuclease digestion and to remove secondary structure. B. Hexameric UvsY protein weakens Gp32-ssDNA interactions by binding to the complex and wrapping the ssDNA lattice. C. ATP-bound UvsX is recruited to the tripartite UvsY-Gp32-ssDNA intermediate. ATP and UvsY both contribute to a synergistic increase in UvsX-ssDNA binding affinity that allows the recombinase to locally displace Gp32 from the lattice. D. Propagation occurs in the 5' → 3' direction as ATP-bound UvsX subunits slowly add to the 3' filament end, displacing more Gp32 subunits in the process. E. The first UvsX subunits to bind are the first to hydrolyze ATP, generating a relatively aged, ADP-capped 5' filament end. The ADP-bound UvsX subunits are now vulnerable to displacement by Gp32. Differential competitive effects between Gp32 and the ATP- vs. ADP-capped filament ends creates dynamic instability in the complex, which could lead to filament treadmilling.