Presynapsis pathway in bacteriophage T4 homologous recombination. (A) A dsDNA end may be nucleolytically resected to expose a 3' ssDNA tail. The Gp46 and Gp47 proteins are thought to be the major enzymes involved in the resection step. (B) The exposed ssDNA is sequestered by the Gp32 ssDNA-binding protein, which denatures secondary structure in ssDNA and keeps it in an extended conformation. (C) The UvsY recombination mediator protein forms a tripartite complex with Gp32 and ssDNA and "primes" the complex for recruitment of UvsX recombinase. (D) UvsY recruits ATP-bound UvsX protein and nucleates presynaptic filament formation. Gp32 is displaced in the process.