THE UVSY RECOMBINATION PROTEIN OF BACTERIOPHAGE-T4 FORMS HEXAMERS IN THE PRESENCE AND ABSENCE OF SINGLE-STRANDED-DNA

A prerequisite to genetic recombination in the T4 bacteriophage is the formation of the presynaptic filament-a helical nucleoprotein filamen t containing stoichiometric amounts of the uvsX recombinase in complex with single-stranded DNA(ssDNA). Once formed, the filament is compete nt to catalyze homologous pairing and DNA strand exchange reactions. A n important component in the formation of the presynaptic filament is the uvsY protein, which is required for optimal uvsX-ssDNA assembly in vitro, and essential for phage recombination in vivo. uvsY enhances u vsX activities by promoting filament formation and stabilizing filamen ts under conditions of low uvsX, high salt, and/or high gp32 (ssDNA-bi nding protein) concentrations. The molecular properties of uvsY includ e noncooperative binding to ssDNA and specific protein-protein interac tions with both uvsX and gp32. Evidence suggests that all of these het ero-associations of the uvsY protein are important for presynaptic fil ament formation. However, there is currently no structural information available on the uvsY protein itself. In this study, we present the f irst characterization of the self-association of uvsY. Using hydrodyna mic methods, we demonstrate that uvsY associates into a stable hexamer (s(20,w)(0) = 6.0, M = 95 kDa) in solution and that this structure is competent to bind ssDNA. We further demonstrate that uvsY hexamers ar e capable of reversible association into higher aggregates in a manner dependent on both salt and protein concentration. The implications fo r presynaptic filament formation are discussed.