The Escherichia coli recA protein has been shown to hydrolyze several nucleoside triphosphates in the presence of ssDNA. The substitution of dATP for rATP has significant effects on various recA protein biochemical properties. In the presence of dATP, recA protein can invade more secondary structure in native ssDNA than it can in the presence of rATP. The dATP-recA protein complex can compete more effectively with the E . coli ssDNA binding protein (SSB) for ssDNA binding sites compared with

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... rATP-recA protein complex. Finally, the rate of dATP hydrolysis stimulated by dsDNA is greater than the rate of rATP hydrolysis. These effects, in turn, are observed as alterations in the recA protein catalyzed D N A strand exchange reaction. In the absence of SSB protein, the rate of joint molecule and product formation in the D N A strand exchange reaction is greater in the presence of dATP than in the presence of rATP. The rate of product formation in the dATP-dependent reaction is also faster than the rATP-dependent reaction when SSB protein is added to the ssDNA before recA protein; the rate of rATP-dependent product formation is inhibited 10-fold under these conditions. This nucleotide, dATP, was previously shown to induce an apparent affinity of recA protein for ssDNA which is higher than any other NTP. These results suggest that the observed enhancement of enzymatic activity may be related to the steady-state properties of the high-affinity ssDNA binding state of recA protein. In addition, the data suggest that recA protein functions in N T P hydrolysis as a dimer of protein filaments and that the binding of ssDNA to only one of the recA filaments is sufficient to activate all recA protein molecules in the dimeric filament. The implications of this finding to the enzymatic function of recA protein are discussed. R e c A protein can catalyze the complete exchange of one single-stranded DNA (ssDNA) molecule for a homologous strand within a duplex DNA molecule in a reaction called DNA strand exchange (Cox & Lehman, 1981a) . This reaction requires the hydrolysis of a nucleoside triphosphate cofactor, typically rATP (Cox & Lehman, 1981a) ; however, the actual mechanistic requirement for rATP hydrolysis is unknown. Hypotheses for the role of rATP binding and hydrolysis in recA protein function include mediating the polar polymerization of recA protein on DNA (Griffith et al., 1984; Kowalczykowski et al., 1987) , providing energy for concerted rotation of a recA protein filament to drive branch migration of heteroduplex joints (Schutte & Cox, 1987) , and a way to ' Abbreviations: PEP, phosphoenolpyruvate; ssDNA, single-stranded DNA; dsDNA, double-stranded DNA; etheno M13 DNA, modified M13 ssDNA containing 1 ,M-ethenoadenosine and 3,N"-ethenocytidine residues: poly(dT), poly(thymidy1ic acid); poly(dA), poly(deoxyadeny1ic acid); poly(dA-dT), alternating copolymer of deoxyadenylic and thymidylic acid.