RecA protein binding to duplex DNA occurs by a multi-step process. The tau analysis, originally developed to examine the binding of RNA polymerase to promoter DNA, is adapted here to study two kinetically distinguishable reaction segments of RecA-double stranded (ds) DNA complex formation in greater detail. One, which is probably a rapid preequilibrium in which RecA protein binds weakly to native dsDNA, is found to have the following properties: (1) a sensitivity to pH, involving a net release of approximately one proton; (2) a sensitivity to salts; (3) little or no dependence on temperature; (4) little or no dependence on DNA length. The second reaction segment, the rate-limiting nucleation of nucleoprotein filament formation accompanied by partial DNA unwinding, is found to have the following properties: (1) a sensitivity to pH, involving a net uptake of approximately three protons; (2) a sensitivity to salts; (3) a relatively large dependence on temperature, with an Arrhenius activation energy of 39 kcal mol(-1); (4) a sensitivity to DNA topology; (5) a dependence on DNA length. These results contribute to a general mechanism for RecA protein binding to duplex DNA, which can provide a rationale for the apparent preferential binding to altered DNA structures such as pyrimidine dimers and Z-DNA.