Figure 6.

RecA strand exchange model with the polarity intrinsic to the RecA-ssDNA filaments. Structure of the RecA filament groove mediates an intrinsic strand exchange polarity. Successful strand exchange is initiated near the 5′-proximal end of RecA-ssDNA filaments, with an initial paired region of 8–9 bp (three RecA subunits). Strand exchange is propagated in 3 nt (one RecA subunit) steps. These steps are propagated unidirectionally along the filament, in the 5′-to-3′ direction relative to the initially bound ssDNA, in a process that does not require ATP hydrolysis. A conformation change in one RecA subunit makes a similar conformation change more likely in the adjacent subunit, producing a cooperative and largely unidirectional wave traveling down the length of the filament.