The PriA protein of Escherichia coli provides a vital link between recombination and DNA replication. To establish the molecular basis for this link, we investigated the ability of PriA to target DNA substrates modelled on D-loops, the intermediates formed during the early stages of RecA-mediated recombination. We show that PriA binds D-loops and unwinds the DNA in reactions that rely on its ability to function as a helicase. The minimal structure that binds PriA is a duplex DNA molecule with unpaired single strands at one end, an arrangement likely to occur at a D-loop. It resembles features of the stem-loop formed by primosome assembly site (PAS) sequences in the DNA of bacteriophage φX174 and plasmid ColE1, and which enable PriA to assemble active primosomes for the initiation of lagging strand synthesis. We suggest that PAS sequences may have evolved to mimic the natural D-loop target for PriA formed in the chromosome of E. coli during recombination and DNA repair. Genetic studies have revealed an interaction between PriA and RecG, a DNA helicase that drives branch migration of recombination intermediates. We therefore compared PriA and RecG for their ability to bind and unwind DNA. RecG, like PriA, binds D-loops and unwinds the DNA. However, it prefers branched structures with at least two duplex components. The possibility that it competes with PriA for binding recombination intermediates is discussed.
