A PriB variant defective for ssDNA binding stimulates the helicase activity of PriA. A) Ribbon diagrams of the crystal structures of E. coli PriB complexed with ssDNA (top, PDB code 2CCZ) and N. gonorrhoeae PriB (bottom, PDB code 3K8A). The two monomers of the PriB dimers are colored red and blue, and the ssDNA is rendered as a cyan tube. The ssDNA modeled above the red chain of E. coli PriB is derived from a symmetry-related molecule in the crystal structure. Amino acid residue K34 of N. gonorrhoeae PriB, and the structurally-analogous R34 amino acid residue of E. coli PriB are rendered as sticks with carbon atoms colored violet and nitrogen atoms colored orange. B) Unwinding of 1 nM Fork 3 by 2 nM PriA in the presence of wild type N. gonorrhoeae PriB (circles) or PriB:K34A (squares). Measurements are reported in triplicate and error bars represent one standard deviation of the mean.