Auxiliary ATP binding sites power rapid unwinding by RecBCD

RecBCD, responsible for the initiation of double stranded break repair in bacteria, is a processive DNA helicase with an unwinding rate approaching [~]1,600 bp{middle dot}s-1. The mechanism enabling RecBCD to achieve such fast unwinding rate is not known. We employed a combination of equilibrium and time-resolved binding experiments, and ensemble and single molecule activity assays to uncover the molecular mechanism underlying RecBCDs rapid catalysis. We report the existence of auxiliary binding sites, where ATP binds with lower affinity and with distinct chemical interactions as compared to the known catalytic sites. The catalytic rate of RecBCD is reduced both by preventing and by strengthening ATP binding to these sites, suggesting that the dynamics of ATP at these sites modulates the enzymes rate. We propose a model by which RecBCD achieves its fast unwinding rate by utilizing the weaker binding sites to increase the flux of ATP to its catalytic sites.