Interaction of RFC with PCNA, ptDNA stimulates ATP binding and hydrolysis. (A) RFC (0.5 μM) alone (○) binds 3 ATPγS, whereas in the presence of PCNA (□) or ptDNA (△), or both (●), it binds 5 ATPγS molecules; PCNA also helps increase the binding affinity of RFC for ATPγS. (B) Steady state ATPase assays show that ptDNA increases RFC kcat by ~ 20-fold, while PCNA increases it by only ~ 2-fold (calculated as Vmax/(4*[RFC]), assuming 4 ATPase active sites in RFC).