Binding of adenosine (3-thiotriphosphate) (ATPgammaS), a nonhydrolyzable analog of ATP, to replication factor C with a N-terminal truncation (Delta2-273) of the Rfc1 subunit (RFC) was studied by filter binding. RFC alone bound 1.8 ATPgammaS molecules. However, when either PCNA or primer-template DNA were also present 2.6 or 2.7 ATPgammaS molecules, respectively, were bound. When both PCNA and DNA were present 3.6 ATPgammaS molecules were bound per RFC. Order of addition experiments using surface plasmon resonance indicate that RFC forms an ATP-mediated binary complex with PCNA prior to formation of a ternary DNA.PCNA.RFC complex. An ATP-mediated complex between RFC and DNA was not competent for binding PCNA, and the RFC.DNA complex dissociated with hydrolysis of ATP. Based on these experiments a model is proposed in which: (i) RFC binds two ATPs (RFC.ATP(2)); (ii) this complex binds PCNA (PCNA.RFC.ATP(2)), which goes through a conformational change to reveal a binding site for one additional ATP (PCNA.RFC.ATP(3)); (iii) this complex can bind DNA to yield DNA.PCNA.RFC.ATP(3); (iv) a conformational change in the latter complex reveals a fourth binding site for ATP; and (v) the DNA.PCNA.RFC.ATP(4) complex is finally competent for completion of PCNA loading and release of RFC upon hydrolysis of ATP.