A, an SDS-PAGE analysis of S. cerevisiae RFC complex comprising five subunits in 1:1 stoichiometry. B, PCNA clamp assembly on DNA, assayed by incubating 0.03 μM 32P-PCNA with 0.02 μM RFC and 0.03 μM SSB-coated circular primed M13 ssDNA in the presence (●) or absence (○) of 0.5 mM ATP, unprimed M13 ssDNA with ATP (□), and circular duplex pBluescript DNA with ATP (△) for 5 min at 30 °C, followed by gel filtration as described under “Experimental Procedures.” PCNA loaded on DNA elutes in fractions 7–12 and free PCNA elutes in fractions 14–25.