PCNA opening in equilibrium binding assays with RFC. A, AF488 fluorescence was measured in solutions containing increasing concentrations of RFC and either 1 nm (red) or 5 nm (blue) PCNA-AF4882 and 0.5 mm ATP. B, AF488 fluorescence was measured in a competition binding assay containing 20 nm PCNA-AF4882, 20 nm RFC, 0.5 mm ATP and increasing concentrations of unlabeled PCNA. RFC, at a constant concentration, was added to the mixture of labeled and unlabeled PCNA. The average value of relative intensity of AF488 from three independent experiments is plotted with standard deviations.