X-ray derived model of PCNA bound to DNA. (A) Crystals of single-chain PCNA-DNA. The DNA is labeled on the 5' end of the primer strand with a Cy5 chromophore. (B) Unbiased electron density for the DNA produced from single-chain PCNA-DNA crystal, calculated before DNA was placed into the model. 2Fo-Fc map is displayed at a contour level of 0.7σ. Positions of the DNA phosphates in the final model are shown as spheres. (C) PCNA-DNA model derived from X-ray diffraction data. The DNA makes a ~40° angle with the central axis of PCNA.