cryo-EM structures of the E. coli replicative DNA polymerase reveal its dynamic interactions with the DNA sliding clamp, exonuclease and τ

The replicative DNA polymerase Pollll alpha from Escherichia coli is a uniquely fast and processive enzyme. For its activity it relies on the DNA sliding clamp beta, the proofreading exonuclease E and the C-terminal domain of the clamp loader subunit tau. Due to the dynamic nature of the four-protein complex it has long been refractory to structural characterization. Here we present the 8 angstrom resolution cryo-electron microscopy structures of DNA-bound and DNA-free states of the Pollll alpha-clamp-exonuclease-tau(c) complex. The structures show how the polymerase is tethered to the DNA through multiple contacts with the clamp and exonuclease. A novel contact between the polymerase and clamp is made in the DNA bound state, facilitated by a large movement of the polymerase tail domain and tau(c). These structures provide crucial insights into the organization of the catalytic core of the replisome and form an important step towards determining the structure of the complete holoenzyme.