3-DIMENSIONAL STRUCTURE OF THE BETA-SUBUNIT OF ESCHERICHIA-COLI DNA POLYMERASE-III HOLOENZYME - A SLIDING DNA CLAMP

The crystal structure of the beta-subunit (processivity factor) of DNA polymerase III holoenzyme has been determined at 2.5 angstrom resolution. A dimer of the beta-subunit (M(r) = 2 x 40.6 kd, 2 x 366 amino acid residues) forms a ring-shaped structure lined by 12-alpha-helices that can encircle duplex DNA. The structure is highly symmetrical, with each monomer containing three domains of identical topology. The charge distribution and orientation of the helices indicate that the molecule functions by forming a tight clamp that can slide on DNA, as shown biochemically. A potential structural relationship is suggested between the beta-subunit and proliferating cell nuclear antigen (PCNA, the eukaryotic polymerase-delta [and epsilon] processivity factor), and the gene 45 protein of the bacteriophage T4 DNA polymerase.