Polymerase III b Subunit - DNA Clamp

<> 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 (Kong et al, 1992).

• <> 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 (Kuriyan and O'Donnell, 1993) is suggested between the beta subunit and other processivity factors for polymerases
  • <> proliferating cell nuclear antigen (PCNA, the eukaryotic polymerase delta [and epsilon] processivity factor; Krishna et al, 1993)
    • as you can see, a trimer of this structure would closely resemble the dimer of  the polIII b-subunit
  • the gene 45 protein of the bacteriophage T4 DNA polymerase.