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  • The EMBO Journal (1999) 18, 771 - 783
  • doi:10.1093/emboj/18.3.771

The internal workings of a DNA polymerase clamp-loading machine

Jennifer Turner1, Manju M. Hingorani2, Zvi Kelman1,3 and Mike O'Donnell2,4

  1. Cornell University Medical College, 1300 York Avenue, New York, NY 10021, USA
  2. The Rockefeller University, New York, NY 10021, USA
  3. Sloan-Kettering Institute, 1275 York Avenue, New York, NY 10021, USA
  4. The Howard Hughes Medical Institute, 1230 York Avenue, New York, NY 10021, USA

Correspondence to:

Mike O'Donnell, E-mail: odonnel@rockvax.rockefeller.edu

Received 11 September 1998; Accepted 27 November 1998; Revised 27 November 1998

Replicative DNA polymerases are multiprotein machines that are tethered to DNA during chain extension by sliding clamp proteins. The clamps are designed to encircle DNA completely, and they are manipulated rapidly onto DNA by the ATP-dependent activity of a clamp loader. We outline the detailed mechanism of gamma complex, a five-protein clamp loader that is part of the Escherichia coli replicase, DNA polymerase III holoenzyme. The gamma complex uses ATP to open the beta clamp and assemble it onto DNA. Surprisingly, ATP is not needed for gamma complex to crack open the beta clamp. The function of ATP is to regulate the activity of one subunit, delta, which opens the clamp simply by binding to it. The delta' subunit acts as a modulator of the interaction between delta and beta. On binding ATP, the gamma complex is activated such that the delta' subunit permits delta to bind beta and crack open the ring at one interface. The clamp loader–open clamp protein complex is now ready for an encounter with primed DNA to complete assembly of the clamp around DNA. Interaction with DNA stimulates ATP hydrolysis which ejects the gamma complex from DNA, leaving the ring to close around the duplex.

  • Keywords:

    • ATPase,
    • clamp loader,
    • DNA polymerase,
    • processivity,
    • sliding clamp