Figure 1. Structure of the Escherichia coli sliding clamp. The clamp encircles the DNA molecule (with its axis orientated perpendicular to the plane of the diagram, not shown), associates with the polIII core and holds it firmly in place so that primer extension can proceed. This ribbon diagram of the polypeptide backbone of a dimer of subunits was generated from the coordinates from crystallographic data. Reproduced with permission from Kong et al. (1992).

Figure 2. The steps that occur when complex loads onto DNA. Biochemical evidence suggests that most likely exists as a dimer in solution. The complex assembles the dimeric clamp onto DNA in a reaction that requires the hydrolysis of ATP. In the presence of ATP, the complex binds and may open one or both dimer interfaces to assemble onto the DNA. Then the complex dissociates so that the –DNA complex is free to interact with the core polymerase.

Figure 3. Stimulation of the activities of the Escherichia coli – DNA polymerase III core subunits and of DNA polymerase II by accessory factors. SSB is E. coli single-stranded binding protein. Reproduced with permission from Bonner et al. (1992).