Clamp loaders and sliding clamps.

A coherent view of the structure and function of DNA polymerase processivity factors (sliding clamps and clamp loaders) is emerging from recent structural studies. Crystal structures of sliding clamps from the T4 and RB69 bacteriophages, and from an archaebacterium expand the gallery of ring-shaped processivity factors and clarify how the clamp interacts with the DNA polymerase. Crystallographic and electron microscopic views of clamp loaders from bacteria, archaebacteria and eukaryotes emphasize their common architecture and have produced models of how ATPbinding might be coupled to clamp opening/loading.