Structure of a Sliding Clamp on DNA

The structure of the E. coli β clamp polymerase processivity factor has been solved in complex with primed DNA. Interestingly, the clamp directly binds the DNA duplex and also forms a crystal contact with the ssDNA template strand, which binds into the protein-binding pocket of the clamp. We demonstrate that these clamp-DNA interactions function in clamp loading, perhaps by inducing the ring to close around DNA. Clamp binding to template ssDNA may also serve to hold the clamp at a primed site after loading or during switching of multiple factors on the clamp. Remarkably, the DNA is highly tilted as it passes through the β ring. The pronounced 22° angle of DNA through β may enable DNA to switch between multiple factors bound to a single clamp simply by alternating from one protomer of the ring to the other.