Mechanism of processivity clamp opening by the delta subunit wrench of theclamp loader complex of E-coli DNA polymerase III

The dimeric ring-shaped sliding clamp of E. coli DNA polymerase III (beta s ubunit, homolog of eukaryotic PCNA) is loaded onto DNA by the clamp loader gamma complex (homolog of eukaryotic Replication Factor C, RFC). The delta subunit of the gamma complex binds to the beta ring and opens it. The cryst al structure of a beta:delta complex shows that delta, which is structurall y related to the delta' and gamma subunits of the gamma complex, is a molec ular wrench that induces or traps a conformational change in beta such that one of its dimer interfaces is destabilized. Structural comparisons and mo lecular dynamics simulations suggest a spring-loaded mechanism in which the beta ring opens spontaneously once a dimer interface is perturbed by the d elta wrench.