The sliding clamp tethers the endonuclease domain of MutL to DNA

The sliding clamp enhances polymerase processivity and coordinates DNA replication with other critical DNA processing events including translesion synthesis, Okazaki fragment maturation and DNA repair. The relative binding affinity of the sliding clamp for its partners determines how these processes are orchestrated and is essential to ensure the correct processing of newly replicated DNA. However, while stable clamp interactions have been extensively studied; dynamic interactions mediated by the sliding clamp remain poorly understood. Here, we characterize the interaction between the bacterial sliding clamp (beta-clamp) and one of its weak-binding partners, the DNA mismatch repair protein MutL. Disruption of this interaction causes a mild mutator phenotype in Escherichia coli, but completely abrogates mismatch repair activity in Bacillus subtilis. We stabilize the MutL-beta interaction by engineering two cysteine residues at variable positions of the interface. Using disulfide bridge crosslinking, we have stabilized the E. coli and B. subtilis MutL-beta complexes and have characterized their structures using small angle X-ray scattering. We find that the MutL-beta interaction greatly stimulates the endonuclease activity of B. subtilis MutL and supports this activity even in the absence of the N-terminal region of the protein.