Unexpected new insights into DNA clamp loaders: Eukaryotic clamp loaders contain a second DNA site for recessed 5' ends that facilitates repair and signals DNA damage: Eukaryotic clamp loaders contain a second DNA site for recessed 5' ends that facilitates repair and signals DNA damage.

Clamp loaders are pentameric AAA+ assemblies that use ATP to open and close circular DNA sliding clamps around DNA. Clamp loaders show homology in all organisms, from bacteria to human. The eukaryotic PCNA clamp is loaded onto 3' primed DNA by the replication factor C (RFC) hetero-pentameric clamp loader. Eukaryotes also have three alternative RFC-like clamp loaders (RLCs) in which the Rfc1 subunit is substituted by another protein. One of these is the yeast Rad24-RFC (Rad17-RFC in human) that loads a 9-1-1 heterotrimer clamp onto a recessed 5' end of DNA. Recent structural studies of Rad24-RFC have discovered an unexpected 5' DNA binding site on the outside of the clamp loader and reveal how a 5' end can be utilized for loading the 9-1-1 clamp onto DNA. In light of these results, new studies reveal that RFC also contains a 5' DNA binding site, which functions in gap repair. These studies also reveal many new features of clamp loaders. As reviewed herein, these recent studies together have transformed our view of the clamp loader mechanism.