Sliding clamps of DNA polymerases.

The determination of the structure of the processivity factor (beta subunit) of Escherichia coli DNA polymerase III holoenzyme showed that this protein acts to clamp the polymerase onto DNA by forming a closed circular structure that can encircle duplex DNA (X.-P. Kong, R. Onrust, M. O'Donnell & J. Kuriyan. (1992). Cell, 69, 425-437). In this review we describe the features of the beta subunit that allow it to be linked tightly but non-specifically to DNA, and discuss the surprisingly symmetrical architecture of the molecule. The simple repeating pattern of the chain fold allows a connection to be made to the as yet unknown structures of eukaryotic proliferating cell nuclear antigen and the gene 45 protein of bacteriophage T4, which are the processivity factors of the corresponding DNA polymerases.