Assembly of a chromosomal replication machine: Two DNA polymerases, a clamp loader, and sliding clamps in one holoenzyme particle. III. Interface between two polymerases and the clamp loader

Abstract

The nine-subunit DNA polymerase (Pol) III* coupled to its β sliding clamp is a rapid and highly processive replicating machine. The multiple subunits are needed for the complicated task of duplicating the Escherichia coli chromosome. In this report, Pol III* was constituted from individual pure proteins, and its structure was studied. Constitution of the Pol III* particle requires an ordered addition of the subunits, and the final structure contains 14 polypeptides in the ratio α2ε2θ2τ2γ2δ1δ'1χ1ψ1. The structure can be summarized as being composed of two core polymerases (αεθ) held together by a dimer of τ and one γ complex clamp loader (γ2δ1δ'1χ1ψ1) for loading β onto DNA. At the center of the structure, the related τ and γ subunits form a heterotetramer upon which the two core polymerases and clamp loader proteins assemble. The single copy nature of the δ, δ', χ, and ψ subunits confers a structural asymmetry with respect to the two polymerases, presumably for the different functions of replicating the leading and lagging strands.