ASSEMBLY OF A CHROMOSOMAL REPLICATION MACHINE - 2 DNA-POLYMERASES, A CLAMP LOADER, AND SLIDING CLAMPS IN ONE HOLOENZYME PARTICLE .3. INTERFACE BETWEEN THE POLYMERASES AND THE CLAMP LOADER

The nine-subunit DNA polymerase (Pol) III coupled to its beta sliding clamp is a rapid and highly processive replicating machine. The multi ple subunits are needed for the complicated task of duplicating the Es cherichia coli chromosome. In this report, Pol III was constituted fr om individual pure proteins, and its structure was studied. Constituti on of the Pol III particle requires an ordered addition of the subuni ts, and the final structure contains 14 polypeptides in the ratio alph a(2) epsilon(2) theta(2) tau(2) gamma(2) delta(1) delta(1)' chi(1) psi (1). The structure can be summarized as being composed of two core pol ymerases (alpha epsilon theta) held together by a dimer of tau and one gamma complex clamp loader (gamma(2) delta(1) delta(1)'chi(1) psi(1)) for loading beta onto DNA. At the center of the structure, the relate d tau and gamma subunits form a heterotetramer upon which the two core polymerases and clamp loader proteins assemble. The single copy natur e of the delta, delta', chi, and psi subunits confers a structural asy mmetry with respect to the two polymerases, presumably for the differe nt functions of replicating the leading and lagging strands.