ASSEMBLY OF A CHROMOSOMAL REPLICATION MACHINE - 2 DNA-POLYMERASES, A CLAMP LOADER, AND SLIDING CLAMPS IN ONE HOLOENZYME PARTICLE .3. INTERFACE BETWEEN TWO 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 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 alpha(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 polymerases (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 related tau and gamma subunits form a heterotetramer upon which the two core polymerases and clamp loader proteins assemble. The single copy nature of the delta, delta', chi, and psi subunits confers a structural asymmetry with respect to the two polymerases, presumably for the different functions of replicating the leading and lagging strands.