Pol III Holoenzyme is a big molecule that consists of 10 different subunits. Three subunits alpha, epsilon and theta form the core and there are 2 cores in a Pol III Holo enzyme complex that linked by tau subunit. Each core is responsible for leading and lagging synthesis. The gamma complex (clamp loader) is composed of gamma, delta, delta prime, chi and psi subunits; take in charge in loading beta clamp onto primed DNA. The complex consist of two cores and the gamma complex is called as Polymerase III*. When beta clamp is added to each cores of Polymerase III*, it becomes Pol III Holo enzyme. The beta clamp is very important subunit, which forms a donut shaped ring around the DNA and helps to anchor the holoenzyme to the DNA during DNA replication. By acting as a sliding clamp, beta clamp helps the Holoenzyme to replicate with high processivity (over 50 kb per binding event) and high speed (about 1kb s−1), while Pol III* alone has a very low processivity (<10 base pairs per DNA binding event) and replicates DNA at low rate (<20 nucleotides s−1). Pol III Holoenzyme also have 3′-5′ exonuclease proofreading activity (by epsilon subunit and stimulated by theta subunit), this characteristic helps PolIII Holoenzyme can synthesize with the minimum mistake.
Schematic of DNA polymerase III holoenzyme. DNA polymerase III holoenzyme contains 10 different subunits; arrows identify each of them. The three functional components of the holoenzyme are shown to the right of the diagram [clamp (β), core polymerase (αεθ), γ complex (γδδ’χψ)]. The τ subunit dimer acts as a glue to bind together two core polymerases and one γ complex to form the Pol III* subassembly (the holoenzyme lacking β), as shown in the diagram at the far right (Zvi Kelman and Mike O’Donnell et al 1998, The EMBO Journal Vol.17 No.8 pp.2436–2449).
“Replisome mechanics: insights into a twin DNA polymerase machine”. Richard T. Pomerantz and Mike O’Donnell 2007. Trends in microbiology, Vol.15 No.4.
“Devoted to the lagging strand—the χ subunit of DNA polymerase III holoenzyme contacts SSB to promote processive elongation and sliding clamp assembly”. Zvi Kelman et al 1998, The EMBO Journal Vol.17 No.8 pp.2436–2449.