The beta subunit of the Polymerase III holoenzyme is one of ten subunits in the Pol III multisubunit complex, which catalyzes the synthesis of both the leading and lagging strands of DNA in E. Coli. While the core Pol III enzyme (consisting of the alpha, sigma and theta subunits) dissociates from the DNA after replicating approximately 12 residues, the holoenzyme complex has a processivity of more than 5,000 residues. The increase in replication processing is due to the beta subunit.
The beta subunit binds DNA by forming a ring around the DNA helix, essentially acting as a sliding clamp, also known as a beta clamp. This conformation allows the enzyme to move along the DNA structure without diffusing away, thereby increasing the processivity and rate of nucleotide polymerization. The beta complex consists of a dimer of monomers that form a donut-type structure with an ~80A total diameter. This creates an inner ring of approximately 35A, in which A- and B-DNAs have room to bind. The dimeric ring is a pseudo-symmetrical six pointed star, with each monomer made up of three structurally similar domains. These domains align head to tail to form the ring shape. Model studies of B-DNA binding this enzyme
