Proofreading exonuclease on a tether: the complex between the E. coli DNA polymerase III subunits α, ε, θ and β reveals a highly flexible arrangement of the proofreading domain

A complex of the three (αeθ) core subunits and the β2 sliding clamp is responsible for DNA synthesis by Pol III, the Escherichia coli chromosomal DNA replicase. The 1.7 A crystal structure of a complex between the PHP domain of α (polymerase) and the C-terminal segment of e (proofreading exonuclease) subunits shows that e is attached to α at a site far from the polymerase active site. Both α and e contain clamp-binding motifs (CBMs) that interact simultaneously with β2 in the polymerization mode of DNA replication by Pol III. Strengthening of both CBMs enables isolation of stable αeθ:β2 complexes. Nuclear magnetic resonance experiments with reconstituted αeθ:β2 demonstrate retention of high mobility of a segment of 22 residues in the linker that connects the exonuclease domain of e with its α-binding segment. In spite of this, small-angle X-ray scattering data show that the isolated complex with strengthened CBMs has a compact, but still flexible, structure. Photo-crosslinking with p-benzoyl-L-phenylalanine incorporated at different sites in the α-PHP domain confirm the conformational variability of the tether. Structural models of the αeθ:β2 replicase complex with primer-template DNA combine all available structural data.