Insights into the replisome from the structure of a ternary complex of the DNA polymerase III α-subunit

The crystal structure of the catalytic alpha-subunit of the DNA polymerase III (PolIII alpha) holoenzyme bound to primer-template DNA and an incoming deoxy-nucleoside 5'-triphosphate has been determined at 4.6-angstrom resolution. The polymerase interacts with the sugar-phosphate backbone of the DNA across its minor groove, which is made possible by significant movements of the thumb, finger, and beta-binding domains relative to their orientations in the unliganded polymerase structure. Additionally, the DNA and incoming nucleotide are bound to the active site of PolIII alpha nearly identically as they are in their complex with DNA polymerase beta, thereby proving that the eubacterial replicating polymerase, but not the eukaryotic replicating polymerase, is homologous to DNA polymerase beta. Finally, superimposing a recent structure of the clamp bound to DNA on this PolIII alpha complex with DNA places a loop of the beta-binding domain into the appropriate clamp cleft and supports a mechanism of polymerase switching. (C) 2008 Elsevier Ltd. All rights reserved.