DNA Replicases from a Bacterial Perspective

Bacterial replicases are complex, tripartite replicative machines. They contain a polymerase, polymerase III (Pol III), a β2 processivity factor, and a DnaX complex ATPase that loads β2 onto DNA and chaperones Pol III onto the newly loaded β2. Bacterial replicases are highly processive, yet cycle rapidly during Okazaki fragment synthesis in a regulated way. Many bacteria encode both a full-length τ and a shorter γ form of DnaX by a variety of mechanisms. γ appears to be uniquely placed in a single position relative to two τ protomers in a pentameric ring. The polymerase catalytic subunit of Pol III, α, contains a PHP domain that not only binds to a prototypical ε Mg²⁺-dependent exonuclease, but also contains a second Zn²⁺-dependent proofreading exonuclease, at least in some bacteria. This review focuses on a critical evaluation of recent literature and concepts pertaining to the above issues and suggests specific areas that require further investigation.