Bacterial replicases and related polymerases

Bacterial replicases are complex, tripartite replicative machines. They contain a polymerase, 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. Many bacteria encode both a full length τ and a shorter γ form of DnaX by a variety of mechanisms. The polymerase catalytic subunit of Pol III, α, contains a PHP domain that not only binds to prototypical ɛ Mg2+-dependent exonuclease, but also contains a second Zn2+-dependent proofreading exonuclease, at least in some bacteria. Replication of the chromosomes of low GC Gram-positive bacteria require two Pol IIIs, one of which, DnaE, appears to extend RNA primers a only short distance before handing the product off to the major replicase, PolC. Other bacteria encode a second Pol III (ImuC) that apparently replaces Pol V, required for induced mutagenesis in E. coli. Approaches that permit simultaneous biochemical screening of all components of complex bacterial replicases promise inhibitors of specific protein targets and reaction stages.

► Structures show Pol III active site has the fold like eukaryotic Pol β. ► The PHP domain of Pol III serves as a second proofreading exonuclease. ► ψ binds 3 DnaX protomers asymmetrically and stabilizes an active conformation. ► τ-containing DnaX complex chaperones Pol III onto newly loaded β. ► The second Gram-positive DnaE elongates RNA primers before handoff to PolC.