Bacterial replicases are complex, tripartite replicative machines. They contain a polymerase, Pol III, a β(2) processivity factor and a DnaX complex ATPase that loads β(2) onto DNA and chaperones Pol III onto the newly loaded β(2). 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 Mg(2+)-dependent exonuclease, but also contains a second Zn(2+)-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.
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