Blue-White Selection

Abstract

Synopsis Bacterial replicases are complex, tripartite repli-cative machines. They contain a polymerase, Pol III; a processivity factor, b 2 ; and an ATPase, DnaX complex, which loads b 2 onto DNA and chaperones Pol III onto the loaded b 2. Bacterial replicases are highly processive yet cycle rapidly during Okazaki fragment synthesis in a regulated way. Many bacteria encode both a full-length t and a shorter g form of DnaX by a variety of mechanisms. g is uniquely placed relative to two t protomers in a pentameric ring. The catalytic subunit of Pol III, a, contains a PHP domain that not only binds to prototypical e, a Mg ++-dependent exonuclease but also contains a second Zn ++-containing proofreading exonuclease, at least in some bacteria. Replication of the chromosomes of low-GC Gram-positive bacteria requires two Pol IIIs, one of which, DnaE, appears to only extend RNA primers a 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, which is required for induced mutagenesis in E. coli.