We have investigated the question whether during chromosomal DNA replication in Escherichia coli the two DNA strands may be replicated with differential accuracy. This possibility of differential replication fidelity arises from the distinct modes of replication in the two strands, one strand (the leading strand) being synthesized continuously, the other (the lagging strand) discontinuously in the form of short Okazaki fragments. We have constructed a series of lacZ strains in which the lac operon is inserted into the bacterial chromosome in the two possible orientations with regard to the chromosomal replication origin oriC. Measurement of lac reversion frequencies for the two orientations, under conditions in which mutations ref lect replication errors, revealed distinct differences in mutability between the two orientations. As gene inversion causes a switching of leading and lagging strands, these findings indicate that leading and lagging strand replication have differential fidelity. Analysis of the possible mispairs underlying each specific base pair substitution suggests that the lagging strand replication on the E. coli chromosome may be more accurate than leading strand replication.The question as to how organisms duplicate their DNA with high accuracy is of fundamental interest. Previous studies have revealed the functioning of at least three separate steps, base selection, proofreading, and DNA mismatch repair, which, by their sequential action, are responsible for the low error rate of Ϸ10 Ϫ10 per base replicated (1, 2). The most detailed information about this process is available for the bacterium E. coli based on both enzymological and genetical data. Replication of the E. coli chromosome is performed by DNA polymerase III holoenzyme, an asymmetric dimeric enzyme composed of 18 subunits (10 distinct) that simultaneously replicates the leading and lagging strand of the replication fork (for review, see ref.3). It contains two polymerase core units, one for each strand, each consisting of three tightly associated subunits, ␣, , and . Of these, ␣ is the polymerase (dnaE gene product), (dnaQ gene product) is a 3Ј 3 5Ј exonuclease that performs an editing function, and is a small subunit of unknown function. Additional components of the holoenzyme include the subunit ( 2 ) that dimerizes the two cores, the ␤ subunit (␤ 2 ) that encircles the DNA and tethers each DNA polymerase to the DNA to ensure high processivity, and the five-subunit ␥ complex (␥, ␦, ␦Ј, , and ) that loads the ␤ rings onto the DNA.With regard to the fidelity of polymerase III holoenzyme, as studied both in vivo and in vitro, the main focus has been on the role of the ␣ and subunits. The ␣ (polymerase) subunit plays a critical role through the process of base selection, selecting with great preference correct nucleotides at the nucleotide insertion step. The subunit, in conjunction with the polymerase, is responsible for the subsequent proofreading step, in which by virtue of its 3Ј exonuclease activity incorrectly inserted ...

High accuracy (fidelity) of DNA replication is important for cells to preserve the genetic identity and to prevent the accumulation of deleterious mutations. The error rate during DNA replication is as low as 10−9 to 10−11 errors per base pair. How this low level is achieved is an issue of major interest. This review is concerned with the mechanisms underlying the fidelity of the chromosomal replication in the model system Escherichia coli by DNA polymerase III holoenzyme, with further emphasis on participation of the other, accessory DNA polymerases, of which E. coli contains four (Pols I, II, IV, and V). Detailed genetic analysis of mutation rates revealed that (1) Pol II has an important role as a back‐up proofreader for Pol III, (2) Pols IV and V do not normally contribute significantly to replication fidelity, but can readily do so under conditions of elevated expression, (3) participation of Pols IV and V, in contrast to that of Pol II, is specific to the lagging strand, and (4) Pol I also makes a lagging‐strand‐specific fidelity contribution, limited, however, to the faithful filling of the Okazaki fragment gaps. The fidelity role of the Pol III τ subunit is also reviewed.

SummaryEscherichia coli DNA polymerase III holoenzyme (HE) is the main replicase responsible for replication of the bacterial chromosome. E. coli contains four additional polymerases, and it is a relevant question whether these might also contribute to chromosomal replication and its fidelity. Here, we have investigated the role of DNA polymerase II (Pol II) ( polB gene product). Mismatch repair-defective strains containing the polBex1 allele -encoding a polymerase-proficient but exonucleolytically defective Pol II -displayed a mutator activity for four different chromosomal lac mutational markers. The mutator effect was dependent on the chromosomal orientation of the lacZ gene. The results indicate that Pol II plays a role in chromosomal replication and that its role is not equal in leadingversus lagging-strand replication. In particular, the role of Pol II appeared larger in the lagging strand. When combined with dnaQ or dnaE mutator alleles, polBex1 showed strong, near multiplicative effects. The results fit a model in which Pol II acts as proofreader for HE-produced misinsertion errors. A second role of Pol II is to protect mismatched 3 ¢ ¢ ¢ ¢ termini against the mutagenic action of polymerase IV ( dinB product). Overall, Pol II may be considered a main player in the polymerase trafficking at the replication fork.