In Escherichia coli, the dinB gene is required for the SOS-induced lambda untargeted mutagenesis pathway and confers a mutator phenotype to the cell when the gene product is overexpressed. Here, we report that the purified DinB protein is a DNA polymerase. This novel E. coli DNA polymerase (pol IV) is shown to be strictly distributive, devoid of proofreading activity, and prone to elongate bulged (misaligned) primer/template structures. Site-directed mutagenesis experiments of dinB also demonstrate that the polymerase activity of DinB is required for its in vivo mutagenicity. Along with the sequence homologies previously found within the UmuC-like protein family, these results indicate that the uncovered DNA polymerase activity may be a common feature of all these homologous proteins.

The recent discovery of a new family of ubiquitous DNA polymerases involved in translesion synthesis has shed new light onto the biochemical basis of mutagenesis. Among these polymerases, the dinB gene product (Pol IV) is involved in mutagenesis in Escherichia coli. We show here that the activity of native Pol IV is drastically modified upon interaction with the β subunit, the processivity factor of DNA Pol III. In the absence of the β subunit Pol IV is strictly distributive and no stable complex between Pol IV and DNA could be detected. In contrast, the β clamp allows Pol IV to form a stable initiation complex (t 1/2 ≈ 2.3 min), which leads to a dramatic increase in the processivity of Pol IV reaching an average of 300-400 nucleotides. In vivo, the β processivity subunit may target DNA Pol IV to its substrate, generating synthesis tracks much longer than previously thought.

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The dinB gene of Escherichia coli is known to be involved in the untargeted mutagenesis of phage. Recently, we have demonstrated that this damage-inducible and SOS-controlled gene encodes a novel DNA polymerase, DNA Pol IV, which is able to dramatically increase the untargeted mutagenesis of F plasmid. At the amino acid level, DNA Pol IV shares sequence homologies with E. coli UmuC (DNA Pol V), Rev1p, and Rad30p (DNA polymerase ) of Saccharomyces cerevisiae and human Rad30A (XPV) proteins, all of which are involved in translesion DNA synthesis. To better characterize the Pol IV-dependent untargeted mutagenesis, i.e., the DNA Pol IV mutator activity, we analyzed the genetic requirements of this activity and determined the forward mutation spectrum generated by this protein within the cII gene of phage. The results indicated that the DNA Pol IV mutator activity is independent of polA, polB, recA, umuDC, uvrA, and mutS functions. The analysis of more than 300 independent mutations obtained in the wild-type or mutS background revealed that the mutator activity clearly promotes single-nucleotide substitutions as well as one-base deletions in the ratio of about 1:2. The base changes were strikingly biased for substitutions toward G:C base pairs, and about 70% of them occurred in 5-GX-3 sequences, where X represents the base (T, A, or C) that is mutated to G. These results are discussed with respect to the recently described biochemical characteristics of DNA Pol IV.Stability of genetic information is a key element in the maintenance of proper cell biology and the perpetuation of species. On the other hand, evolution obviously proceeds with the help of mutations. It is thus of great interest to understand the mechanisms underlying DNA replication fidelity and its modulation. In both eukaryotic and prokaryotic organisms, DNA replication is a highly accurate process, allowing only one error among 10 9 to 10 10 incorporation events (8). However, this low error frequency of DNA replication may be enhanced either upon modification of the substrate molecule by endogenous or exogenous DNA-damaging agents, i.e., targeted mutagenesis, or through modulation of the replication fidelity in the absence of any DNA damage, i.e., untargeted mutagenesis.In the bacterium Escherichia coli, the induction of the socalled SOS system allows the organism to cope with adverse conditions in various ways. One of the consequences of its activation is an increase in both targeted and untargeted mutagenesis (for a review, see reference 12). SOS-dependent targeted mutagenesis in E. coli relies on an increase in the frequency of DNA synthesis past DNA lesions, i.e., translesion synthesis. Although their precise mechanisms are not yet established, two distinct translesion synthesis pathways have been described to date. One relies on the activity, together with the replicative polymerase III, of the UmuDC and RecA proteins (12), whereas the other occurs independently of these accessory proteins but requires another, yet unidentified, SOS function terme...