DNA base excision repair potentiates the protective effect of Salmonella Pathogenicity Island 2 within macrophages

Oxidative stress activates transcription of Salmonella pathogenicity island-2 genes in macrophages

The type III secretion system encoded in the Salmonella pathogenicity island-2 (SPI-2) gene cluster facilitates intracellular growth of nontyphoidal Salmonella by interfering with the maturation of Salmonella-containing vacuoles along the degradative pathway. SPI-2 gene products also protect Salmonella against the antimicrobial activity of reactive oxygen species (ROS) synthesized by the phagocyte NADPH oxidase 2 (NOX2). However, a potential relationship between inflammatory ROS and the activation of transcription of SPI-2 genes by intracellular Salmonella is unclear. Here, we show that ROS engendered in the innate host response stimulate SPI-2 gene transcription. We found that the expression of SPI-2 genes in Salmonella-sustaining oxidative stress conditions involves DksA, a protein otherwise known to regulate the stringent response of bacteria to nutritional stress. We also demonstrate that the J and zinc-2-oxidoreductase domains of DnaJ as well as the ATPase activity of the DnaK chaperone facilitate loading of DksA onto RNA polymerase complexed with SPI-2 promoters. Furthermore, the DksA-driven transcription of SPI-2 genes in Salmonella experiencing oxidative stress is contingent on upstream OmpR, PhoP, and SsrB signaling events that participate in the removal of nucleoid proteins while simultaneously recruiting RNA polymerase to SPI-2 promoter regions. Taken together, our results suggest the activation of SPI-2 gene transcription in Salmonella subjected to ROS produced by the respiratory burst of macrophages protects this intracellular pathogen against NOX2-mediated killing. We propose that Salmonella have co-opted inflammatory ROS to induce SPI-2-mediated protective responses against NOX2 host defenses.

Molecular characterization of newly identified Klebsiella PKBSG14 and analysis of its effect on immune response and cell cycle progression using common catfish (Channa punctatus) as a model

Microbial studies on Catfish revealed that Klebsiella is the most common pathogen causing prevalence of ulcers, fin erosion, and other lesions. During this study, a new strain of bacteria was isolated from Channa punctatus, and molecular identification by 16srRNA revealed the strain was Klebsiella PKBSG14 (Accession no KJ162158). The strain was also PCR positive for two virulent gene wcaG (Accession no LN606595) and rmpA (Accession no LN606594) responsible for inflammatory reactions and induction of innate immune response in the host cell. To study innate immune response induced by pathogenic infection the phagocytic interactive process between the spleen macrophages and KlebsiellaPKBSG14 was investigated using optical microscopy. FACS of splenic macrophages revealed that the phagocytic interaction leads to the process of macrophage cell cycle progression. A detailed study on the macrophage DNA content by performing DNA fragmentation and comet allowed us to study simultaneously host cell division as a function of phagocytosis and the findings unveiled the fact that Phagocytosis of KlebsiellaPKBSG14 aided in macrophage cell cycle progression but was less likely to complete mitosis. Here we also report the cytotoxic effect linked to the infection with KlebsiellaPKBSG14 by performing Cell viability assay, intracellular production of ROS, and mitochondrial transmembrane potential where it manifested itself in impaired cellular function. So, in summary, we simultaneously discovered a new strain of bacteria ie. Klebsiella PKBSG14 as well as deliberately attempted to study the immunomodulatory effect of isolated new stain on Channa punctatus by performing host-pathogen phagocytic interactive experiments, the cell cycle state of the host cell and pathogen-mediated cytotoxicity along with genotoxicity, and our results evidence a new immunomodulatory effect of KlebsiellaPKBSG14 infection on fish splenic macrophages.

Phylogenomics and antimicrobial resistance of the leprosy bacillus Mycobacterium leprae

Leprosy is a chronic human disease caused by the yet-uncultured pathogen Mycobacterium leprae. Although readily curable with multidrug therapy (MDT), over 200,000 new cases are still reported annually. Here, we obtain M. leprae genome sequences from DNA extracted directly from patients’ skin biopsies using a customized protocol. Comparative and phylogenetic analysis of 154 genomes from 25 countries provides insight into evolution and antimicrobial resistance, uncovering lineages and phylogeographic trends, with the most ancestral strains linked to the Far East. In addition to known MDT-resistance mutations, we detect other mutations associated with antibiotic resistance, and retrace a potential stepwise emergence of extensive drug resistance in the pre-MDT era. Some of the previously undescribed mutations occur in genes that are apparently subject to positive selection, and two of these (ribD, fadD9) are restricted to drug-resistant strains. Finally, nonsense mutations in the nth excision repair gene are associated with greater sequence diversity and drug resistance.

Leprosy is caused by the yet-uncultured pathogen Mycobacterium leprae. Here, Benjak et al. obtain M. leprae genome sequences from DNA extracted from patients' skin biopsies and, by analysing 154 genomes from 25 countries, provide insight into the pathogen’s evolution and antimicrobial resistance.

Antioxidant Defense by Thioredoxin Can Occur Independently of Canonical Thiol-Disulfide Oxidoreductase Enzymatic Activity

The thiol-disulfide oxidoreductase CXXC catalytic domain of thioredoxin contributes to antioxidant defense in phylogenetically diverse organisms. We find that although the oxidoreductase activity of thioredoxin-1 protects Salmonella enterica serovar Typhimurium from hydrogen peroxide in vitro, it does not appear to contribute to Salmonella's antioxidant defenses in vivo. Nonetheless, thioredoxin-1 defends Salmonella from oxidative stress resulting from NADPH phagocyte oxidase macrophage expression during the innate immune response in mice. Thioredoxin-1 binds to the flexible linker, which connects the receiver and effector domains of SsrB, thereby keeping this response regulator in the soluble fraction. Thioredoxin-1, independently of thiol-disulfide exchange, activates intracellular SPI2 gene transcription required for Salmonella resistance to both reactive species generated by NADPH phagocyte oxidase and oxygen-independent lysosomal host defenses. These findings suggest that the horizontally acquired virulence determinant SsrB is regulated post-translationally by ancestrally present thioredoxin.

DNA Repair Is Associated with Information Content in Bacteria, Archaea, and DNA Viruses

The concept of a "proteomic constraint" proposes that DNA repair capacity is positively correlated with the information content of a genome, which can be approximated to the size of the proteome (P). This in turn implies that DNA repair genes are more likely to be present in genomes with larger values of P. This stands in contrast to the common assumption that informational genes have a core function and so are evenly distributed across organisms. We examined the presence/absence of 18 DNA repair genes in bacterial genomes. A positive relationship between gene presence and P was observed for 17 genes in the total dataset, and 16 genes when only nonintracellular bacteria were examined. A marked reduction of DNA repair genes was observed in intracellular bacteria, consistent with their reduced value of P. We also examined archaeal and DNA virus genomes, and show that the presence of DNA repair genes is likewise related to a larger value of P. In addition, the products of the bacterial genes mutY, vsr, and ndk, involved in the correction of GC/AT mutations, are strongly associated with reduced genome GC content. We therefore propose that a reduction in information content leads to a loss of DNA repair genes and indirectly to a reduction in genome GC content in bacteria by exposure to the underlying AT mutation bias. The reduction in P may also indirectly lead to the increase in substitution rates observed in intracellular bacteria via loss of DNA repair genes.

Direct measurement of oxidative and nitrosative stress dynamics in Salmonella inside macrophages

Many significant bacterial pathogens have evolved virulence mechanisms to evade degradation and exposure to reactive oxygen (ROS) and reactive nitrogen species (RNS), allowing them to survive and replicate inside their hosts. Due to the highly reactive and short-lived nature of ROS and RNS, combined with limitations of conventional detection agents, the mechanisms underlying these evasion strategies remain poorly understood. In this study, we describe a system that uses redox-sensitive GFP to nondisruptively measure real-time fluctuations in the intrabacterial redox environment. Using this system coupled with high-throughput microscopy, we report the intrabacterial redox dynamics of Salmonella enterica Typhimurium (S. Typhimurium) residing inside macrophages. We found that the bacterial SPI-2 type III secretion system is required for ROS evasion strategies and this evasion relies on an intact Salmonella-containing vacuole (SCV) within which the bacteria reside during infection. Additionally, we found that cytosolic bacteria that escape the SCV experience increased redox stress in human and murine macrophages. These results highlight the existence of specialized evasion strategies used by intracellular pathogens that either reside inside a vacuole or "escape" into the cytosol. Taken together, the use of redox-sensitive GFP inside Salmonella significantly advances our understanding of ROS and RNS evasion strategies during infection. This technology can also be applied to measuring bacterial oxidative and nitrosative stress dynamics under different conditions in a wide variety of bacteria.

The contribution of Nth and Nei DNA glycosylases to mutagenesis in Mycobacterium smegmatis

The increased prevalence of drug resistant strains of Mycobacterium tuberculosis (Mtb) indicates that significant mutagenesis occurs during tuberculosis disease in humans. DNA damage by host-derived reactive oxygen/nitrogen species is hypothesized to be critical for the mutagenic process in Mtb thus, highlighting an important role for DNA repair enzymes in maintenance of genome fidelity. Formamidopyrimidine (Fpg/MutM/Fapy) and EndonucleaseVIII (Nei) constitute the Fpg/Nei family of DNA glycosylases and together with EndonucleaseIII (Nth) are central to the base excision repair pathway in bacteria. In this study we assess the contribution of Nei and Nth DNA repair enzymes in Mycobacterium smegmatis (Msm), which retains a single nth homologue and duplications of the Fpg (fpg1 and fpg2) and Nei (nei1 and nei2) homologues. Using an Escherichia coli nth deletion mutant, we confirm the functionality of the mycobacterial nth gene in the base excision repair pathway. Msm mutants lacking nei1, nei2 and nth individually or in combination did not display aberrant growth in broth culture. Deletion of nth individually results in increased UV-induced mutagenesis and combinatorial deletion with the nei homologues results in reduced survival under oxidative stress conditions and an increase in spontaneous mutagenesis to rifampicin. Deletion of nth together with the fpg homolgues did not result in any growth/survival defects or changes in mutation rate. Furthermore, no differential emergence of the common rifampicin resistance conferring genotypes were noted. Collectively, these data confirm a role for Nth in base excision repair in mycobacteria and further highlight a novel interplay between the Nth and Nei homologues in spontaneous mutagenesis.

Dynamics of spread of Salmonella enterica in the systemic compartment

Traditional microbiological and immunological tools, combined with modern imaging, and molecular and mathematical approaches, have revealed the dispersive nature of Salmonella infections. Bacterial escape from infected cells, spread in the tissues and attempts to restrain this process by the host give rise to fascinating scenarios that underpin the pathogenesis of salmonelloses.

Low-molecular-weight thiol-dependent antioxidant and antinitrosative defences in Salmonella pathogenesis

We found herein that the intracytoplasmic pool of the low-molecular-weight (LMW) thiol glutathione (GSH) is readily oxidized in Salmonella exposed to nitric oxide (NO). The hypersusceptibility of gshA and gshB mutants lacking γ-glutamylcysteine and glutathione synthetases to NO and S-nitrosoglutathione indicates that GSH antagonizes the bacteriostatic activity of reactive nitrogen species. Metabolites of the GSH biosynthetic pathway do not affect the enzymatic activity of classical NO targets such as quinol oxidases. In contrast, LMW thiols diminish the nitrosative stress experienced by enzymes, such as glutamine oxoglutarate amidotransferase, that contain redox active cysteines. LMW thiols also preserve the transcription of Salmonella pathogenicity island 2 gene targets from the inhibitory activity of nitrogen oxides. These findings are consistent with the idea that GSH scavenges reactive nitrogen species (RNS) other than NO. Compared with the adaptive response afforded by inducible systems such as the hmp-encoded flavohaemoprotein, gshA, encoding the first step of GSH biosynthesis, is constitutively expressed in Salmonella. An acute model of salmonellosis has revealed that the antioxidant and antinitrosative properties associated with the GSH biosynthetic pathway represent a first line of Salmonella resistance against reactive oxygen and nitrogen species engendered in the context of a functional NRAMP1(R) divalent metal transporter.

Redox active thiol sensors of oxidative and nitrosative stress

SIGNIFICANCE

The reactivity of the thiol in the side chain of cysteines is exploited by bacterial regulatory proteins that sense and respond to reactive oxygen and nitrogen species.

RECENT ADVANCES

Charged residues and helix dipoles diminish the pKa of redox active cysteines, resulting in a thiolate that is stabilized by neighboring polar amino acids. The reaction of peroxides with thiolates generates a sulfenic acid (-SOH) intermediate that often gives rise to a reversible disulfide bond. Peroxide-induced intramolecular and intermolecular disulfides and intermolecular mixed disulfides modulate the signaling activity of members of the LysR/OxyR, MarR/OhrR, and RsrA family of transcriptional regulators. Thiol-dependent regulators also help bacteria resist the nitrosative and nitroxidative stress. -SOHs, mixed disulfides, and S-nitrosothiols are some of the post-translational modifications induced by nitrogen oxides in the thiol groups of OxyR and SsrB bacterial regulatory proteins. Sulfenylation, disulfide bond formation, S-thiolation, and S-nitrosylation are reversible modifications amenable to feedback regulation by antioxidant and antinitrosative repair systems. The structural and functional changes engaged in the thiol-dependent sensing of reactive species have been adopted by several regulators to foster bacterial virulence during exposure to products of NADPH phagocyte oxidase and inducible nitric oxide synthase.

CRITICAL ISSUES

Investigations with LysR/OxyR, MarR/OhrR, and RsrA family members have helped in an understanding of the mechanisms by which thiols in regulatory proteins react with reactive species, thereby activating antioxidant and antinitrosative gene expression.

FUTURE DIRECTIONS

To define the determinants that provide selectivity of redox active thiolates for some reactive species but not others is an important challenge for future investigations.

Distinct mechanisms of DNA repair in mycobacteria and their implications in attenuation of the pathogen growth

About a third of the human population is estimated to be infected with Mycobacterium tuberculosis. Emergence of drug resistant strains and the protracted treatment strategies have compelled the scientific community to identify newer drug targets, and to develop newer vaccines. In the host macrophages, the bacterium survives within an environment rich in reactive nitrogen and oxygen species capable of damaging its genome. Therefore, for its successful persistence in the host, the pathogen must need robust DNA repair mechanisms. Analysis of M. tuberculosis genome sequence revealed that it lacks mismatch repair pathway suggesting a greater role for other DNA repair pathways such as the nucleotide excision repair, and base excision repair pathways. In this article, we summarize the outcome of research involving these two repair pathways in mycobacteria focusing primarily on our own efforts. Our findings, using Mycobacterium smegmatis model, suggest that deficiency of various DNA repair functions in single or in combinations severely compromises their DNA repair capacity and attenuates their growth under conditions typically encountered in macrophages.

Antimicrobial actions of reactive oxygen species

Everything should be as simple as it can be, but not simpler.

—Attributed to Albert Einstein (1)

Reactive oxygen species (ROS) are produced by host phagocytes and exert antimicrobial actions against a broad range of pathogens. The observable antimicrobial actions of ROS are highly dependent on experimental conditions. This perspective reviews recent controversies regarding ROS in Salmonella-phagocyte interactions and attempts to reconcile conflicting observations from different laboratories.

Molecular mechanisms of Salmonella virulence and host resistance

Salmonella species can cause typhoid fever and gastroenteritis in humans and pose a global threat to human health. In order to establish a successful infection, Salmonella utilize a large number of genes encoding a variety of virulence factors. Different animal models of infection have been used to better understand the mechanisms underlying each disease including cattle, rodents, and nematodes. To date, a number of different bacterial virulence factors have been identified using such animal models, most of which are secreted by two type three secretion systems (T3SS) encoded within Salmonella pathogenicity islands (SPI) 1 and 2. These proteins alter various host cell pathways, facilitating the invasion of epithelial cells during infection, as well as the survival and replication of Salmonella inside phagocytic cells. On the other hand, host genetics and resistance also play a role in the susceptibility to Salmonella infection. The natural resistance-associated macrophage protein 1 (Nramp1), for example, is critical for host defense, since mice lacking Nramp1 fail to control bacterial replication and succumb to low doses of S. Typhimurium. In this chapter, we analyze the different pathogen and host factors that play a role in the dynamic interaction between Salmonella and its host and their impact on disease.

Codependent and independent effects of nitric oxide-mediated suppression of PhoPQ and Salmonella pathogenicity island 2 on intracellular Salmonella enterica serovar typhimurium survival

Here we show that the Salmonella enterica serovar Typhimurium PhoQ sensor kinase lessens the cytotoxicity of reactive nitrogen species (RNS) generated by inducible nitric oxide synthase (iNOS) in the innate response of mononuclear phagocytic cells. This observation is consistent with the expression patterns of PhoP-activated genes during moderate nitrosative stress in the innate host response. In contrast, RNS synthesized during high-NO fluxes of gamma interferon (IFN-gamma)-activated macrophages repress PhoP-activated lpxO, pagP, and phoP gene transcription. Because PhoP-regulated Salmonella pathogenicity island 2 (SPI2) genes are also repressed by high-order RNS (39), we investigated whether the NO-mediated inhibition of PhoPQ underlies the repression of SPI2. Our studies indicate that a third of the expression of the SPI2 spiC gene recorded in nonactivated macrophages depends on PhoQ. Transcription of spiC is repressed in IFN-gamma-primed macrophages in an iNOS-dependent manner, irrespective of the phoQ status of the bacteria. Transcription of spiC is restored in IFN-gamma-treated, iNOS-deficient macrophages to levels sustained by a phoQ mutant in nonactivated phagocytes, suggesting that most NO-dependent repression of spiC is due to the inhibition of PhoPQ-independent targets. Comparison of the intracellular fitness of spiC, phoQ, and spiC phoQ mutants revealed that PhoPQ and SPI2 have codependent and independent effects on S. Typhimurium survival during innate nitrosative stress. However, the intracellular survival of most S. Typhimurium bacteria is conferred by the PhoPQ two-component regulator, and the SPI2 type III secretion system is repressed by high-order RNS of IFN-gamma-activated macrophages.

Phagocytic superoxide specifically damages an extracytoplasmic target to inhibit or kill Salmonella

Background

The phagocytic oxidative burst is a primary effector of innate immunity that protects against bacterial infection. However, the mechanism by which reactive oxygen species (ROS) kill or inhibit bacteria is not known. It is often assumed that DNA is a primary target of oxidative damage, consistent with known effects of endogenously produced ROS in the bacterial cytoplasm. But most studies fail to distinguish between effects of host derived ROS versus damage caused by endogenous bacterial sources. We took advantage of both the ability of Salmonella enterica serovar Typhimurium to survive in macrophages and the genetic tractability of the system to test the hypothesis that phagocytic superoxide damages cytoplasmic targets including DNA.

Methodology/Principal Findings

SodCI is a periplasmic Cu-Zn superoxide dismutase (SOD) that contributes to the survival of Salmonella Typhimurium in macrophages. Through competitive virulence assays, we asked if sodCI has a genetic interaction with various cytoplasmic systems. We found that SodCI acts independently of cytoplasmic SODs, SodA and SodB. In addition, SodCI acts independently of the base excision repair system and RuvAB, involved in DNA repair. Although sodCI did show genetic interaction with recA, this was apparently independent of recombination and is presumably due to the pleiotropic effects of a recA mutation.

Conclusions/Significance

Taken together, these results suggest that bacterial inhibition by phagocytic superoxide is primarily the result of damage to an extracytoplasmic target.

N(2)O(3) enhances the nitrosative potential of IFNgamma-primed macrophages in response to Salmonella

We show here that the nitric oxide (NO)-detoxifying Hmp flavohemoprotein increases by 3-fold the transcription of the Salmonella pathogenicity island 2 (SPI2) in macrophages expressing a functional inducible NO synthase (iNOS). However, Hmp does not prevent NO-related repression of SPI2 transcription in IFNgamma-primed phagocytes, despite preserving intracellular transcription of sdhA sdhB subunits of Salmonella succinate dehydrogenase within both control and IFNgamma-primed phagocytes. To shed light into the seemingly paradoxical role that Hmp plays in protecting intracellular SPI2 expression in various populations of macrophages, N(2)O(3) was quantified as an indicator of the nitrosative potential of Salmonella-infected phagocytes in different states of activation. Hmp was found to prevent the formation of 300nM N(2)O(3)/h/bacteria in IFNgamma-primed macrophages, accounting for about a 60% reduction of the nitrosative power of activated phagocytes. Utilization of the vacuolar ATPase inhibitor bafilomycin indicates that a fourth of the approximately 200nM N(2)O(3)/h sustained by IFNgamma-primed macrophages is generated in endosomal compartments via condensation of HNO(2). In sharp contrast, control macrophages infected with wild-type Salmonella produce as little N(2)O(3) as iNOS-deficient controls. Collectively, these findings indicate that the NO-metabolizing activity of Salmonella Hmp is functional in both control and IFNgamma-primed macrophages. Nonetheless, a substantial amount of the NO generated by IFNgamma-primed macrophages gives rise to N(2)O(3), a species that not only enhances the nitrosative potential of activated phagocytes but also avoids detoxification by Salmonella Hmp.

Phylogenomic analysis of natural selection pressure in Streptococcus genomes

BACKGROUND

In comparative analyses of bacterial pathogens, it has been common practice to discriminate between two types of genes: (i) those shared by pathogens and their non-pathogenic relatives (core genes), and (ii) those found exclusively in pathogens (pathogen-specific accessory genes). Rather than attempting to a priori delineate genes into sets more or less relevant to pathogenicity, we took a broad approach to the analysis of Streptococcus species by investigating the strength of natural selection in all clusters of homologous genes. The genus Streptococcus is comprised of a wide variety of both pathogenic and commensal lineages, and we relate our findings to the pre-existing knowledge of Streptococcus virulence factors.

RESULTS

Our analysis of 1730 gene clusters revealed 136 cases of positive Darwinian selection, which we suggest is most likely to result from an antagonistic interaction between the host and pathogen at the molecular level. A two-step validation procedure suggests that positive selection was robustly identified in our genomic survey. We found no evidence to support the notion that pathogen specific accessory genes are more likely to be subject to positive selection than core genes. Indeed, we even uncovered a few cases of essential gene evolution by positive selection. Among the gene clusters subject to positive selection, a large fraction (29%) can be connected to virulence. The most striking finding was that a considerable fraction of the positively selected genes are also known to have tissue specific patterns of expression during invasive disease. As current expression data is far from comprehensive, we suggest that this fraction was underestimated.

CONCLUSION

Our findings suggest that pathogen specific genes, although a popular focus of research, do not provide a complete picture of the evolutionary dynamics of virulence. The results of this study, and others, support the notion that the products of both core and accessory genes participate in complex networks that comprise the molecular basis of virulence. Future work should seek to understand the evolutionary dynamics of both core and accessory genes as a function of the networks in which they participate.

AP endonuclease paralogues with distinct activities in DNA repair and bacterial pathogenesis

Oxidative stress is a principal cause of DNA damage, and mechanisms to repair this damage are among the most highly conserved of biological processes. Oxidative stress is also used by phagocytes to attack bacterial pathogens in defence of the host. We have identified and characterised two apurinic/apyrimidinic (AP) endonuclease paralogues in the human pathogen Neisseria meningitidis. The presence of multiple versions of DNA repair enzymes in a single organism is usually thought to reflect redundancy in activities that are essential for cellular viability. We demonstrate here that these two AP endonuclease paralogues have distinct activities in DNA repair: one is a typical Neisserial AP endonuclease (NApe), whereas the other is a specialised 3'-phosphodiesterase Neisserial exonuclease (NExo). The lack of AP endonuclease activity of NExo is shown to be attributable to the presence of a histidine side chain, blocking the abasic ribose-binding site. Both enzymes are necessary for survival of N. meningitidis under oxidative stress and during bloodstream infection. The novel functional pairing of NExo and NApe is widespread among bacteria and appears to have evolved independently on several occasions.

Role of Pseudomonas aeruginosa dinB-encoded DNA polymerase IV in mutagenesis

Pseudomonas aeruginosa is a human opportunistic pathogen that chronically infects the lungs of cystic fibrosis patients and is the leading cause of morbidity and mortality of people afflicted with this disease. A striking correlation between mutagenesis and the persistence of P. aeruginosa has been reported. In other well-studied organisms, error-prone replication by Y family DNA polymerases contributes significantly to mutagenesis. Based on an analysis of the PAO1 genome sequence, P. aeruginosa contains a single Y family DNA polymerase encoded by the dinB gene. As part of an effort to understand the mechanisms of mutagenesis in P. aeruginosa, we have cloned the dinB gene of P. aeruginosa and utilized a combination of genetic and biochemical approaches to characterize the activity and regulation of the P. aeruginosa DinB protein (DinB(Pa)). Our results indicate that DinB(Pa) is a distributive DNA polymerase that lacks intrinsic proofreading activity in vitro. Modest overexpression of DinB(Pa) from a plasmid conferred a mutator phenotype in both Escherichia coli and P. aeruginosa. An examination of this mutator phenotype indicated that DinB(Pa) has a propensity to promote C-->A transversions and -1 frameshift mutations within poly(dGMP) and poly(dAMP) runs. The characterization of lexA+ and DeltalexA::aacC1 P. aeruginosa strains, together with in vitro DNA binding assays utilizing cell extracts or purified P. aeruginosa LexA protein (LexA(Pa)), indicated that the transcription of the dinB gene is regulated as part of an SOS-like response. The deletion of the dinB(Pa) gene sensitized P. aeruginosa to nitrofurazone and 4-nitroquinoline-1-oxide, consistent with a role for DinB(Pa) in translesion DNA synthesis over N2-dG adducts. Finally, P. aeruginosa exhibited a UV-inducible mutator phenotype that was independent of dinB(Pa) function and instead required polA and polC, which encode DNA polymerase I and the second DNA polymerase III enzyme, respectively. Possible roles of the P. aeruginosa dinB, polA, and polC gene products in mutagenesis are discussed.