NADPH-specific quinone reductase is induced by 2-methylene-4-butyrolactone in Escherichia coli

The Azotobacter vinelandii AlgU regulon during vegetative growth and encysting conditions: A proteomic approach

In the Pseduomonadacea family, the extracytoplasmic function sigma factor AlgU is crucial to withstand adverse conditions. Azotobacter vinelandii, a closed relative of Pseudomonas aeruginosa, has been a model for cellular differentiation in Gram-negative bacteria since it forms desiccation-resistant cysts. Previous work demonstrated the essential role of AlgU to withstand oxidative stress and on A. vinelandii differentiation, particularly for the positive control of alginate production. In this study, the AlgU regulon was dissected by a proteomic approach under vegetative growing conditions and upon encystment induction. Our results revealed several molecular targets that explained the requirement of this sigma factor during oxidative stress and extended its role in alginate production. Furthermore, we demonstrate that AlgU was necessary to produce alkyl resorcinols, a type of aromatic lipids that conform the cell membrane of the differentiated cell. AlgU was also found to positively regulate stress resistance proteins such as OsmC, LEA-1, or proteins involved in trehalose synthesis. A position-specific scoring-matrix (PSSM) was generated based on the consensus sequence recognized by AlgU in P. aeruginosa, which allowed the identification of direct AlgU targets in the A. vinelandii genome. This work further expands our knowledge about the function of the ECF sigma factor AlgU in A. vinelandii and contributes to explains its key regulatory role under adverse conditions.

SifR is an Rrf2-family quinone sensor associated with catechol iron uptake in Streptococcus pneumoniae D39

Streptococcus pneumoniae (pneumococcus) is a Gram-positive commensal and human respiratory pathogen. How this bacterium satisfies its nutritional iron (Fe) requirement in the context of endogenously produced hydrogen peroxide is not well understood. Here, we characterize a novel virulence-associated Rrf2-family transcriptional repressor that we term SifR (streptococcal IscR-like family transcriptional repressor) encoded by spd_1448 and conserved in Streptococci. Global transcriptomic analysis of a ΔsifR strain defines the SifR regulon as genes encoding a candidate catechol dioxygenase CatE, an uncharacterized oxidoreductase YwnB, a candidate flavin-dependent ferric reductase YhdA, a candidate heme-based ferric reductase domain-containing protein and the Piu (pneumococcus iron uptake) Fe transporter (piuBCDA). Previous work established that membrane-anchored PiuA binds Fe^III-bis-catechol or monocatechol complexes with high affinity, including the human catecholamine stress hormone, norepinephrine. We demonstrate that SifR senses quinone via a single conserved cysteine that represses its regulon when in the reduced form. Upon reaction with catechol-derived quinones, we show that SifR dissociates from the DNA leading to regulon derepression, allowing the pneumococcus to access a catechol-derived source of Fe while minimizing reactive electrophile stress induced by quinones. Consistent with this model, we show that CatE is an Fe^II-dependent 2,3-catechol dioxygenase with broad substrate specificity, YwnB is an NAD(P)H-dependent quinone reductase capable of reducing the oxidized and cyclized norepinephrine, adrenochrome, and YhdA is capable of reducing a number of Fe^III complexes, including PiuA-binding transport substrates. These findings are consistent with a model where Fe^III-catechol complexes serve as significant nutritional Fe sources in the host.

MdaB and NfrA, two novel reductases important in the survival and persistence of the major enteropathogen Campylobacter jejuni

The paralogues RrpA and RrpB, which are members of the MarR family of DNA binding proteins, are important for the survival of the global bacterial foodborne pathogen Campylobacter jejuni under redox stress. We report that RrpA is a positive regulator of mdaB, encoding a flavin-dependent quinone reductase that contributes to the protection from redox stress mediated by structurally diverse quinones, while RrpB negatively regulates the expression of cj1555c (renamed nfrA for NADPH-flavin reductase A), encoding a flavin reductase. NfrA reduces riboflavin at a greater rate than its derivatives, suggesting that exogenous free flavins are the natural substrate. MdaB and NfrA both prefer NADPH as an electron donor. Cysteine substitution and posttranslational modification analyses indicated that RrpA and RrpB employ a cysteine-based redox switch. Complete genome sequence analyses revealed that mdaB is frequently found in Campylobacter and related Helicobacter spp., while nfrA is predominant in C. jejuni strains. Quinones and flavins are redox cycling agents secreted by a wide range of cell types that can form damaging superoxide by one-electron reactions. We propose a model for stress adaptation where MdaB and NfrA facilitate a two-electron reduction mechanism to the less toxic hydroquinones, thus aiding survival and persistence of this major pathogen.

IMPORTANCE Changes in cellular redox potential result in alteration in the oxidation state of intracellular metabolites and enzymes; consequently, cells make adjustments that favor growth and survival. The work we present here answers some of the many questions that have remained elusive over the years of investigation into the enigmatic microaerophile bacterium Campylobacter jejuni. We employed molecular approaches to understand the regulation mechanisms and functional analyses to reveal the roles of two novel quinone and flavin reductases; both serve as major pools of cellular redox-active molecules. This work extends our knowledge on bacterial redox sensing mechanisms and the significance of hemostasis.

Identification of novel members of the bacterial azoreductase family in Pseudomonas aeruginosa

Azoreductases are a family of diverse enzymes found in many pathogenic bacteria as well as distant homologues being present in eukarya. In addition to having azoreductase activity, these enzymes are also suggested to have NAD(P)H quinone oxidoreductase (NQO) activity which leads to a proposed role in plant pathogenesis. Azoreductases have also been suggested to play a role in the mammalian pathogenesis of Pseudomonas aeruginosa. In view of the importance of P. aeruginosa as a pathogen, we therefore characterized recombinant enzymes following expression of a group of putative azoreductase genes from P. aeruginosa expressed in Escherichia coli. The enzymes include members of the arsenic-resistance protein H (ArsH), tryptophan repressor-binding protein A (WrbA), modulator of drug activity B (MdaB) and YieF families. The ArsH, MdaB and YieF family members all show azoreductase and NQO activities. In contrast, WrbA is the first enzyme to show NQO activity but does not reduce any of the 11 azo compounds tested under a wide range of conditions. These studies will allow further investigation of the possible role of these enzymes in the pathogenesis of P. aeruginosa.

NarJ subfamily system specific chaperone diversity and evolution is directed by respiratory enzyme associations

Background

Redox enzyme maturation proteins (REMPs) describe a diverse family of prokaryotic chaperones involved in the biogenesis of anaerobic complex iron sulfur molybdoenzyme (CISM) respiratory systems. Many REMP family studies have focused on NarJ subfamily members from Escherichia coli: NarJ, NarW, DmsD, TorD and YcdY. The aim of this bioinformatics study was to expand upon the evolution, distribution and genetic association of these 5 REMP members within 130 genome sequenced taxonomically diverse species representing 324 Prokaryotic sequences. NarJ subfamily member diversity was examined at the phylum-species level and at the amino acid/nucleotide level to determine how close their genetic associations were between their respective CISM systems within phyla.

Results

This study revealed that NarJ members possessed unique motifs that distinguished Gram-negative from Gram-positive/Archaeal species and identified a strict genetic association with its nitrate reductase complex (narGHI) operon compared to all other members. NarW appears to be found specifically in Gammaproteobacteria. DmsD also showed close associations with the dimethylsulfoxide reductase (dmsABC) operon compared to TorD. Phylogenetic analysis revealed that YcdY has recently evolved from DmsD and that YcdY has likely diverged into 2 subfamilies linked to Zn- dependent alkaline phosphatase (ycdX) operons and a newly identified operon containing part of Zn-metallopeptidase FtsH complex component (hflC) and NADH-quinone dehydrogenase (mdaB). TorD demonstrated the greatest diversity in operon association. TorD was identifed within operons from either trimethylamine-N-oxide reductase (torAC) or formate dehydrogenase (fdhGHI), where each type of TorD had a unique motif. Additionally a subgroup of dmsD and torD members were also linked to operons with biotin sulfoxide (bisC) and polysulfide reductase (nrfD) indicating a potential role in the maturation of diverse CISM.

Conclusion

Examination of diverse prokaryotic NarJ subfamily members demonstrates that the evolution and genetic association of each member is uniquely biased by its CISM operon association.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0412-3) contains supplementary material, which is available to authorized users.

Identification of NAD(P)H quinone oxidoreductase activity in azoreductases from P. aeruginosa: azoreductases and NAD(P)H quinone oxidoreductases belong to the same FMN-dependent superfamily of enzymes

Water soluble quinones are a group of cytotoxic anti-bacterial compounds that are secreted by many species of plants, invertebrates, fungi and bacteria. Studies in a number of species have shown the importance of quinones in response to pathogenic bacteria of the genus Pseudomonas. Two electron reduction is an important mechanism of quinone detoxification as it generates the less toxic quinol. In most organisms this reaction is carried out by a group of flavoenzymes known as NAD(P)H quinone oxidoreductases. Azoreductases have previously been separate from this group, however using azoreductases from Pseudomonas aeruginosa we show that they can rapidly reduce quinones. Azoreductases from the same organism are also shown to have distinct substrate specificity profiles allowing them to reduce a wide range of quinones. The azoreductase family is also shown to be more extensive than originally thought, due to the large sequence divergence amongst its members. As both NAD(P)H quinone oxidoreductases and azoreductases have related reaction mechanisms it is proposed that they form an enzyme superfamily. The ubiquitous and diverse nature of azoreductases alongside their broad substrate specificity, indicates they play a wide role in cellular survival under adverse conditions.

Genomics of rapid adaptation to antibiotics: convergent evolution and scalable sequence amplification

Evolutionary adaptation can be extremely fast, especially in response to high selection intensities. A prime example is the surge of antibiotic resistance in bacteria. The genomic underpinnings of such rapid changes may provide information on the genetic processes that enhance fast responses and the particular trait functions under selection. Here, we use experimentally evolved Escherichia coli for a detailed dissection of the genomics of rapid antibiotic resistance evolution. Our new analyses demonstrate that amplification of a sequence region containing several known antibiotic resistance genes represents a fast genomic response mechanism under high antibiotic stress, here exerted by drug combination. In particular, higher dosage of such antibiotic combinations coincided with higher copy number of the sequence region. The amplification appears to be evolutionarily costly, because amplification levels rapidly dropped after removal of the drugs. Our results suggest that amplification is a scalable process, as copy number rapidly changes in response to the selective pressure encountered. Moreover, repeated patterns of convergent evolution were found across the experimentally evolved bacterial populations, including those with lower antibiotic selection intensities. Intriguingly, convergent evolution was identified on different organizational levels, ranging from the above sequence amplification, high variant frequencies in specific genes, prevalence of individual nonsynonymous mutations to the unusual repeated occurrence of a particular synonymous mutation in Glycine codons. We conclude that constrained evolutionary trajectories underlie rapid adaptation to antibiotics. Of the identified genomic changes, sequence amplification seems to represent the most potent, albeit costly genomic response mechanism to high antibiotic stress.

Structural and biochemical characterization of MdaB from cariogenicStreptococcus mutansreveals an NADPH-specific quinone oxidoreductase

Thesmu.1420 gene from the cariogenic pathogenStreptococcus mutansencodes a putative protein which has sequence homology to NQO [NAD(P)H:quinone oxidoreductase] family members, including mammalian NQO and bacterial MdaB (modulator of drug activity B). NQO can detoxify quinones by converting them to hydroquinones and prevent the generation of reactive oxygen species. Thus, comprehensive studies on Smu.1420 will be important for uncovering the antioxidation and antidrug mechanisms ofS. mutans. Here, the catalytic properties of Smu.1420 have been characterized, and its structure was determined in complexes with NADP+and menadione, respectively. Smu.1420 binds menadione directly and exhibits a pronounced preference for NADPH over NADH as a substrate, demonstrating that it is an NADPH-specific quinone oxidoreductase. The structure of Smu.1420 shows a compact homodimer with two substrate pockets located in the cleft of the dimer interface. The nicotinamide moiety of NADP+is bound on top of the isoalloxazine moiety of the FAD cofactor and overlaps with the binding site of menadione, suggesting a hydride-transfer process from NADPH to FAD and then to menadione. Two strongly basic patches near the substrate pocket are expected to confer the preference for NADPH over NADH. These studies shed light on future drug development against the cariogenic pathogenS. mutans.

KefF, the regulatory subunit of the potassium efflux system KefC, shows quinone oxidoreductase activity

Escherichia coli and many other Gram-negative pathogenic bacteria protect themselves from the toxic effects of electrophilic compounds by using a potassium efflux system (Kef). Potassium efflux is coupled to the influx of protons, which lowers the internal pH and results in immediate protection. The activity of the Kef system is subject to complex regulation by glutathione and its S conjugates. Full activation of KefC requires a soluble ancillary protein, KefF. This protein has structural similarities to oxidoreductases, including human quinone reductases 1 and 2. Here, we show that KefF has enzymatic activity as an oxidoreductase, in addition to its role as the KefC activator. It accepts NADH and NADPH as electron donors and quinones and ferricyanide (in addition to other compounds) as acceptors. However, typical electrophilic activators of the Kef system, e.g., N-ethyl maleimide, are not substrates. If the enzymatic activity is disrupted by site-directed mutagenesis while retaining structural integrity, KefF is still able to activate the Kef system, showing that the role as an activator is independent of the enzyme activity. Potassium efflux assays show that electrophilic quinones are able to activate the Kef system by forming S conjugates with glutathione. Therefore, it appears that the enzymatic activity of KefF diminishes the redox toxicity of quinones, in parallel with the protection afforded by activation of the Kef system.

Oxidative stress in Campylobacter jejuni: responses, resistance and regulation

Campylobacter jejuni is a major food-borne human pathogen that paradoxically is an oxygen-sensitive microaerophile, yet must resist the oxidative stresses encountered both in the host and in the environment. Recent studies suggest that, perhaps surprisingly, C. jejuni contains a wide range of enzymes involved in oxidative stress defense, and this review focuses on the properties and roles of these proteins. Although the mechanisms of gene regulation are still poorly understood in C. jejuni, several regulators of the oxidative stress response have been identified and their properties are discussed here. We suggest that future studies should be directed towards identifying the role of additional and less well characterized components involved in oxidative stress resistance, as well as providing a more complete picture of the underlying sensing and regulatory mechanisms.

Mutation of cytotoxin-associated gene A affects expressions of antioxidant proteins of Helicobacter pylori

AIM: To determine if disruption of the cagA gene of Helicobacter pylori (H pylori) has an effect on the expression of other proteins at proteome level.

METHODS: Construction of a cagA knock out mutant Hp27_ΔcagA (cagA^-) via homologous recombination with the wild-type strain Hp27 (cagA⁺) as a recipient was performed. The method of sonication-urea-CHAPS-DTT was employed to extract bacterial proteins from both strains. Soluble proteins were analyzed by two-dimensional electrophoresis (2-DE). Images of 2-DE gels were digitalized and analyzed. Only spots that had a statistical significance in differential expression were selected and analyzed by matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF-MS). Biological information was used to search protein database and identify the biological function of proteins.

RESULTS: The proteome expressions between wild-type strain and isogenic mutant with the cagA gene knocked-out were compared. Five protein spots with high abundance in bacteria proteins of wild-type strains, down-regulated or absently expressed in bacteria proteins of mutants, were identified and analyzed. From a quantitative point of view, the identified proteins are related to the cagA gene and important antioxidant proteins of H pylori, including alkyl hydroperoxide reductase (Ahp), superoxide dismutase (SOD) and modulator of drug activity (Mda66), respectively, suggesting that cagA is important to maintain the normal activity of antioxidative stress and ensure H pylori persistent colonization in the host.

CONCLUSION: cagA gene is relevant to the expressions of antioxidant proteins of H pylori, which may be a novel mechanism involved in H pylori cagA pathogenesis.

Vinyl ketone reduction by three distinct Gluconobacter oxydans 621H enzymes

Three cytosolic NADPH-dependent flavin-associated proteins (Gox2107, Gox0502, and Gox2684) from Gluconobacter oxydans 621H were overproduced in Escherichia coli, and the recombinant enzymes were purified and characterized. Apparent native molecular masses of 65.2, 78.2, and 78.4 kDa were observed for Gox2107, Gox0502, and Gox2684, corresponding to a trimeric structure for Gox2107 and dimers for Gox0502 and Gox2684. Analysis of flavin content revealed Gox2107 was flavin adenine dinucleotide dependent, whereas Gox0502 and Gox2684 contained flavin mononucleotide. The enzymes were able to reduce vinyl ketones and quinones, reducing the olefinic bond of vinyl ketones as shown by (1)H nuclear magnetic resonance. Additionally, Gox0502 and Gox2684 stereospecifically reduced 5S-(+)-carvone to 2R,5S-dihydrocarvone. All enzymes displayed highest activities with 3-butene-2-one and 1,4-naphthoquinone. Gox0502 and Gox2684 displayed a broader substrate spectrum also reducing short-chain alpha-diketones, whereas Gox2107 was most catalytically efficient.

The NADPH quinone reductase MdaB confers oxidative stress resistance to Helicobacter hepaticus

An mdaB mutant strain in a quinone reductase (MdaB) of Helicobacter hepaticus type strain ATCC51449 was constructed by insertional mutagenesis, and the MdaB protein was purified and compared to the Helicobacter pylori enzyme. While wild type H. hepaticus cells could tolerate 6% O(2) for growth, the mdaB strain was clearly inhibited at this oxygen level. Disruption of the gene downstream of mdaB (HH1473) did not affect the oxidative stress phenotype of the strain. The mdaB mutant was also more sensitive to oxidative stress reagents such as H(2)O(2), cumene hydroperoxide, t-butyl hydroperoxide, and paraquat. All H. hepaticus mdaB strains isolated constitutively up-expressed another oxidative stress-combating enzyme, superoxide dismutase; this is in contrast to H. pylori mdaB strains. H. hepaticus MdaB is a flavoprotein catalyzing quinone reduction using a two-electron transfer mechanism from NAD(P)H to quinone. The H. hepaticus enzyme specific activity was far less than for the H. pylori enzyme purified in the same manner.

Piecing together the structure–function puzzle: Experiences in structure-based functional annotation of hypothetical proteins

The combination of genomic sequencing with structural genomics has provided a wealth of new structures for previously uncharacterized ORFs, more commonly referred to as hypothetical proteins. This rapid growth has been the direct result of high-throughput, automated approaches in both the identification of new ORFs and the determination of high-resolution 3-D protein structures. A significant bottleneck is reached, however, at the stage of functional annotation in that the assignment of function is not readily automatable. It is often the case that the initial structural analysis at best indicates a functional family for a given hypothetical protein, but further identification of a relevant ligand or substrate is impeded by the diversity of function in a particular structural classification of proteins family, a highly selective and specific ligand-binding site, or the identification of a novel protein fold. Our approach to the functional annotation of hypothetical proteins relies on the combination of structural information with additional bioinformatics evidence garnered from operon prediction, loose functional information of additional operon members, conservation of catalytic residues, as well as cocrystallization trials and virtual ligand screening. The synthesis of all available information for each protein has permitted the functional annotation of several hypothetical proteins from Escherichia coli and each assignment has been confirmed through generally accepted biochemical methods.

Cytotoxicity of lawsone and cytoprotective activity of antioxidants in catalase mutant Escherichia coli

Lawsone is an active naphthoquinone derivative isolated from henna (Lawsonia inermis L.), a widely used hair dye. Previous study on the toxicity of lawsone remains unclear since the involvement of oxidative stress and the kind of ROS (reactive oxygen species) involved have not been fully resolved yet. This present study reports the cytotoxic effects of lawsone and henna. We carried out CAT assay (a zone of inhibition test of bacterial growth and colony-forming efficiency test of transformant Escherichia coli strains that express mammalian catalase gene derived from normal catalase mice (Cs(a)) and catalase-deficient mutant mice (Cs(b))), Ames mutagenicity assay and H(2)O(2) generation assay. Lawsone generated H(2)O(2) slightly in phosphate buffer system and was not mutagenic in Ames assay using TA 98, TA 100 and TA 102, both in the absence and presence of metabolic activation. Lawsone exposure inhibited the growth of both Cs(a) and Cs(b) strains in a dose-dependent manner. Mean zone diameter for Cs(a) was 9.75+/-0.96 mm and 12.75+/-1.5 mm for Cs(b). Natural henna leaves did not show toxic effects, whereas two out of four samples of marketed henna products were shown toxicity effects. Catalase abolished zone of inhibition (ZOI) of marketed henna products, eliminated ZOI of lawsone in a dose-dependent manner and low concentration of exogenous MnSOD and Cu/ZnSOD eliminated the toxicity. Histidine and DTPA, the metal chelator; BHA and low concentration of capsaicin, the inducer of NADH-quinone reductase, effectively protected Cs(a) and Cs(b) against lawsone in this study. We suggest that lawsone cytotoxicity is probably mediated, at least in part, by the release of O(2)(-), H(2)O(2) and OH(-).

The diverse antioxidant systems of Helicobacter pylori

The gastric pathogen Helicobacter pylori induces a strong inflammatory host response, yet the bacterium maintains long-term persistence in the host. H. pylori combats oxidative stress via a battery of diverse activities, some of which are unique or newly described. In addition to using the well-studied bacterial oxidative stress resistance enzymes superoxide dismutase and catalase, H. pylori depends on a family of peroxiredoxins (alkylhydroperoxide reductase, bacterioferritin co-migratory protein and a thiol-peroxidase) that function to detoxify organic peroxides. Newly described antioxidant proteins include a soluble NADPH quinone reductase (MdaB) and an iron sequestering protein (NapA) that has dual roles - host inflammation stimulation and minimizing reactive oxygen species production within H. pylori. An H. pylori arginase attenuates host inflammation, a thioredoxin required as a reductant for many oxidative stress enzymes is also a chaperon, and some novel properties of KatA and AhpC were discovered. To repair oxidative DNA damage, H. pylori uses an endonuclease (Nth), DNA recombination pathways and a newly described type of bacterial MutS2 that specifically recognizes 8-oxoguanine. A methionine sulphoxide reductase (Msr) plays a role in reducing the overall oxidized protein content of the cell, although it specifically targets oxidized Met residues. H. pylori possess few stress regulator proteins, but the key roles of a ferric uptake regulator (Fur) and a post-transcriptional regulator CsrA in antioxidant protein expression are described. The roles of all of these antioxidant systems have been addressed by a targeted mutant analysis approach and almost all are shown to be important in host colonization. The described antioxidant systems in H. pylori are expected to be relevant to many bacterial-associated diseases, as genes for most of the enzymes carrying out the newly described roles are present in a number of pathogenic bacteria.

Modulator of drug activity B from Escherichia coli: crystal structure of a prokaryotic homologue of DT-diaphorase

Modulator of drug activity B (MdaB) is a putative member of the DT-diaphorase family of NAD(P)H:oxidoreductases that afford cellular protection against quinonoid compounds. While there have been extensive investigations of mammalian homologues, putative prokaryotic members of this enzyme family have received little attention. The three-dimensional crystal structure of apo-MdaB reported herein exhibits significant structural similarity to a number of flavoproteins, including the mammalian DT-diaphorases. We have shown by mass spectrometry that the endogenously associated cofactor is flavin adenine dinucleotide and we present here the structure of MdaB in complex with this compound. Growth of Escherichia coli carrying null mutations in the genes encoding MdaB or quinol monooxygenase, the gene for which shares the mdaB promoter, were not affected by the presence of menadione. However, over-expression of recombinant quinol monooxygenase conferred a state of resistance against both tetracycline and adriamycin. This work suggests that the redox cycle formed by these proteins protects E. coli from the toxic effects of polyketide compounds rather than the oxidative stress of menadione alone.

CrgA is an inducible LysR-type regulator of Neisseria meningitidis, acting both as a repressor and as an activator of gene transcription

The crgA gene of Neisseria meningitidis, which codes for a LysR-type regulator, is divergently oriented with respect to the mdaB gene, which codes for a hypothetical NADPH-quinone oxidoreductase. Transcriptional studies of the intergenic region between crgA and mdaB showed that two overlapping and divergent promoters, P(crgA) and P(mdaB), control transcription of these genes. Deletion of the crgA gene led to a strong increase in transcription from the P(crgA) promoter and a concomitant strong decrease in transcription from the P(mdaB) promoter, indicating that CrgA acts both as an autorepressor of transcription at its own promoter and as an activator of transcription at the mdaB promoter. Addition of alpha-methylene-gamma-butyrolactone (MBL), an inducer of NADPH-quinone oxidoreductase, to wild-type N. meningitidis cells specifically resulted in further activation of transcription of the P(mdaB) promoter and more repression of transcription of the P(crgA) promoter. No such regulation was observed when MBL was added to crgA-deficient cells, indicating that the transcriptional response to MBL is CrgA mediated. Under the same experimental conditions, no regulation of transcription by either CrgA or MBL was detected at the pilus and capsule genes. The role of CrgA in the regulation of gene expression during the infectious cycle of N. meningitidis is discussed.

Structural and Biochemical Evidence for an Enzymatic Quinone Redox Cycle in Escherichia coli

Naturally synthesized quinones perform a variety of important cellular functions. Escherichia coli produce both ubiquinone and menaquinone, which are involved in electron transport. However, semiquinone intermediates produced during the one-electron reduction of these compounds, as well as through auto-oxidation of the hydroxyquinone product, generate reactive oxygen species that stress the cell. Here, we present the crystal structure of YgiN, a protein of hitherto unknown function. The three-dimensional fold of YgiN is similar to that of ActVA-Orf6 monooxygenase, which acts on hydroxyquinone substrates. YgiN shares a promoter with "modulator of drug activity B," a protein with activity similar to that of mammalian DT-diaphorase capable of reducing mendione. YgiN was able to reoxidize menadiol, the product of the "modulator of drug activity B" (MdaB) enzymatic reaction. We therefore refer to YgiN as quinol monooxygenase. Modulator of drug activity B is reported to be involved in the protection of cells from reactive oxygen species formed during single electron oxidation and reduction reactions. The enzymatic activities, together with the structural characterization of YgiN, lend evidence to the possible existence of a novel quinone redox cycle in E. coli.

MdaB from Escherichia coli: cloning, purification, crystallization and preliminary X-ray analysis

The gene mdaB from Escherichia coli encodes an enzyme with activity similar to that of mammalian DT-diaphorase. It has been reported that the protein is able to confer resistance to the antibiotics DMP 840, adriamycin and etoposide. The gene was cloned and overexpressed in E. coli, allowing purification of the protein to homogeneity. The protein co-purified with an unidentified flavin. Suitable crystals for X-ray diffraction experiments were obtained by hanging-drop vapour diffusion. Their space group was triclinic P1, with unit-cell parameters a = 48.664, b = 52.099, c = 86.584 A, alpha = 87.106, beta = 86.889, gamma = 63.526 degrees. X-ray diffraction data were collected to 2.5 A.

An NADPH quinone reductase of Helicobacter pylori plays an important role in oxidative stress resistance and host colonization

Oxidative stress resistance is one of the key properties that enable pathogenic bacteria to survive the toxic reactive oxygen species released by the host. In a previous study characterizing oxidative stress resistance mutants of Helicobacter pylori, a novel potential antioxidant protein (MdaB) was identified by the observation that the expression of this protein was significantly upregulated to compensate for the loss of other major antioxidant components. In this study, we characterized an H. pylori mdaB mutant and the MdaB protein. While the wild-type strain can tolerate 10% oxygen for growth, the growth of the mdaB mutant was significantly inhibited by this oxygen condition. The mdaB mutant is also more sensitive to H(2)O(2), organic hydroperoxides, and the superoxide-generating agent paraquat. Although the wild-type strain can survive more than 10 h of air exposure, exposure of the mutant strain to air for 8 h resulted in recovery of no viable cells. The oxidative stress sensitivity of the mdaB mutant resulted in a deficiency in the ability to colonize mouse stomachs. H. pylori was recovered from 10 of 11 mouse stomachs inoculated with the wild-type strain, with about 5,000 to 45,000 CFU/g of stomach. However, only 3 of 12 mice that were inoculated with the mdaB mutant strain were found to harbor any H. pylori, and these 3 contained less than 2,000 CFU/g of stomach. A His-tagged MdaB protein was purified and characterized. It was shown to be a flavoprotein that catalyzes two-electron transfer from NAD(P)H to quinones. It reduces both ubiquinones and menaquinones with similar efficiencies and preferably uses NADPH as an electron donor. We propose that the physiological function of the H. pylori MdaB protein is that of an NADPH quinone reductase that plays an important role in managing oxidative stress and contributes to successful colonization of the host.

Vibrio harveyi nitroreductase is also a chromate reductase

The chromate reductase purified from Pseudomonas ambigua was found to be homologous with several nitroreductases. Escherichia coli DH5alpha and Vibrio harveyi KCTC 2720 nitroreductases were chosen for the present study, and their chromate-reducing activities were determined. A fusion between glutathione S-transferase (GST) and E. coli DH5alpha NfsA (GST-EcNfsA), a fusion between GST and E. coli DH5alpha NfsB (GST-EcNfsB), and a fusion between GST and V. harveyi KCTC 2720 NfsA (GST-VhNfsA) were prepared for their overproduction and easy purification. GST-EcNfsA, GST-EcNFsB, and GST-VhNFsA efficiently reduced nitrofurazone and 2,4,6-trinitrotoluene (TNT) as their nitro substrates. The K(m) values for GST-EcNfsA, GST-EcNfsB, and GST-VhNfsA for chromate reduction were 11.8, 23.5, and 5.4 micro M, respectively. The V(max) values for GST-EcNfsA, GST-EcNfsB, and GST-VhNfsA were 3.8, 3.9, and 10.7 nmol/min/mg of protein, respectively. GST-VhNfsA was the most effective of the three chromate reductases, as determined by each V(max)/K(m) value. The optimal temperatures of GST-EcNfsA, GST-EcNfsB, and GST-VhNfsA for chromate reduction were 55, 30, and 30 degrees C, respectively. Thus, it is confirmed that nitroreductase can also act as a chromate reductase. Nitroreductases may be used in chromate remediation. GST-EcNfsA, GST-EcNfsB, and GST-VhNfsA have a molecular mass of 50 kDa and exist as a monomer in solution. Thin-layer chromatography showed that GST-EcNfsA, GST-EcNfsB, and GST-VhNfsA contain FMN as a cofactor. GST-VhNfsA reduced Cr(VI) to Cr(III). Cr(III) was much less toxic to E. coli than Cr(VI).

Oxidative-stress resistance mutants of Helicobacter pylori

Within a large family of peroxidases, one member that catalyzes the reduction of organic peroxides to alcohols is known as alkyl hydroperoxide reductase, or AhpC. Gene disruption mutations in the gene encoding AhpC of Helicobacter pylori (ahpC) were generated by screening transformants under low-oxygen conditions. Two classes of mutants were obtained. Both types lack AhpC protein, but the major class (type I) isolated was found to synthesize increased levels (five times more than the wild type) of another proposed antioxidant protein, an iron-binding, neutrophil-activating protein (NapA). The other class of mutants, the minor class (type II), produced wild-type levels of NapA. The two types of AhpC mutants differed in their frequencies of spontaneous mutation to rifampin resistance and in their sensitivities to oxidative-stress chemicals, with the type I mutants exhibiting less sensitivity to organic hydroperoxides as well as having a lower mutation frequency. The napA promoter regions of the two types of AhpC mutants were identical, and primer extension analysis revealed their transcription start site to be the same as for the wild type. Gene disruption mutations were obtained in napA alone, and a double mutant strain (ahpC napA) was also created. All four of the oxidative-stress resistance mutants could be distinguished from the wild type in oxygen sensitivity or in some other oxidative-stress resistance phenotype (i.e., in sensitivity to stress-related chemicals and spontaneous mutation frequency). For example, growth of the NapA mutant was more sensitive to oxygen than that of the wild-type strain and both of the AhpC-type mutants were highly sensitive to paraquat and to cumene hydroperoxide. Of the four types of mutants, the double mutant was the most sensitive to growth inhibition by oxygen and by organic peroxides and it had the highest spontaneous mutation frequency. Notably, two-dimensional gel electrophoresis combined with protein sequence analysis identified another possible oxidative-stress resistance protein (HP0630) that was up-regulated in the double mutant. However, the transcription start site of the HP0630 gene was the same for the double mutant as for the wild type. It appears that H. pylori can readily modulate the expression of other resistance factors as a compensatory response to loss of a major oxidative-stress resistance component.