Heterologous growth phase- and temperature-dependent expression and H2O2 toxicity protection of a superoxide-inducible monofunctional catalase gene from Xanthomonas oryzae pv. oryzae

PXO_RS20535, Encoding a Novel Response Regulator, Is Required for Chemotactic Motility, Biofilm Formation, and Tolerance to Oxidative Stress in Xanthomonas oryzae pv. oryzae

Xanthomonas oryzae pv. oryzae (Xoo), a causal agent of bacterial leaf blight of rice, possesses two-component regulatory systems (TCSs) as an intracellular signaling pathway. In this study, we observed changes in virulence, biofilm formation, motility, chemotaxis, and tolerance against oxidative stress of a knockout mutant strain for the PXO_RS20535 gene, encoding an orphan response regulator (RR). The mutant strain lost virulence, produced significantly less biofilm, and showed remarkably reduced motility in swimming, swarming, and twitching. Furthermore, the mutant strain lost glucose-guided movement and showed clear diminution of growth and survival in the presence of H₂O₂. These results indicate that the RR protein encoded in the PXO_RS20535 gene (or a TCS mediated by the protein) is closely involved in regulation of biofilm formation, all types of motility, chemotaxis, and tolerance against reactive oxygen species (ROS) in Xoo. Moreover we found that the expression of most genes required for a type six secretion system (T6SS) was decreased in the mutant, suggesting that lack of the RR gene most likely leads to defect of T6SS in Xoo.

Inactivation of the Agrobacterium tumefaciens ActSR system affects resistance to multiple stresses with increased H2O2 sensitivity due to reduced expression of hemH

The Agrobacterium tumefaciens ActSR two-component regulatory system is a member of a homologous group of global redox-responsive regulatory systems that adjust the expression of energy-consuming and energy-supplying metabolic pathways in order to maintain cellular redox balance. In this study, the transcriptional organization of the hrpB-actSR locus was determined and the effect of actSR system inactivation on stress resistance was investigated. It was found that hrpB is transcribed as a monocistronic mRNA and actS is transcribed along with actR as a bicistronic mRNA, while actR is also transcribed as a monocistronic message. Each message is initiated from a separate promoter. Inactivation of actR resulted in decreased resistance to membrane stress (sodium dodecyl sulfate), acid stress (pH 5.5), iron starvation (bipyridyl) and iron excess (FeCl₃), and antibiotic stress (tetracycline and ciprofloxacin). Resistance to oxidative stress in the form of organic peroxide (cumene hydroperoxide) increased, while resistance to inorganic peroxide (H₂O₂) decreased. An actR insertion mutant displayed reduced catalase activity, even though transcription of katA and catE remained unchanged. Complementation of the actR inactivation mutant with plasmid-encoded actR or overexpression of hemH, encoding ferrochelatase, restored wild-type catalase activity and H₂O₂ resistance levels. Gel mobility shift and hemH promoter-lacZ fusion results indicated that ActR is a positive regulator of hemH that binds directly to the hemH promoter region. Thus, inactivation of the A. tumefaciens ActSR system affects resistance to multiple stresses, including reduced resistance to H₂O₂ resulting from a reduction in catalase activity due to reduced expression of hemH.

OxyR-regulated catalase CatB promotes the virulence in rice via detoxifying hydrogen peroxide in Xanthomonas oryzae pv. oryzae

BACKGROUND

To facilitate infection, Xanthomonas oryzae pv. oryzae (Xoo), the bacterial blight pathogen of rice, needs to degrade hydrogen peroxide (H₂O₂) generated by the host defense response via a mechanism that is mediated by the transcriptional regulator OxyR. The catalase (CAT) gene catB has previously been shown to belong to the OxyR regulon in Xoo. However, its expression patterns and function in H₂O₂ detoxification and bacterial pathogenicity on rice remain to be elucidated.

RESULTS

The catB gene encodes a putative catalase and is highly conserved in the sequenced strains of Xanthomonas spp. β-galactosidase analysis and electrophoretic mobility shift assays (EMSA) showed that OxyR positively regulated the transcription of catB by directly binding to its promoter region. The quantitative real-time PCR (qRT-PCR) assays revealed that the expression levels of catB and oxyR were significantly induced by H₂O₂. Deletion of catB or oxyR drastically impaired bacterial viability in the presence of extracellular H₂O₂ and reduced CAT activity, demonstrating that CatB and OxyR contribute to H₂O₂ detoxification in Xoo. In addition, ΔcatB and ΔoxyR displayed shorter bacterial blight lesions and reduced bacterial growth in rice compared to the wild-type stain, indicating that CatB and OxyR play essential roles in the virulence of Xoo.

CONCLUSIONS

Transcription of catB is enhanced by OxyR in response to exogenous H₂O₂. CatB functions as an active catalase that is required for the full virulence of Xoo in rice.

Differential Control Efficacies of Vitamin Treatments against Bacterial Wilt and Grey Mould Diseases in Tomato Plants

Bacterial wilt and grey mould in tomato plants are economically destructive bacterial and fungal diseases caused by Ralstonia solanacearum and Botrytis cinerea, respectively. Various approaches including chemical and biological controls have been attempted to arrest the tomato diseases so far. In this study, in vitro growths of bacterial R. solanacearum and fungal B. cinerea were evaluated using four different vitamins including thiamine (vitamin B1), niacin (vitamin B3), pyridoxine (vitamin B6), and menadione (vitamin K3). In planta efficacies of the four vitamin treatments on tomato protection against both diseases were also demonstrated. All four vitamins showed different in vitro antibacterial activities against R. solanacearum in dose-dependent manners. However, treatment with 2 mM thiamine was only effective in reducing bacterial wilt of detached tomato leaves without phytotoxicity under lower disease pressure (106 colony-forming unit [cfu]/ml). Treatment with the vitamins also differentially reduced in vitro conidial germination and mycelial growth of B. cinerea. The four vitamins slightly reduced the conidial germination, and thiamine, pyridoxine and menadione inhibited the mycelial growth of B. cinerea. Menadione began to drastically suppress the conidial germination and mycelial growth by 5 and 0.5 mM, respectively. Grey mould symptoms on the inoculated tomato leaves were significantly reduced by pyridoxine and menadione pretreatments one day prior to the fungal challenge inoculation. These findings suggest that disease-specific vitamin treatment will be integrated for eco-friendly management of tomato bacterial wilt and grey mould.

Pseudomonas syringae Catalases Are Collectively Required for Plant Pathogenesis

The bacterial pathogen Pseudomonas syringae pv. tomato DC3000 must detoxify plant-produced hydrogen peroxide (H(2)O(2)) in order to survive in its host plant. Candidate enzymes for this detoxification include the monofunctional catalases KatB and KatE and the bifunctional catalase-peroxidase KatG of DC3000. This study shows that KatG is the major housekeeping catalase of DC3000 and provides protection against menadione-generated endogenous H(2)O(2). In contrast, KatB rapidly and substantially accumulates in response to exogenous H(2)O(2). Furthermore, KatB and KatG have nonredundant roles in detoxifying exogenous H(2)O(2) and are required for full virulence of DC3000 in Arabidopsis thaliana. Therefore, the nonredundant ability of KatB and KatG to detoxify plant-produced H(2)O(2) is essential for the bacteria to survive in plants. Indeed, a DC3000 catalase triple mutant is severely compromised in its ability to grow in planta, and its growth can be partially rescued by the expression of katB, katE, or katG. Interestingly, our data demonstrate that although KatB and KatG are the major catalases involved in the virulence of DC3000, KatE can also provide some protection in planta. Thus, our results indicate that these catalases are virulence factors for DC3000 and are collectively required for pathogenesis.

A novel manganese efflux system, YebN, is required for virulence by Xanthomonas oryzae pv. oryzae

Manganese ions (Mn²⁺) play a crucial role in virulence and protection against oxidative stress in bacterial pathogens. Such pathogens appear to have evolved complex mechanisms for regulating Mn²⁺ uptake and efflux. Despite numerous studies on Mn²⁺ uptake, however, only one efflux system has been identified to date. Here, we report on a novel Mn²⁺ export system, YebN, in Xanthomonas oryzae pv. oryzae (Xoo), the causative agent of bacterial leaf blight. Compared with wild-type PXO99, the yebN mutant was highly sensitive to Mn²⁺ and accumulated high concentrations of intracellular manganese. In addition, we found that expression of yebN was positively regulated by Mn²⁺ and the Mn²⁺-dependent transcription regulator, MntR. Interestingly, the yebN mutant was more tolerant to methyl viologen and H₂O₂ in low Mn²⁺ medium than PXO99, but more sensitive in high Mn²⁺ medium, implying that YebN plays an important role in Mn²⁺ homoeostasis and detoxification of reactive oxygen species (ROS). Notably, deletion of yebN rendered Xoo sensitive to hypo-osmotic shock, suggesting that YebN may protect against such stress. That mutation of yebN substantially reduced the Xoo growth rate and lesion formation in rice implies that YebN could be involved in Xoo fitness in host. Although YebN has two DUF204 domains, it lacks homology to any known metal transporter. Hence, this is the first report of a novel metal export system that plays essential roles in hypo-osmotic and oxidative stress, and virulence. Our results lay the foundations for elucidating the complex and fascinating relationship between metal homeostasis and host-pathogen interactions.

Mini-Tn7 vectors as genetic tools for gene cloning at a single copy number in an industrially important and phytopathogenic bacteria, Xanthomonas spp

Transposon mini-Tn7 vectors insert into the chromosome of several Gram-negative bacteria in a site-specific manner. Here, we showed the application of mini-Tn7 as single copy site-specific integration vector system for Xanthomonas campestris pv. campestris. The transposition of the mini-Tn7 into the bacterial genome was detected at a Tn7 attachment (attTn7) site located downstream of glmS1. Furthermore, using a newly constructed vector pBBR1FLP2 containing the flipase (FLP) recombinase for site-specific excision of the sequence between the FLP recombinase target (FRT) sites, and a sacB counter selection marker, an unmarked mini-Tn7 insertion mutant was created. Mini-Tn7 insertion did not affect bacterial virulence on the tested plant. The mini-Tn7 and FLP-FRT systems also work well in Xanthomonas oryzae pv. oryzae.

Effect of culture temperature on the heterologous expression of Pleurotus eryngii versatile peroxidase in Aspergillus hosts

Production of recombinant versatile peroxidase in Aspergillus hosts was optimized through the modification of temperature during bioreactor cultivations. To further this purpose, the cDNA encoding a versatile peroxidase of Pleurotus eryngii was expressed under control of the alcohol dehydrogenase (alcA) promoter of Aspergillus nidulans. A dependence of recombinant peroxidase production on cultivation temperature was found. Lowering the culture temperature from 28 to 19 degrees C enhanced the level of active peroxidase 5.8-fold and reduced the effective proteolytic activity twofold. Thus, a maximum peroxidase activity of 466 U L(-1) was reached. The same optimization scheme was applied to a recombinant Aspergillus niger that bore the alcohol dehydrogenase regulator (alcR), enabling transformation with the peroxidase cDNA under the same alcA promoter. However, with this strain, the peroxidase activity was not improved, while the effective proteolytic activity was increased between 3- and 11-fold compared to that obtained with A. nidulans.

HpdR is a transcriptional activator of Sinorhizobium meliloti hpdA, which encodes a herbicide-targeted 4-hydroxyphenylpyruvate dioxygenase

Sinorhizobium meliloti hpdA, which encodes the herbicide target 4-hydroxyphenylpyruvate dioxygenase, is positively regulated by HpdR. Gel mobility shift and DNase I footprinting analyses revealed that HpdR binds to a region that spans two conserved direct-repeat sequences within the hpdR-hpdA intergenic space. HpdR-dependent hpdA transcription occurs in the presence of 4-hydroxyphenylpyruvate, tyrosine, and phenylalanine, as well as during starvation.

Elucidation of the hrp clusters of Xanthomonas oryzae pv. oryzicola that control the hypersensitive response in nonhost tobacco and pathogenicity in susceptible host rice

Xanthomonas oryzae pv. oryzicola, the cause of bacterial leaf streak in rice, possesses clusters of hrp genes that determine its ability to elicit a hypersensitive response (HR) in nonhost tobacco and pathogenicity in host rice. A 27-kb region of the genome of X. oryzae pv. oryzicola (RS105) was identified and sequenced, revealing 10 hrp, 9 hrc (hrp conserved), and 8 hpa (hrp-associated) genes and 7 regulatory plant-inducible promoter boxes. While the region from hpa2 to hpaB and the hrpF operon resembled the corresponding genes of other xanthomonads, the hpaB-hrpF region incorporated an hrpE3 gene that was not present in X. oryzae pv. oryzae. We found that an hrpF mutant had lost the ability to elicit the HR in tobacco and pathogenicity in adult rice plants but still caused water-soaking symptoms in rice seedlings and that Hpa1 is an HR elicitor in nonhost tobacco whose expression is controlled by an hrp regulator, HrpX. Using an Hrp phenotype complementation test, we identified a small hrp cluster containing the hrpG and hrpX regulatory genes, which is separated from the core hrp cluster. In addition, we identified a gene, prhA (plant-regulated hrp), that played a key role in the Hrp phenotype of X. oryzae pv. oryzicola but was neither in the core hrp cluster nor in the hrp regulatory cluster. A prhA mutant failed to reduce the HR in tobacco and pathogenicity in rice but caused water-soaking symptoms in rice. This is the first report that X. oryzae pv. oryzicola possesses three separate DNA regions for HR induction in nonhost tobacco and pathogenicity in host rice, which will provide a fundamental base to understand pathogenicity determinants of X. oryzae pv. oryzicola compared with those of X. oryzae pv. oryzae.

Transformation of Xanthomonas axonopodis pv. citri by electroporation

This study describes the use of electroporation for transforming Xanthomonas axonopodis pv. citri (Xac), the causal agent of citrus (Citrus spp.) canker. It also evaluates the methodology used for this species under different electrical parameters. The bacterium used in the study (Xac 306) was the same strain used for recent complete sequencing of the organism. The use of a plasmid (pUFR047, gentamycin r) is reported here to be able to replicate in cells of Xac. Following the preparation and resuspension of competent cells of Xac at a density of ~4 x 10(10) cfu/ml, in 10% glycerol, and the addition of the replicative plasmid, an electrical pulse was applied to each treatment. Selection of transformants showed a high efficiency of transformation (1.1 x 10(6) transformants/mug DNA), which indicates an effective, and inverse, combination between electrical resistance (50 W) and capacitance (50 µF) for this species, with an electrical field strength of 12.5 kV.cm-1 and 2.7-ms pulse duration. Besides the description of a method for electroporation of Xac 306, this study provides additional information for the use of the technique on studies for production of mutants of this species.

The unique glutathione reductase from Xanthomonas campestris: Gene expression and enzyme characterization

The glutathione reductase gene, gor, was cloned from the plant pathogen Xanthomonas campestris pv. phaseoli. Its gene expression and enzyme characteristics were found to be different from those of previously studied homologues. Northern blot hybridization, promoter-lacZ fusion, and enzyme assay experiments revealed that its expression, unlike in Escherichia coli, is OxyR-independent and constitutive upon oxidative stress conditions. The deduced amino acid sequence shows a unique NADPH binding motif where the most highly conserved arginine residue, which is critical for NADPH binding, is replaced by glutamine. Interestingly, a search of the available Gor amino acid sequences from various sources, including other Xanthomonas species, revealed that this replacement is specific to the genus Xanthomonas. Recombinant Gor enzyme was purified and characterized, and was found to have a novel ability to use both, NADPH and NADH, as electron donor. A gor knockout mutant was constructed and shown to have increased expression of the organic peroxide-inducible regulator gene, ohrR.

Protection of Xanthomonas against arsenic toxicity involves the peroxide-sensing transcription regulator OxyR

Arsenic has been shown to mediate its toxicity through induced generation of reactive oxygen species. Here, we examined the role of oxidative stress-inducible genes (katA, ahpC and ohr) and their regulators (oxyR and ohrR) in the response to arsenic treatment in a plant pathogenic bacterium, Xanthomonas campestris pv. phaseoli (Xp). Overproduction of peroxide-scavenging enzymes (KatA, AhpCF and Ohr) did not enhance arsenic tolerance in wild-type Xp. Furthermore, inactivation of katA, ahpC, ohr, and ohrR genes had no effect on the level of arsenic resistance. By contrast, an oxyR mutant (Xp oxyR) showed increased sensitivity to both pentavalent arsenate and, to a greater extent, trivalent arsenite. The resistance of cells to arsenite treatment was significantly affected by the level of iron. Cells were 10-fold more sensitive to arsenite killing in the presence of excess iron, while removal of iron by an iron chelator (2,2'-dipyridyl) protected Xanthomonas from arsenite toxicity. The arsenite-sensitive phenotype of Xp oxyR could be complemented by the expression of functional OxyR from a plasmid vector, but not by the expression of other known OxyR-regulated peroxide-scavenging enzymes such as KatA and AhpCF, Ohr and OhrR. The data suggested that as yet unidentified, OxyR-regulated gene(s) are involved in conferring arsenic resistance in Xp. To our knowledge, this is the first report showing that the peroxide-sensing regulator OxyR is involved in arsenic resistance.

Genetic and physiological analysis of the major OxyR-regulated katA from Xanthomonas campestris pv. phaseoli

katAencodes the major catalase that accounts for 90 % of the total catalase activity present inXanthomonas campestrispv.phaseoli.katAis located upstream of an ORF designatedankAencoding a cytoplasmic membrane protein homologous to eukaryotic ankyrin. Transcriptional analysis ofkatAandankAidentified twokatAtranscripts: a major monocistronickatAtranscript and a minor bicistronickatA–ankAtranscript. KatA expression was induced in the presence of various oxidants including H2O2, organic hydroperoxides and the superoxide-generating agent menadione, in an OxyR-dependent manner. Analysis of thekatApromoter region showed a putative OxyR binding site located upstream of anEscherichia coli-likeσ70−35 region that is likely to be responsible for transcription activation in response to oxidant treatment. Gel mobility shift experiments confirmed that purified OxyR specifically binds to thekatApromoter. AkatAmutant was highly sensitive to H2O2during both the exponential and stationary phases of growth. This phenotype could be complemented by functionalkatA, confirming the essential role of the gene in protectingX. campestrisfrom H2O2toxicity. Unexpectedly, inactivation ofankAalso significantly reduced resistance to H2O2and the phenotype could be complemented by plasmid-borne expression ofankA. Physiological analyses showed thatkatAplays an important role in, but is not solely responsible for, both the adaptive and menadione-induced cross-protective responses to H2O2killing inX. campestris.

DpsA protects the human pathogen Burkholderia pseudomallei against organic hydroperoxide

The human pathogen, Burkholderia pseudomalle, is able to survive and multiply in hostile environments such as within macrophages. In an attempt to understand its strategy to cope with oxidative stress, the physiological role and gene regulation of a nonspecific DNA-binding protein (DpsA) was investigated. Expression of dpsA increases in response to oxidative stress through increased transcription from the upstream katG (catalase-peroxidase) promoter, which is OxyR dependent. dpsA is also transcribed from its own promoter, which is activated by osmotic stress in an OxyR-independent manner. DpsA-deficient mutants are hypersensitive to tert-butyl hydroperoxide, while overexpression of DpsA leads to increased resistance to organic oxidants. B. pseudomallei DpsA can also protect Escherichia coli against organic hydroperoxide toxicity. The mechanism of DpsA-mediated resistance to organic hydroperoxides was shown to differ from that of alkyl hydroperoxide reductase.

Regulation of the katG-dpsA operon and the importance of KatG in survival of Burkholderia pseudomallei exposed to oxidative stress

Homologues of the catalase-peroxidase gene katG and the gene for the non-specific DNA binding protein dpsA were identified downstream of oxyR in Burkholderia pseudomallei. Northern experiments revealed that both katG and dpsA are co-transcribed during oxidative stress. Under conditions where the katG promoter is not highly induced, dpsA is transcribed from a second promoter located within the katG-dpsA intergenic region. A katG insertion mutant was found to be hypersensitive to various oxidants. Analysis of katG expression in the oxyR mutant indicates that OxyR is a dual function regulator that represses the expression of katG during normal growth and activates katG during exposure to oxidative stress. Both reduced and oxidized OxyR were shown to bind to the katG promoter.

Transaldolase exhibits a protective role against menadione toxicity in Xanthomonas campestris pv. phaseoli

A talA gene encoded transaldolase, a rate-limiting enzyme in the non-oxidative branch of the pentose-phosphate pathway, was cloned from Xanthomonas campestris pv. phaseoli. talA located in a region of the bacterial genome rich in genes involved in oxidative stress protection and regulation. TalA from X. campestris pv. phaseoli showed a high degree of homology to many previously reported transaldolases from both prokaryotic and eukaryotic sources. The expression of X. campestris pv. phaseoli talA was high at log-phase of growth, then declined at stationary phase, and could not be induced by oxidants. A talA mutant constructed by insertional inactivation did not possess any detectable transaldolase activity. Lack of a functional talA gene did not affect bacterial growth in a rich medium containing glucose or sucrose as a carbon source. However, the talA knockout mutant showed increased sensitivity to the superoxide generator menadione, but not to other oxidants. This increased menadione sensitivity phenotype could be complemented by expression of talA in a plasmid vector. The data demonstrated a novel and essential role of transaldolase in protection against menadione toxicity in X. campestris.

Unusual adaptive, cross protection responses and growth phase resistance against peroxide killing in a bacterial shrimp pathogen, Vibrio harveyi

Oxidant induced protection against peroxide killing was investigated in a prawn bacterial pathogen, Vibrio harveyi. Exposure to 250 microM H(2)O(2) induced adaptive protection against subsequent exposure to killing concentrations of H(2)O(2). In addition, 200 microM t-butyl hydroperoxide (tBOOH) induced cross protection to H(2)O(2) killing. On the other hand, peroxide pretreatment did not induce protection against tBOOH killing. Peroxide induced adaptive and cross protection responses required new protein synthesis and were abolished by addition of a protein synthesis inhibitor. Pretreatments of V. harveyi with 250 microM H(2)O(2) and 200 microM tBOOH induced an increase in peroxide scavenging enzymes, catalase and alkyl hydroperoxide reductase subunit C. In addition, stationary phase cells of V. harveyi were more resistant to H(2)O(2) and iodoacetamide killing but highly susceptible to tBOOH killing compared to exponential phase cells. Many aspects of the oxidative stress response of V. harveyi are different from those of other bacteria and these factors may be important for bacterial survival in the environment and during interactions with host shrimp.

Catalase has a novel protective role against electrophile killing of Xanthomonas

The ability of XANTHOMONAS: campestris pv. phaseoli to protect itself against lethal concentrations of man-made (N:-ethylmaleimide, NEM) and endogenously produced (methylglyoxal, MG) electrophiles was investigated. Pretreatment of X. c. pv. phaseoli with a low concentration of NEM induced protection against lethal concentrations of NEM and MG. MG pretreatment weakly induced protection against NEM but not against MG itself. NEM-induced protection against electrophile killing required new protein synthesis and was abolished by the addition of a protein synthesis inhibitor. By contrast, MG-induced protection against NEM killing was independent of de novo protein synthesis. X. c. pv. phaseoli harbouring an expression vector carrying a catalase gene was over 100-fold more resistant to MG and NEM killing. High expression levels of genes for other peroxide-protective enzymes, such as those for alkyl hydroperoxide reductase (ahpC and ahpF) and ohr, failed to protect against electrophile killing. Thus, catalase appears to have a novel protective role(s) against electrophile toxicity. This finding suggests that in X. c. pv. phaseoli NEM and MG toxicity might involve accumulation and/or increased production of H(2)O(2). This idea was supported by the observation that addition of 10 mM sodium pyruvate, a compound that can react chemically with peroxide or hydroxyl radical scavengers (DMSO and glycerol), was found to protect XANTHOMONAS: from electrophile killing. The protective role of catalase and the role of H(2)O(2) in electrophile toxicity are novel observations and could be generally important in other bacteria. In addition, unlike other bacteria, XANTHOMONAS: in stationary phase was more susceptible to electrophile killing compared to cells in exponential phase.

In vitro resistance of Burkholderia cepacia complex isolates to reactive oxygen species in relation to catalase and superoxide dismutase production

The Burkholderia cepacia complex comprises groups of genomovars (genotypically distinct strains with very similar phenotypes) that have emerged as important opportunistic pathogens in cystic fibrosis (CF) patients. The inflammatory response against bacteria in the airways of CF individuals is dominated by polymorphonuclear cells and involves the generation of oxidative stress, which leads to further inflammation and tissue damage. Bacterial catalase, catalase-peroxidase and superoxide dismutase activities may contribute to the survival of B. cepacia following exposure to reactive oxygen metabolites generated by host cells in response to infection. In the present study the authors investigated the production of catalase, peroxidase and SOD by isolates belonging to various genomovars of the B. cepacia complex. Production of both catalase and SOD was maximal during late stationary phase in almost all isolates examined. Native PAGE identified 13 catalase electrophoretotypes and two SOD electrophoretotypes (corresponding to an Fe-SOD class) in strains belonging to the six genomovars of the B. cepacia complex. Seven out of 11 strains displaying high-level survival after H(2)O(2) treatment in vitro had a bifunctional catalase/peroxidase, and included all the genomovar III strains examined. These isolates represent most of the epidemic isolates that are often associated with the cepacia syndrome. The majority of the isolates from all the genomovars were resistant to extracellular O(-)(2), while resistance to intracellularly generated O(-)(2)was highly variable and could not be correlated with the detected levels of SOD activity. Altogether the results suggest that resistance to toxic oxygen metabolites from extracellular sources may be a factor involved in the persistence of B. cepacia in the airways of CF individuals.

Molecular and physiological analysis of an OxyR-regulated ahpC promoter in Xanthomonas campestris pv. phaseoli

In Xanthomonas campestris pv. phaseoli, a gene for the alkyl hydroperoxide reductase subunit C (ahpC) had unique patterns of regulation by various forms of OxyR. Reduced OxyR repressed expression of the gene, whereas oxidized OxyR activated its expression. This dual regulation of ahpC is unique and unlike all other OxyR-regulated genes. The ahpC transcription start site was determined. Analysis of the region upstream of the site revealed promoter sequences that had high homology to the Xanthomonas consensus promoter sequence. Data from gel shift experiments indicated that both reduced and oxidized OxyR could bind to the ahpC regulatory region. Moreover, the reduced and the oxidized forms of OxyR gave different DNase I footprint patterns, indicating that they bound to different sites. The oxidized OxyR binding site overlapped the -35 region of the ahpC promoter by a few bases. This position is consistent with the role of the protein in activating transcription of the gene. Binding of reduced OxyR to the ahpC promoter showed an extended DNase I footprint and DNase I hypersensitive sites, suggesting that binding of the protein caused a shift in the binding site and bending of the target DNA. In addition, binding of reduced OxyR completely blocked the -35 region of the ahpC promoter and prevented binding of RNA polymerase, leading to repression of the gene. Monitoring of the ahpC promoter activity in vivo confirmed the location of the oxidized OxyR binding site required for activation of the promoter. A mutant that separated OxyR regulation from basal ahpC promoter activity was constructed. The mutant was unable to respond to oxidants by increasing ahpC expression. Physiologically, it had a slower aerobic growth rate and was more sensitive to organic peroxide killing. This indicated that oxidant induction of ahpC has important physiological roles in normal growth and during oxidative stress.

Exposure of phytopathogenic Xanthomonas spp. to lethal concentrations of multiple oxidants affects bacterial survival in a complex manner

During plant-microbe interactions and in the environment, Xanthomonas campestris pv. phaseoli is likely to be exposed to high concentrations of multiple oxidants. Here, we show that simultaneous exposures of the bacteria to multiple oxidants affects cell survival in a complex manner. A superoxide generator (menadione) enhanced the lethal effect of an organic peroxide (tert-butyl hydroperoxide) by 1, 000-fold; conversely, treatment of cells with menadione plus H(2)O(2) resulted in 100-fold protection compared to that for cells treated with the individual oxidants. Treatment of X. campestris with a combination of H(2)O(2) and tert-butyl hydroperoxide elicited no additive or protective effect. High levels of catalase alone are sufficient to protect cells against the lethal effect of menadione plus H(2)O(2) and tert-butyl hydroperoxide plus H(2)O(2). These data suggest that H(2)O(2) is the lethal agent responsible for killing the bacteria as a result of these treatments. However, increased expression of individual genes for peroxide (alkyl hydroperoxide reductase, catalase)- and superoxide (superoxide dismutase)-scavenging enzymes or concerted induction of oxidative stress-protective genes by menadione gave no protection against killing by a combination of menadione plus tert-butyl hydroperoxide. However, X. campestris cells in the stationary phase and a spontaneous H(2)O(2)-resistant mutant (X. campestris pv. phaseoli HR) were more resistant to killing by menadione plus tert-butyl hydroperoxide. These findings give new insight into oxidant killing of Xanthomonas spp. that could be generally applied to other bacteria.

Mutations in oxyR resulting in peroxide resistance in Xanthomonas campestris

A spontaneous Xanthomonas campestris pv. phaseoli H(2)O(2)-resistant mutant emerged upon selection with 1 mM H(2)O(2). In this report, we show that growth of this mutant under noninducing conditions gave high levels of catalase, alkyl hydroperoxide reductase (AhpC and AhpF), and OxyR. The H(2)O(2) resistance phenotype was abolished in oxyR-minus derivatives of the mutant, suggesting that elevated levels and mutations in oxyR were responsible for the phenotype. Nucleotide sequence analysis of the oxyR mutant showed three nucleotide changes. These changes resulted in one silent mutation and two amino acid changes, one at a highly conserved location (G197 to D197) and the other at a nonconserved location (L301 to R301) in OxyR. Furthermore, these mutations in oxyR affected expression of genes in the oxyR regulon. Expression of an oxyR-regulated gene, ahpC, was used to monitor the redox state of OxyR. In the parental strain, a high level of wild-type OxyR repressed ahpC expression. By contrast, expression of oxyR5 from the X. campestris pv. phaseoli H(2)O(2)-resistant mutant and its derivative oxyR5G197D with a single-amino-acid change on expression vectors activated ahpC expression in the absence of inducer. The other single-amino-acid mutant derivative of oxyR5L301R had effects on ahpC expression similar to those of the wild-type oxyR. However, when the two single mutations were combined, as in oxyR5, these mutations had an additive effect on activation of ahpC expression.

Expression analysis and characterization of the mutant of a growth-phase- and starvation-regulated monofunctional catalase gene from Xanthomonas campestris pv. phaseoli

Analysis of the Xanthomonas campestris pv. phaseoli (Xp) catalase profile using an activity gel revealed at least two distinct monofunctional catalase isozymes denoted Kat1 and Kat2. Kat1 was expressed throughout growth, whereas Kat2 was expressed only during the stationary phase of growth. The nucleotide sequence of a previously isolated monofunctional catalase gene, Xp katE, was determined. The deduced amino acid sequence of Xp KatE showed a high percentage identity to an atypical group of monofunctional catalases that includes the well-characterized E. coli katE. Expression of Xp katE was growth phase-dependent but was not inducible by oxidants. In addition, growth of Xp in a carbon-starvation medium induced expression of the gene. An Xp katE mutant was constructed, and analysis of its catalase enzyme pattern showed that Xp katE coded for the Kat2 isozyme. Xp katE mutant had resistance levels similar to the parental strain against peroxide and superoxide killing at both exponential and stationary phases of growth. Interestingly, the level of total catalase activity in the mutant was similar to that of the parental strain even in stationary phase. These results suggest the existence of a novel compensatory mechanism for the activity of Xp catalase isozymes.

Construction and physiological analysis of a Xanthomonas oryzae pv. oryzae recA mutant

A Xoo recA insertion inactivation mutant was constructed. The mutant, lacking RecA, showed increased sensitivity towards mutagen killing. This phenotype could be complemented by a cloned, functional recA. Unlike other bacteria, both the recA mutant and the parental strain had similar level of resistance to H2O2 killing and peroxide-induced mutagenesis.

Oxygen supply without gas-liquid film resistance to xanthomonas campestris cultivation

Alternative methods of oxygen supply are of crucial importance, especially in viscous fermentations and shear-sensitive fermentations. A method of oxygen supply that completely eliminates the gas-liquid transport resistance has been presented. The method involves a need-based liquid-phase decomposition of hydrogen peroxide to provide the necessary oxygen. When Xanthomonas campestris was cultivated (viscous cultivation) using this method of oxygen supply, dissolved oxygen (DO) levels were maintained above the setpoint of 50% throughout the cultivation, whereas the conventional cultivation was able to meet culture oxygen demand only for about 6 h in a 72-h fermentation. Furthermore, the maximum specific growth rate and xanthan yields in the novel cultivation were 89% and 169%, respectively, of those obtained in conventional cultivation. A mathematical model was also developed to simulate and predict results in fermentations employing the presented methodology. In addition, studies with HOCl pretreatments indicated that monofunctional catalase may be responsible for the decomposition of H2O2 supplied externally to cells; HOCl pretreatments also increased the tolerance of cells to H2O2. The decomposition kinetics of externally supplied H2O2 was Michaelis-Menten in nature with vmax = 1.196 x 10(-6) M s-1 and Km = 0.21 mM. The catalase concentration was estimated to be 3.4 x 10(-10) mol/g of cells. Copyright 1998 John Wiley & Sons, Inc.

Construction and physiological analysis of a Xanthomonas mutant to examine the role of the oxyR gene in oxidant-induced protection against peroxide killing

We constructed and characterized a Xanthomonas campestris pv. phaseoli oxyR mutant. The mutant was hypersensitive to H2O2 and menadione killing and had reduced aerobic plating efficiency. The oxidants' induction of the catalase and ahpC genes was also abolished in the mutant. Analysis of the adaptive responses showed that hydrogen peroxide-induced protection against hydrogen peroxide was lost, while menadione-induced protection against hydrogen peroxide was retained in the oxyR mutant. These results show that X. campestris pv. phaseoli oxyR is essential to peroxide adaptation and revealed the existence of a novel superoxide-inducible peroxide protection system that is independent of OxyR.

Identification and characterization of a new organic hydroperoxide resistance (ohr) gene with a novel pattern of oxidative stress regulation from Xanthomonas campestris pv. phaseoli

We have isolated a new organic hydroperoxide resistance (ohr) gene from Xanthomonas campestris pv. phaseoli. This was done by complementation of an Escherichia coli alkyl hydroperoxide reductase mutant with an organic hydroperoxide-hypersensitive phenotype. ohr encodes a 14.5-kDa protein. Its amino acid sequence shows high homology with several proteins of unknown function. An ohr mutant was subsequently constructed, and it showed increased sensitivity to both growth-inhibitory and killing concentrations of organic hydroperoxides but not to either H2O2 or superoxide generators. No alterations in sensitivity to other oxidants or stresses were observed in the mutant. ohr had interesting expression patterns in response to low concentrations of oxidants. It was highly induced by organic hydroperoxides, weakly induced by H2O2, and not induced at all by a superoxide generator. The novel regulation pattern of ohr suggests the existence of a second organic hydroperoxide-inducible system that differs from the global peroxide regulator system, OxyR. Expression of ohr in various bacteria tested conferred increased resistance to tert-butyl hydroperoxide killing, but this was not so for wild-type Xanthomonas strains. The organic hydroperoxide hypersensitivity of ohr mutants could be fully complemented by expression of ohr or a combination of ahpC and ahpF and could be partially complemented by expression ahpC alone. The data suggested that Ohr was a new type of organic hydroperoxide detoxification protein.

Isolation and analysis of the Xanthomonas alkyl hydroperoxide reductase gene and the peroxide sensor regulator genes ahpC and ahpF-oxyR-orfX

From Xanthomonas campestris pv. phaseoli, we have isolated by two independent methods genes involved in peroxide detoxification (ahpC and ahpF), a gene involved in peroxide sensing and transcription regulation (oxyR), and a gene of unknown function (orfX). Amino acid sequence analysis of AhpC, AhpF, and OxyR showed high identity with bacterial homologs. OrfX was a small cysteine-rich protein with no significant homology to known proteins. The genes ahpC, ahpF, oxyR, and orfX were arranged in a head-to-tail fashion. This unique arrangement was conserved in all of the Xanthomonas strains tested. The functionalities of both the ahpC and oxyR genes were demonstrated. In X. campestris pv. phaseoli, increased expression of ahpC alone conferred partial protection against growth retardation and killing by organic hydroperoxides but not by H2O2 or superoxide generators. These genes are likely to have important physiological roles in protection against peroxide toxicity in Xanthomonas.

Characterization of transcription organization and analysis of unique expression patterns of an alkyl hydroperoxide reductase C gene (ahpC) and the peroxide regulator operon ahpF-oxyR-orfX from Xanthomonas campestris pv. phaseoli

We have analyzed the transcription organization of ahpC, ahpF, oxyR, and orfX from Xanthomonas campestris pv. phaseoli. ahpC was transcribed as a monocistronic 0.6-kb mRNA, while ahpF-oxyR-orfX were transcribed as a polycistronic approximately 3.0-kb-long mRNA. The novel transcription organization of these genes has not observed in other bacteria. Western analysis showed that oxidants (peroxides and superoxide anions), a thiol reagent (N-ethylmaleimide), and CdCl2 caused large increases in the steady-state level of AhpC. Growth at alkaline pH also moderately induced AhpC accumulation. Thermal and osmotic stresses did not alter the levels of AhpC. Northern blotting results confirmed that oxidant- and CdCl2-induced AhpC accumulation was due to increased levels of ahpC transcripts. Analysis of oxyR expression revealed a unique pattern. Unlike other bacterial systems, peroxides and a superoxide generator induced accumulation of OxyR. Northern blotting results confirmed that these oxidants induced expression of oxyR operon. This novel regulatory pattern could be generally important. The transcription organization and patterns of chemicals and stress induction of ahpC and oxyR differed from those of other bacteria and are likely to be important for X. campestris pv. phaseoli survival during exposure to oxidants.