A highly sensitive biosensor with a single-copy evolved sensing cassette for chlorpyrifos pesticide detection

A formylglycine-generating enzyme (FGE)-sulfatase-based whole-cell biosensor was genetically improved into a single-copy system by integrating the Sinorhizobium meliloti transcriptional activator ChpR and the chpA promoter-FGE-sulfatase fusion into the Escherichia coli chromosome. The sensitivity was further enhanced through a random mutagenesis of the chpR. The new integrated biosensor offered both a lower detection limit [5 nM chlorpyrifos (CPF)] and fluorescence background. The ready-to-use kit was developed using silica gel for on-field detection. The biosensor kit was stable for 20 days when stored at 4 °C. Moreover, a 1-(1-naphthylmethyl)-piperazine (NMP) efflux pump inhibitor can improve the sensitivity by 57 %.

Potential use of two aryl sulfotransferase cell-surface display systems to detoxify the endocrine disruptor bisphenol A

Bisphenol A (BPA) is one of the most common toxic endocrine disruptors in the environment. A fast, efficient and environmental-friendly method for BPA detoxification is urgently needed. In this study, we show that the enzymatic transformation of BPA into a non-estrogenic BPA sulfate can be performed by the aryl sulfotransferase (ASTB) from Desulfitobacterium hafniense. We developed and compared two Escherichia coli ASTB cell-surface displaying systems using the outer membrane porin F (OprF) and the lipoprotein outer membrane A (Lpp-OmpA) as carriers. The surface localization of both fusion proteins was confirmed by Western blot and flow cytometry analysis as well as the enzymatic activity assay of the outer membrane fractions. Unfortunately, Lpp-OmpA-ASTB cells had an adverse effect on cell growth. In contrast, the OprF-ASTB cell biocatalyst was stable, expressing 70% of enzyme activity for 7 days. It also efficiently sulfated 90% of 5 mM BPA (1 mg/mL) in wastewater within 6 h.

Cefoperazone induces esterase B expression by EstR and esterase B enhances cefoperazone activity at the periplasm

The occurrence of antibiotic resistance bacteria has become a major threat to public health. We have recently discovered a transcriptional activator that belongs to MarR family, EstR, and an esterase B (EstB) with a newly proposed de-arenethiolase activity from Sphingobium sp. SM42. De-arenethiolase activity involves the removal of the small aromatic side chain of cephalosporin antibiotics as an excellent leaving group by the enzymatic CS bond cleavage. Here, we report the regulation of estB through EstR as an activator in response to a third generation cephalosporin, cefoperazone, antibiotic. Cefoperazone induced the expression of estB in wild type Sphingobium sp., but not in the estR knockout strain, and the induction was restored in the complemented strain. Moreover, we revealed the importance of EstB localization in periplasm. Since EsB has the ability to inactivate selected β-lactam antibiotics in vitro, it is possible that the enzyme works at the periplasmic space of Gram negative bacteria similar to β-lactamases. EstB was genetically engineered by incorporating NlpA binding motif, or OmpA signal sequence, or SpyTag-SpyCatcher to the estB gene to mobilize it to different compartments of periplasm; inner membrane, outer membrane, and periplasmic space, respectively. Surprisingly, we found that Sphingobium sp. SM42 and E. coli expressing EstB at the periplasm were more sensitive to cefoperazone. The possible drug enhancement mechanism by enzyme was proposed. This work might lead to a novel strategy to tackle antibiotic resistance problem.

Specific detection of the pesticide chlorpyrifos by a sensitive genetic-based whole cell biosensor

The Sinorhizobium meliloti chpA promoter is highly induced in the presence of the pesticide chlorpyrifos (CPF) through the action of the transcriptional activator, ChpR. A whole-cell biosensor for the detection of CPF was developed and is composed of an Escherichia coli strain carrying a chpR expression vector and a chpA promoter-atsBA transcriptional fusion plasmid encoding sulfatase (atsA) and formylglycine generating enzyme (atsB) from Klebsiella sp. The sulfatase is posttranslationally activated by formylglycine generating enzyme (FGE) and then converts 4-methylumbelliferyl sulfate (4-MUS) to the fluorescent product, 4-methyllumbelliferone (4-MU). This biosensor system exhibited a linear response range from 25 to 500 nM CPF.

ChpR is a chlorpyrifos-responsive transcription regulator in Sinorhizobium meliloti

The broad-spectrum organophosphate insecticide chlorpyrifos (CPF)-inducible locus, chpAB, was identified on the endogenous plasmid pSymB in Sinorhizobium meliloti. The S. meliloti chpA promoter was highly induced by CPF and was induced at much lower levels by diazinon and ethion. Transcription of chpA was dependent on chpR, a CadC family transcriptional regulator located upstream of, and divergently transcribed from, chpAB. ChpR was able to mediate the CPF-inducible expression of the S. melilotichpA promoter in Escherichia coli through direct interaction with the chpAB promoter. The chpR-chpA intergenic regions of several bacterial chpRAB operons were aligned and a putative ChpR-binding sequence was proposed. Both the ChpR transcription factor and chpA promoter constitute a good candidate system for genetic-based biosensor development.

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.

Analysis of mutations that alter HO sensing and transcription regulation properties of a global peroxide regulator OxyR in Xanthomonas campestris pv. phaseoli

OxyR5, from a Xanthomonas campestris pv. phaseoli H(2)O(2)-resistant mutant, contains the two mutations G197D and L301R. The protein exists in its oxidized-like form in the absence of oxidants as judged by the protein's ability to activate the ahpC promoter. Analysis of DNase I footprint patterns indicates that under reducing conditions OxyR5 and OxyRG197D bind to the target site in the ahpC promoter in a manner similar to oxidized wild-type OxyR. Site-directed mutagenesis showed that OxyR5 behaves like oxidized OxyR, independent of the highly conserved C residues at positions 199 and 208 where, in normal OxyR, a disulfide bond between these residues converts the protein from its reduced to the oxidized form. The presence of aspartic acid or valine residue at position 197 caused OxyR to behave like the oxidized form in uninduced cells. Changing D197 to A or T in OxyR5 resulted in proteins with similar properties to native OxyR. In vivo, OxyR5 probably locked in an oxidized-like conformation, resulting in continuous high-level activation of target genes in the OxyR regulon.

Important role for methionine sulfoxide reductase in the oxidative stress response of Xanthomonas campestris pv. phaseoli

A methionine sulfoxide reductase gene (msrA) from Xanthomonas campestris pv. phaseoli has unique expression patterns and physiological function. msrA expression is growth dependent and is highly induced by exposure to oxidants and N-ethylmaleimide in an OxyR- and OhrR-independent manner. An msrA mutant showed increased sensitivity to oxidants but only during stationary phase.

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.

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.

Compensatory increase in ahpC gene expression and its role in protecting Burkholderia pseudomallei against reactive nitrogen intermediates

In the human pathogen Burkholderia pseudomallei, katG encodes the antioxidant defense enzyme catalase-peroxidase. Interestingly, a B. pseudomallei mutant, disrupted in katG, is hyperresistant to organic hydroperoxide. This hyperresistance is due to the compensatory expression of the alkyl hydroperoxide reductase gene ( ahpC) and depends on a global regulator OxyR. The KatG-deficient mutant is also highly resistant to reactive nitrogen intermediates (RNI). When overproduced, the B. pseudomallei AhpC protein, protected cells against killing by RNI. The levels of resistance to both organic peroxide and RNI returned to those of the wild-type when the katG mutant was complemented with katG. These studies establish the partially overlapping defensive activities of KatG and AhpC.

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.

A suppressor of the menadione-hypersensitive phenotype of a Xanthomonas campestris pv. phaseoli oxyR mutant reveals a novel mechanism of toxicity and the protective role of alkyl hydroperoxide reductase

We isolated menadione-resistant mutants of Xanthomonas campestris pv. phaseoli oxyR (oxyR(Xp)). The oxyRR2(Xp) mutant was hyperresistant to the superoxide generators menadione and plumbagin and was moderately resistant to H(2)O(2) and tert-butyl hydroperoxide. Analysis of enzymes involved in oxidative-stress protection in the oxyRR2(Xp) mutant revealed a >10-fold increase in AhpC and AhpF levels, while the levels of superoxide dismutase (SOD), catalase, and the organic hydroperoxide resistance protein (Ohr) were not significantly altered. Inactivation of ahpC in the oxyRR2(Xp) mutant resulted in increased sensitivity to menadione killing. Moreover, high levels of expression of cloned ahpC and ahpF in the oxyR(Xp) mutant complemented the menadione hypersensitivity phenotype. High levels of other oxidant-scavenging enzymes such as catalase and SOD did not protect the cells from menadione toxicity. These data strongly suggest that the toxicity of superoxide generators could be mediated via organic peroxide production and that alkyl hydroperoxide reductase has an important novel function in the protection against the toxicity of these compounds in X. campestris.

Evaluation of the roles that alkyl hydroperoxide reductase and Ohr play in organic peroxide-induced gene expression and protection against organic peroxides in Xanthomonas campestris

Alkyl hydroperoxide reductase (ahpC) and organic hydroperoxide resistance (ohr) are distinct genes, structurally and regulatory, but have similar physiological functions. In Xanthomonas campestris pv. phaseoli inactivation of either gene results in increased sensitivity to killing with organic peroxides. An ahpC1-ohr double mutant was highly sensitive to both growth inhibition and killing treatment with organic peroxides. High level expression of ahpC or ohr only partially complemented the phenotype of the double mutant, suggesting that these genes function synergistically, but through different pathways, to protect Xanthomonas from organic peroxide toxicity. Functional analyses of Ohr and AhpC abilities to degrade organic hydroperoxides revealed that both Ohr and AhpC could degrade tert-butyl hydroperoxide (tBOOH) while the former was more efficient at degrading cumene hydroperoxide (CuOOH). Expression analysis of these genes in the mutants showed no compensatory alterations in the levels of AhpC or Ohr. However, CuOOH induced expression of these genes in the mutants was affected. CuOOH induced ahpC expression was higher in the ohr mutant than in the parental strain; in contrast, the ahpC mutation has no effect on the level of induced ohr expression. These analyses reveal complex physiological roles and expression patterns of seemingly functionally similar genes.

OhrR, a transcription repressor that senses and responds to changes in organic peroxide levels in Xanthomonas campestris pv. phaseoli

We report the physiological role of OhrR as an organic peroxide sensor and transcription repressor in Xanthomonas campestris pv. phaseoli. In vivo exposure of X. campestris pv. phaseoli to either tert-butyl or cumene hydroperoxides efficiently neutralized OhrR repression of expression from the OhrR-regulated P1 promoter. H2O2 was a weak and non-physiological inducer of the system while other oxidants and metabolites of organic peroxide metabolism did not induce the expression from the P1. Northern blotting results indicated a correlation between concentrations of tert-butyl hydroperoxide used in the treatment and the induction of ohr (an OhrR-regulated gene) expression. In addition, the levels of ohr mRNA in cultures induced by various concentrations of tert-butyl hydroperoxide were reduced in cells with high levels of an organic peroxide metabolising enzyme (AhpC-AhpF) but not in cells with high catalase levels suggesting that organic peroxide interacts with OhrR. DNA band shift experiments using purified OhrR and the P1 promoter fragment showed that organic peroxide treatment prevented binding of the protein to the P1 promoter by oxidation of OhrR, as the inhibition of binding to the P1 promoter was reversed by addition of a reducing agent, DTT. The highly conserved cysteine residue C22 of OhrR is required for organic peroxide inducible gene expression. A mutant protein, OhrRC22S can repress the P1 promoter activity but is insensitive to organic peroxide treatment. Thus, OhrR is the first transcription repressor characterized that appeared to evolve to physiologically sense organic peroxides.

The Burkholderia pseudomallei oxyR gene: expression analysis and mutant characterization

Burkholderia pseudomallei (Bp) is the causative agent of the life-threatening melioidosis in humans. The global transcription factor oxyR gene was isolated and characterized. It is located between recG, encoding a putative DNA helicase, and katG, encoding a putative catalase-peroxidase. oxyR is expressed as a monocistronic 1 kb mRNA and is induced by oxidative stress compounds. Northern, primer extension, and transcription reporter fusion analyses showed that oxyR mRNA is induced by 0.2 mM menadione, 2 mM paraquat, and 10 mM H(2)O(2). Two knockout mutants of oxyR were constructed, by single- and double-crossover recombination, and found to be hypersensitive to H(2)O(2) and paraquat. Bp lacking OxyR exhibited autoaggregation when cultured in liquid broth and an increased ability to form biofilms in minimal medium, but not in Luria-Bertani broth. The oxyR mutants also have a decreased level of extracellular protease activity. The altered phenotypes of oxyR deficient mutants were complemented when a copy of oxyR was transposed into the mutant chromosomes on the mini-Tn5 transposon.

The repressor for an organic peroxide-inducible operon is uniquely regulated at multiple levels

ohrR encodes a novel organic peroxide-inducible transcription repressor, and we have demonstrated that ohrR is regulated at the transcriptional and the post-transcriptional levels. Primer extension results show that ohrR transcription initiates at the A residue of the ATG translation initiation codon for the ohrR coding sequence. Thus, the gene has a leaderless mRNA. The ohrR promoter (P1) has high homology to the consensus sequence for Xanthomonas promoters, which is reflected in the high in vivo promoter activity of P1. Deletion of a 139 bp fragment containing the P1 promoter showed that the sequences upstream of -35 regions were required for neither the promoter activity nor OhrR autoregulation. In vitro, purified OhrR specifically binds to the P1 promoter. DNase I footprinting of OhrR binding to the P1 revealed a 44 bp region of protection on both DNA strands. The protected regions include the -35 and -10 regions of P1. We suggest that OhrR represses gene expression by blocking RNA polymerase binding to the promoter. There are two steps in the post-transcriptional regulation of ohrR, namely differential stability and inefficient translation of the mRNA. The bicistronic ohrR-ohr mRNA was highly labile and underwent rapid processing in vivo to give only stable monocistronic ohr mRNA and undetectable ohrR mRNA. Furthermore, the ohrR mRNA was inefficiently translated. We propose that, in uninduced cells, the concentration of OhrR is maintained at low levels by the autoregulation mechanism at the transcriptional levels and by the ohrR mRNA instability coupled with inefficient translation at the post-transcriptional level. Upon exposure to an organic peroxide, the compound probably interacts with OhrR and prevents it from repressing the P1 promoter, thus allowing high-level expression of the ohrR-ohr operon. The rapid processing of bicistronic mRNA gives highly stable ohr mRNA and corresponding high levels of Ohr, which remove an organic per-oxide. Once the peroxide has been removed, the autoregulation mechanism feeds back to inhibit the expression of the operon.

Bacterial Ohr and OsmC paralogues define two protein families with distinct functions and patterns of expression

Xanthomonas campestris Ohr (a protein involved in organic peroxide protection) and Escherichia coli OsmC (an osmotically inducible protein of unknown function) are related proteins. Database searches and phylogenetic analyses reveal that Ohr and OsmC homologues cluster into two related subfamilies of proteins widely distributed in both Gram-negative and Gram-positive bacteria. To determine if these two subfamilies are functionally distinct, ohr and osmC in Pseudomonas aeruginosa (a bacterium with one representative from each subfamily) were analysed. Only ohr mutants are hypersensitive to organic peroxide, and this phenotype can be restored by complementation with ohr but not osmC. In addition, expression of ohr was highly induced only by organic peroxides, and not by other oxidants or stresses. In contrast, osmC was induced by ethanol and osmotic stress. A similar pattern of regulation was observed for Ohr and OsmC homologues in the Gram-positive bacterium Deinococcus radiodurans, though uninduced expression was much higher and induction lower in this species. These data clearly support the conclusion that Ohr and OsmC define two functionally distinct subfamilies with distinct patterns of regulation.

A Xanthomonas alkyl hydroperoxide reductase subunit C (ahpC) mutant showed an altered peroxide stress response and complex regulation of the compensatory response of peroxide detoxification enzymes

Alkyl hydroperoxide reductase subunit C (AhpC) is the catalytic subunit responsible for alkyl peroxide metabolism. A Xanthomonas ahpC mutant was constructed. The mutant had increased sensitivity to organic peroxide killing, but was unexpectedly hyperresistant to H(2)O(2) killing. Analysis of peroxide detoxification enzymes in this mutant revealed differential alteration in catalase activities in that its bifunctional catalase-peroxidase enzyme and major monofunctional catalase (Kat1) increased severalfold, while levels of its third growth-phase-regulated catalase (KatE) did not change. The increase in catalase activities was a compensatory response to lack of AhpC, and the phenotype was complemented by expression of a functional ahpC gene. Regulation of the catalase compensatory response was complex. The Kat1 compensatory response increase in activity was mediated by OxyR, since it was abolished in an oxyR mutant. In contrast, the compensatory response increase in activity for the bifunctional catalase-peroxidase enzyme was mediated by an unknown regulator, independent of OxyR. Moreover, the mutation in ahpC appeared to convert OxyR from a reduced form to an oxidized form that activated genes in the OxyR regulon in uninduced cells. This complex regulation of the peroxide stress response in Xanthomonas differed from that in other bacteria.

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.

Characterization and mutagenesis of fur gene from Burkholderia pseudomallei

A homolog of the ferric uptake regulator gene (fur) was isolated from Burkholderia pseudomallei (Bp) by a reverse genetic technique. Sequencing of a 2.2kb DNA fragment revealed an open reading frame with extensive homology to bacterial Fur proteins. The cloned gene encodes a 16kDa protein that cross-reacts with a polyclonal anti-Escherichia coli Fur serum. The transcription start site was determined by the primer extension technique. Expression analysis of fur showed no increased fur mRNA levels in response to various stresses and iron conditions. A positive selection procedure involving the isolation of manganese-resistant mutants was used to isolate mutants that produce altered Fur proteins. Sequencing of a fur mutant revealed a nucleotide change (G to A) converting a conserved amino acid arginine-69 to histidine. The fur missense mutant produced an elevated level of siderophore that could be complemented by a multicopy plasmid carrying the Bp fur. Interestingly, Fur was found to play roles as a positive regulator of FeSOD and peroxidase. The mutant showed a decreased activity of FeSOD and peroxidase, which could be important in its pathogenicity and survival in macrophages.

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.

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.