A Novel A3 Group Aconitase Tolerates Oxidation and Nitric Oxide*

Background: Aconitases are labile cellular targets of oxidative and nitrosative stresses. Results: Aconitase A3 is resistant to and mitigates impaired growth and NADH and ATP generation by reactive nitrogen stress. Conclusion: Aconitase A3 constitutes a novel group of bacterial aconitases for oxidation and reactive nitrogen tolerance. Significance: The ecology and pathology of bacterial NO response are impacted. Achromobacter denitrificans YD35 is an NO2−-tolerant bacterium that expresses the aconitase genes acnA3, acnA4, and acnB, of which acnA3 is essential for growth tolerance against 100 mm NO2−. Atmospheric oxygen inactivated AcnA3 at a rate of 1.6 × 10−3 min−1, which was 2.7- and 37-fold lower compared with AcnA4 and AcnB, respectively. Stoichiometric titration showed that the [4Fe-4S]2+ cluster of AcnA3 was more stable against oxidative inactivation by ferricyanide than that of AcnA4. Aconitase activity of AcnA3 persisted against high NO2− levels that generate reactive nitrogen species with an inactivation rate constant of k = 7.8 × 10−3 min−1, which was 1.6- and 7.8-fold lower than those for AcnA4 and AcnB, respectively. When exposed to NO2−, the acnA3 mutant (AcnA3Tn) accumulated higher levels of cellular citrate compared with the other aconitase mutants, indicating that AcnA3 is a major producer of cellular aconitase activity. The extreme resistance of AcnA3 against oxidation and reactive nitrogen species apparently contributes to bacterial NO2− tolerance. AcnA3Tn accumulated less cellular NADH and ATP compared with YD35 under our culture conditions. The accumulation of more NO by AcnA3Tn suggested that NADH-dependent enzymes detoxify NO for survival in a high NO2− milieu. This novel aconitase is distributed in Alcaligenaceae bacteria, including pathogens and denitrifiers, and it appears to contribute to a novel NO2− tolerance mechanism in this strain.

Aconitase (aconitate hydratase, EC 4.2.1.3) is an enzyme that contains an Fe-S cluster and converts citrate to isocitrate via cis-aconitate. The reaction is ubiquitous from bacteria to higher eukaryotes and constitutes an indispensable step in TCA cycle metabolism (1). Like other Fe-S-containing enzymes, aconitases are easily inactivated under oxidative conditions through disruption of their Fe-S clusters (2)(3)(4). Such inactivation impairs TCA cycle flux and energy conservation and hence cell proliferation. Bacterial aconitases are classified as aconitases A (AcnA) and B (AcnB) (5). Escherichia coli AcnA is induced by oxidative stress, and its activity resists oxidants in vitro compared with the other housekeeping isozyme, AcnB (5,6). Salmonella enterica also produces these aconitase isozymes (7). E. coli aconitase C is identical to methylcitrate dehydratase (PrpD), which is involved in propionate catabolism and is also found in other propionate-utilizing bacteria (8 -11). Other aconitase-related proteins have been identified, but their physiological functions and persistence against oxidative stress remain unknown.
Aerobic organisms can be exposed to stress caused by reactive oxygen species generated as byproducts of electron transport chains and by exogenously added stressors. Bacterial aconitases, including the relatively oxidation-tolerant AcnA isozymes, are among the most labile cellular targets of reactive oxygen species (3,4,6). Reactive nitrogen species (RNS) 2 are generated both in the environment and in endogenous bacterial cells, and they also damage aconitases and other enzymes containing Fe-S (12,13). Nitric oxide (NO) is an RNS that induces oxidative stress in cells and causes various types of cytotoxicity, such as DNA denaturation, lipid oxidation, and enzyme inactivation (14). The innate immune system of mammals uses NO to kill infecting bacteria (15,16). To accomplish infection, bacteria produce various tolerance mechanisms against attack by NO; flavohemoglobin, flavorubredoxin, and peroxiredoxin are NOdetoxifying enzymes that constitute bacterial NO tolerance mechanisms (17)(18)(19). The global nitrogen cycle generates RNS at high levels when the processes of denitrification and nitrification to nitrate are incomplete (20,21). Thus, understanding the relationship between bacterial physiology and RNS is important.
Microorganisms can use nitrite (NO 2 Ϫ ) both as a nitrogen source and as an electron acceptor for anaerobic respiration (20,21). When protonated under acidic conditions, NO 2 Ϫ generates RNS (22,23). To determine bacterial responses to RNS, we isolated Achromobacter denitrificans YD35 from a water treatment tank as a strain that thrives in the presence of NO 2 Ϫ concentrations as high 100 mM, and we investigated its RNS tolerance mechanisms (24,25). Genetic screening of NO 2 Ϫ -hypersensitive mutants derived from YD35 with transposon insertions identified predicted genes involved in bacterial RNS tolerance. An example is the pyruvate dehydrogenase gene, * This work was supported in part by a grant-in-aid for scientific research from the Ministry of Education, Science, Culture and Sports of Japan. □ S This article contains supplemental Tables 1-3  which provides acetyl-CoA, increases turnover of the TCA cycle, and hence generates NADH, which could be a substrate for enzymes that detoxify NO (25). This constitutes a possible RNS tolerance mechanism of YD35 and implies that the TCA cycle is important for bacterial adaptation to RNS.
Here, we investigated a strain that is hypersensitive to NO 2 Ϫ and harbors a mutation at the predicted aconitase gene (acnA3). We found that AcnA3 is important for TCA cycle metabolism by isomerizing citrate in the presence of high NO 2 Ϫ concentrations. The aconitase activity of AcnA3 was highly resistant to oxidation and NO in vitro compared with other aconitase isozymes from this bacterium. A. denitrificans AcnA3 and its counterparts from the closely related Bordetella constitute a clade with the distinct AcnA of the Alcaligenaceae family of bacteria, and they restored the NO 2 Ϫ -sensitive growth of the acnA3 mutant. These findings show that AcnA3 constitutes a novel group of aconitases that tolerate oxidation and extreme high RNS, and it enables oxidation-tolerant TCA cycle metabolism in bacteria. Table 1 shows the bacterial strains and plasmids used in this study, and they were cultured and propagated in DM (0.5% Polypeptone, 0.3% nutrient broth, 0.4% NaCl, 0.1% K 2 HPO 4 , and 0.2% trace element solution (25), pH 7.0), LB (1% Tryptone, 0.5% yeast extract, and 0.5% NaCl), YENB (0.75% yeast extract and 0.8% nutrient broth), and Super Optimal broth with catabolite repression (2% Tryptone, 0.5% yeast extract, 10 mM NaCl, 2.5 mM KCl, 10 mM MgCl 2 ⅐6H 2 O, 10 mM MgSO 4 ⅐7H 2 O, and 20 mM glucose, pH 7.0) media. Bacterial strains were cultured at 120 rpm and 30°C under aerobic conditions. Gene Disruption of A. denitrificans Genes-The tetracycline resistance gene (tet) from pBSL199 was amplified by PCR using KOD-Plus DNA polymerase (Toyobo Co., Osaka, Japan) and primers (supplemental Table 3), digested with EcoRI, and cloned into the same restriction site of pAcnA4 and pAcnB (see below for construction) to generate pAcnB::tet and pAcnA4::tet, which were digested with XbaI and HindIII. The acnA4::tet gene fragment was cloned into pHSG396 (Takara Bio Inc., Shiga, Japan) to generate pAcnA4 KO . After digesting pAcnB::tet with XbaI and SacI, the acnB::tet gene fragment was cloned into pHSG396 to generate pAcnB KO . These plasmids transformed A. denitrificans YD35 by electroporation (25), and transformants were cultured on YENB plates containing 20 mg/liter tetracycline sulfate. Gene disruptions were confirmed by PCR, and we designated the strains AcnA4⌬ and AcnB⌬ (Fig. 1).
Reverse Transcription and Real-time PCR-Total RNA was purified using RNeasy Protect bacteria mini kits (Qiagen), and cDNA was synthesized using QuantiTect reverse transcription kits (Qiagen). Specific amplicons for the 16 S rRNA, acnA1, acnA2, acnA3, acnA4, and acnB genes were amplified by PCR using cDNA, KOD-Plus DNA polymerase, and primers (supplemental Table 3). Real-time PCR was carried out using iQ SYBR Green Supermix and MiniOpticon version 3.1 (Bio-Rad) with the primers (supplemental Table 3) recommended by the manufacturer. The amounts of generated transcripts are shown relative to those of the 16 S rRNA gene.
Production of Aconitases in the AcnA3Tn Strain-We prepared plasmids to introduce aconitase genes into AcnA3Tn by amplifying aconitase gene fragments using PCR and the primer sets used to prepare recombinant aconitases. The PCR products were digested with NheI and HindIII and cloned into NheI/HindIII-digested pNSGroE2 to generate pNS-xxx (where xxx denotes the protein name of the aconitase produced). Transformants harboring the plasmid were selected and maintained in DM containing 50 -500 mg/liter chloramphenicol. Preparation of AcnA3 Mutant Proteins-The 5Ј-regions of the acnA3 genes were amplified using primer AcnA3_PF and primer C437S_R, C503S_R, or C506S_R. The 3Ј-regions of the acnA3 genes were amplified using primer AcnA3_PR and primer C437S_F, C503S_F, or C506S_F. The respective amplified DNA fragments were fused by PCR using primers AcnA3_PF and AcnA3_PR, digested with PstI and SalI, and cloned into PstI/SalI-digested pAcnA3. Mutant proteins were then expressed in E. coli using the resulting pAcnA3 C437S , pAcnA3 C503S , and pAcnA3 C506S . Supplemental Table 3 shows the primers used in these processes.
NO Determination-Strains were cultured in DM with 1 mM diaminorhodamine-4M acetoxymethyl ester (Sekisui Medical Co., Ltd., Tokyo, Japan). After culture for 12 h, 100 mM NO 2 Ϫ was added, and cultures were continued for 12 h. The cells were collected by centrifugation, washed twice with 20 mM sodium phosphate (pH 7.0) containing 0.8% NaCl, and resuspended at A 595 ϭ 0.4 in the same buffer. NO was determined as fluorescence using a plate reader at an excitation of 560 nm and an emission of 580 nm.
Other Methods-The molecular weights of aconitases were determined by gel filtration chromatography using Superose 6 10/300 GL columns (GE Healthcare). Intracellular NADH/ NAD ϩ ratios and ATP concentrations were quantified as described (25). Cellular citrate was extracted from cells using cold methanol as described (27) and analyzed using a Roche Yellow line kit (R-Biopharm AG, Darmstadt, Germany) or a GC/MS-QP2010 Plus gas chromatograph-mass spectrometer (Shimadzu Co., Kyoto, Japan). Proteins were resolved by SDS-PAGE according to Laemmli (28).

Identification of a Novel acnA3 Gene That Tolerates NO-
We isolated mutants with NO 2 Ϫ -hypersensitive growth by transposon mutagenesis of A. denitrificans YD35 and identified four mutants with transposon insertions at acnA3 loci ( Fig. 2A) (25). One of the mutants (AcnA3Tn) did not grow in the presence of Ͼ60 mM NO 2 Ϫ and Ͼ3 mM acidified NO 2 Ϫ (pH 5.5) (Fig.  2B). At low pH, NO 2 Ϫ is protonated and generates RNS, including NO (22,23), indicating that the growth of AcnA3Tn was sensitive to RNS generated from the acidified NO 2 Ϫ . In addition to the single acnB gene, the YD35 strain genome encoded four potential acnA genes, which we termed acnA1, acnA2, acnA3, and acnA4 according to the designations of the corresponding Bordetella avium 197N genes (accession number AM167904). RT-PCR showed that only acnA3, acnA4, and acnB were expressed in the presence or absence of added NO 2 Ϫ under our culture conditions (Fig. 2C). Quantitative PCR indicated that more acnB and acnA4 were transcribed during the early logarithmic growth phase (6 h), whereas more acnA3 was expressed during the late logarithmic growth phase (12-18 h) (Fig. 2D). The cellular aconitase activity of AcnA3Tn was 30% of that of the YD35 strain during the early logarithmic growth phase, and it decreased to Ͻ10% during the late logarithmic growth phase (Fig. 2E), indicating that AcnA3 was produced throughout the culture period and was expressed predominantly during the late logarithmic growth phase.
AcnA3 Is a Unique NO-tolerant Aconitase Isozyme-We constructed gene disruptants of acnA4 (AcnA4⌬) and acnB (AcnB⌬) (Fig. 1). When cultured in the absence of NO 2 Ϫ , AcnA4⌬, AcnB⌬, and AcnA3Tn proliferated at the same rate as YD35 (Fig. 3A). Adding 100 mM NO 2 Ϫ to the medium partially inhibited the growth of the AcnA4⌬ and AcnB⌬ strains and that of the AcnA3Tn strain more obviously (Fig. 3B). The effect of acidified NO 2 Ϫ on AcnA4⌬ and AcnB⌬ growth was very small, but was more obvious with AcnA3Tn (Fig. 2B). We introduced acnA3, acnA4, and acnB into AcnA3Tn using a multicopy vector, and the resulting transformants produced 8 -14fold more aconitase activity in cell extracts compared with YD35 ( Fig. 3C) due to the gene dosage effect. Producing AcnA4 did not restore the growth defect of AcnA3Tn in the presence of 100 mM NO 2 Ϫ , whereas the effect of AcnB was partial, relative to the strain overproducing AcnA3 (Fig. 3D). These results indicate that AcnA3 is distinguishable from AcnA4 and AcnB by its role in bacterial NO-tolerant growth.
AcnA3 mutants in which the potential iron-binding Cys-437, Cys-503, and Cys-506 residues with replaced with Ser were produced by E. coli and did not bind to the Fe-S cluster (Fig. 4A). Their corresponding genes restored neither the defective cellular aconitase activity nor the NO 2 Ϫ -sensitive growth of AcnA3Tn (Fig. 4, B and C). Aconitases without an Fe-S cluster regulate protein translation and control the translation of ferritin and other iron-related proteins (29). Although the function of AcnA3 as a translation regulator in A. denitrificans remains elusive, this confirms that AcnA3 tolerates NO independently from translation regulation. It rather tolerates NO through the TCA cycle mechanism that generates NADH, as discussed below.
We examined the stability of cellular aconitase activity upon exposure to NO 2 Ϫ . When 100 mM NO 2 Ϫ was added to the culture medium at mid-logarithmic growth phase (t ϭ 0) (Fig. 3E), AcnA3Tn proliferated at a lower rate compared with YD35, AcnA4⌬, and AcnB⌬, confirming that AcnA3 is important for NO 2 Ϫ tolerance. AcnA3Tn almost completely lost cellular aconitase activity after a 12-h exposure, whereas the other strains persisted at 30% of the activity (Fig. 3F). These results indicate that the aconitase activity of AcnA3 is more resistant to NO in vivo than that of AcnA4 and AcnB.
Distribution of AcnA3-type Aconitases among ␤-Proteobacteria-The existence of an RNS-resistant aconitase AcnA3 was intriguing because the activities of common aconitases are sensitive to oxidative stress caused by atmospheric oxygen and NO due to oxidation-labile Fe-S clusters. We therefore phylogenetically analyzed the novel aconitase isozyme to understand its distribution and classification. Bacterial aconitases consist of the AcnA and AcnB families (5,30). The results of the phylogenetic analyses of AcnA family proteins indicated that the AcnA isozymes of ␣-, ␤-, ␥-, and ␦-proteobacteria were grouped into respective clades comprising each proteobacterium, whereas the AcnA of ␥-proteobacteria resided in two separate clades (Fig. 5). These results suggest the emergence of a common ancestor before these proteobacteria diversified. The ␤-proteobacteria group included YD35 AcnA3 and its counterparts from related bacteria, indicating that AcnA3-like aconitases are distributed in ␤-proteobacteria. Potential methylcitrate dehydratase constitutes another clade containing mostly proteins of ␤and ␥-proteobacteria, and YD35 AcnA4 belongs to this clade, which reflects its potential function as methylcitrate dehydratase (Fig. 5). Some of the AcnA isozymes were not placed into any of these groups and were unrelated to any proteins with known function, and their physiological functions were not evident (Fig. 5). These isozymes included YD35 AcnA1 and AcnA2. To date, the catalytic properties, Ϫ . The following strains were used: YD35 (E) and AcnA3Tn harboring pNSGroE2 (•), pNS-AcnA3 (OE), pNS-AcnA4 (f), and pNS-AcnB (ࡗ). E and F, growth (E) and cellular aconitase activity (F) of A. denitrificans strains. The bacterial culture in DM at mid-logarithmic growth phase (t ϭ 0) was supplemented with 100 mM NO 2 Ϫ . E, A. denitrificans YD35; •, AcnA3Tn; OE, AcnA4⌬; f, AcnB⌬.
Oxygen/NO Tolerance of AcnA3 Activity-Oxidants damage the labile [4Fe-4S] 2ϩ cluster to generate [3Fe-4S] ϩ and inactivate aconitases (1,31). We purified recombinant AcnA3, AcnA4, and YD35 AcnB produced in E. coli under aerobic conditions. AcnA3 and AcnA4 exhibited 107 and 27 mol/min/mg aconitase activity, respectively (Table 1). Gel filtration chromatography showed that AcnA3 and AcnA4 were monomers with molecular weights of 78,000 and 97,000, respectively, and they were both active over a broad pH range (Fig. 6). Purified AcnB was less active, but when activated by incubation with dithiothreitol and ferrous ion (Fe 2ϩ ), its activity reached 13 mol/ min/mg ( Table 1). The activation procedures did not much affect AcnA3 and AcnA4. Taken together with the following spectroscopic findings, these results indicate that AcnA3 and AcnA4 are more stable against oxidative inactivation during purification compared with AcnB-like E. coli AcnA (6). Exposure to 20% oxygen revoked the activated AcnB and, to a lesser extent, AcnA3 and AcnA4. This inactivation of AcnA3, AcnA4, and AcnB followed pseudo first-order kinetics, with kinetic constants of 1.6 Ϯ 0.4 ϫ 10 Ϫ3 , 4.3 Ϯ 0.1 ϫ 10 Ϫ3 , and 5.8 Ϯ 0.3 ϫ 10 Ϫ2 min Ϫ1 , respectively (Fig. 7A), indicating that the AcnA3 activity was more stable against oxygen than the AcnA4 and AcnB activities. Visible spectrometry of AcnA3 revealed an absorption peak at 410 nm (Fig. 7B), which is typical of aconitases in the reduced [4Fe-4S] 2ϩ state (31). Adding 10 M ferricyanide increased absorption at 480 nm, indicating that the reduced [4Fe-4S] 2ϩ was oxidized to the [3Fe-4S] ϩ state, as reported previously (31). Adding dithionite reduced AcnA3, decreased absorption at 410 nm, and resulted in a spectrum characteristic of [4Fe-4S] 1ϩ aconitases (31). On the other hand, the [4Fe-4S] 2ϩ of AcnA4 was not reduced to the [4Fe-4S] 1ϩ state by adding 10 M dithionite. The absorption spectrum elicited by ferricyanide-dependent oxidation changed AcnA4 similarly to AcnA3, but the spectral changes against the ferricyanide were more sensitive than those of AcnA3. The apparent rate constant of AcnA4 oxidation by 10 M ferricyanide was 1.9 Ϯ 0.3 min Ϫ1 , which was 10-fold higher than that of AcnA3 (0.19 Ϯ 0.01 min Ϫ1 ) (Fig. 7C). These results indicate that the AcnA3 activity is resistant to oxidants.
A 4-h incubation with 100 mM NO 2 Ϫ decreased AcnA4 and AcnB activities to 13 and 1% of those before incubation, whereas 41% of the AcnA3 activity remained (Fig. 7D). The apparent kinetic constants for AcnA3, AcnA4, and AcnB inactivation were 7.8 Ϯ 0.1 ϫ 10 Ϫ3 , 1.3 Ϯ 0.1 ϫ 10 Ϫ2 , and 6.1 Ϯ 0.1 ϫ 10 Ϫ2 min Ϫ1 , respectively (Fig. 7D). This indicates that AcnA3 activity is more resistant to NO than the other aconitase isozymes and agrees with the contribution of AcnA3 to the NO-tolerant growth of YD35. The redox potential of the [4Fe-4S] 2ϩ cluster of AcnA3 might be lower than that of AcnA4, but further electrochemical studies are required to confirm this speculation.
isomerize citrate in vivo (Fig. 8A). Less citrate accumulation in AcnB⌬ than in the other aconitase mutants is in contrast to the major role of E. coli AcnB in citrate metabolism (5,32). Incubating YD35 and the aconitase mutants with a high concentration of NO 2 Ϫ increased cellular citrate levels. The more obvious increase in the AcnA3Tn cells (Fig. 8A) indicated that AcnA3 played a major role in the cells incubated with NO 2 Ϫ . Incubation with high NO 2 Ϫ concentrations decreased the cellular NADH/ NAD ϩ ratio (Fig. 8B) and the ATP concentration (Fig. 8C), which supports our concept that the up-regulated TCA cycle increases cellular NADH and ATP and adapts the cells to oxidative stress caused by NO 2 Ϫ (25). These results show that AcnA3 rendered the cells NO-tolerant via the TCA cycle. The NADH produced by the TCA cycle also served as a substrate for RNS-detoxifying enzymes, which are up-regulated by NO 2 Ϫ (Fig. 8D) (25) and which decrease cellular NO levels, because we found that 6-fold more NO accumulated in AcnA3Tn than in YD35 (Fig. 8E). The growth of AcnA3Tn was insensitive to other oxidative stressors, such as hydrogen peroxide, t-butyl hydroperoxide, and methyl viologen, relative to the wild type (Fig. 8F). Menadione (3 mM) slightly affected the growth of AcnA3Tn and other mutants. These results indicate that the tolerance mechanism of AcnA3 is specific to NO and menadione among oxidants.
Distribution of Oxidation-tolerant Aconitases-Oxygen sensitivity was investigated by comparing the activities of aerobically purified recombinant ␣-, ␤-, and ␥-proteobacterial sources of AcnA aconitases, four methylcitrate dehydratases, and four AcnB aconitases from various bacteria before and after activation by dithiothreitol and Fe 2ϩ ( Table 1). The results showed that all of the AcnA aconitases, including methylcitrate dehydratases, were active irrespective of the type of activation and that all four AcnB aconitases required activation to generate maximal activity. These results indicate that AcnA activity is more oxygen-stable compared with AcnB activity, like the E. coli isozymes (6). In addition, the specific activity of the ␤-proteobacterial aconitases, including YD35 AcnA3 (Fig. 5), was higher than that of the aconitases from the other sources after activation (Table 1). This might account for their role in bacterial NO tolerance (see below).

DISCUSSION
We screened A. denitrificans YD35 genes that tolerated high NO 2 Ϫ levels and discovered the acnA3 gene. The enzyme activity of aconitases is often sensitive to oxidation by oxygen and reactive oxygen species (such as superoxide) both in vivo and in vitro (3,4,6). E. coli and most likely other bacteria produce the aconitase AcnA isozymes, which are more oxidation-resistant than AcnB and which maintain TCA cycle metabolism to facilitate growth in the presence of superoxide (5,6,32). However, the functions of bacterial aconitase in the presence of NO have rarely been characterized. A. denitrificans YD35 AcnA3 is a newly characterized aconitase A isozyme; its enzyme activity is more resistant to oxidants compared with other aconitase isozymes. Genetic studies indicated that the AcnA3 of this and related bacteria enables bacterial cells to grow in the presence of high NO 2 Ϫ levels. This implies that AcnA3 is indeed an AcnA and that its function in the bacterial NO tolerance mechanism is unique.
The molecular mechanisms through which aconitases produce catalytic activity have been reported. E. coli AcnB pro-duces activity through dimerization (33). However, we purified active YD35 AcnA3 as a monomer (Fig. 6B), indicating that dimerization is not required for the stability of AcnA3 against oxygen. The present spectrometric study showed that atmospheric oxygen inactivated AcnA3 at a lower rate compared with AcnA4 (Fig. 7A) and probably other AcnA and AcnB isozymes, suggesting that the redox potential of the Fe-S cluster is important for stability against oxygen. The overall amino acid sequence similarity between AcnA3 and other aconitases predicts that AcnA3 essentially shares a tertiary structure with other aconitases and consists of four domains (1). Domain 4 binds the fourth iron atom of the Fe-S cluster and regulates aconitase substrate (citrate) and ligand (mRNA) binding when functioning as an iron-responding protein (29,34,35). Amino acid substitutions in domain 4 and the adjacent linker region are likely to affect the redox potential and hence the oxygen and NO stability of AcnA3, through which the bacterium acquired NO tolerance. However, detailed studies are required to confirm this notion.
We identified 16 A. denitrificans genes that are involved in NO tolerance, and they are also involved in various cellular processes (25). Some of them notably encode pyruvate dehydrogenase, as well as ␣-ketoglutarate dehydrogenase and cytochrome bd-type terminal oxidase, which are both involved in energy conservation processes. Together with the role of high level of NO 2 Ϫ contamination, and it is a denitrifier (24,25). The AcnA3 generated by this species of bacteria contributes to the ability of the bacteria to thrive in very high NO 2 Ϫ concentrations, against which conventional bacteria have no resistance. The Alcaligenaceae family also contains the whooping cough pathogen Bordetella pertussis and other Bordetella subspecies (39). Infectious bacteria are considered to produce NO tolerance mechanisms for surviving NO generated by host phagocytes. Our findings suggest that AcnA3 isozymes also constitute a novel NO tolerance mechanism in these pathogens and that AcnA3 might serve as a novel target for developing medical treatments to cure and prevent disease.