Nitrosative stress in Escherichia coli: reduction of nitric oxide

The ability of enteric bacteria to protect themselves against reactive nitrogen species generated by their own metabolism, or as part of the innate immune response, is critical to their survival. One important defence mechanism is their ability to reduce NO (nitric oxide) to harmless products. The highest rates of NO reduction by Escherichia coli K-12 were detected after anaerobic growth in the presence of nitrate. Four proteins have been implicated as catalysts of NO reduction: the cytoplasmic sirohaem-containing nitrite reductase, NirB; the periplasmic cytochrome c nitrite reductase, NrfA; the flavorubredoxin NorV and its associated oxidoreductase, NorW; and the flavohaemoglobin, Hmp. Single mutants defective in any one of these proteins and even the mutant defective in all four proteins reduced NO at the same rate as the parent. Clearly, therefore, there are mechanisms of NO reduction by enteric bacteria that remain to be characterized. Far from being minor pathways, the currently unknown pathways are adequate to sustain almost optimal rates of NO reduction, and hence potentially provide significant protection against nitrosative stress.     

The nitric oxide (NO)-sensing repressor NsrR of Neisseria meningitidis has a compact regulon of genes involved in NO synthesis and detoxification

We have analyzed the extent of regulation by the nitric oxide (NO)-sensitive repressor NsrR from Neisseria meningitidis MC58, using microarray analysis. Target genes that appeared to be regulated by NsrR, based on a comparison between an nsrR mutant and a wild-type strain, were further investigated by quantitative real-time PCR, revealing a very compact set of genes, as follows: norB (encoding NO reductase), dnrN (encoding a protein putatively involved in the repair of nitrosative damage to iron-sulfur clusters), aniA (encoding nitrite reductase), nirV (a putative nitrite reductase assembly protein), and mobA (a gene associated with molybdenum metabolism in other species but with a frame shift in N. meningitidis). In all cases, NsrR acts as a repressor. The NO protection systems norB and dnrN are regulated by NO in an NsrR-dependent manner, whereas the NO protection system cytochrome c' (encoded by cycP) is not controlled by NO or NsrR, indicating that N. meningitidis expresses both constitutive and inducible NO protection systems. In addition, we present evidence to show that the anaerobic response regulator FNR is also sensitive to NO but less so than NsrR, resulting in complex regulation of promoters such as aniA, which is controlled by both FNR and NsrR: aniA was found to be maximally induced by intermediate NO concentrations, consistent with a regulatory system that allows expression during denitrification (in which NO accumulates) but is down-regulated as NO approaches toxic concentrations.     

The roles of the polytopic membrane proteins NarK, NarU and NirC in Escherichia coli K-12: two nitrate and three nitrite transporters

Two polytopic membrane proteins, NarK and NarU, are assumed to transport nitrite out of the Escherichia coli cytoplasm, but how nitrate enters enteric bacteria is unknown. We report the construction and use of four isogenic strains that lack nitrate reductase Z and the periplasmic nitrate reductase, but express all combinations of narK and narU. The active site of the only functional nitrate reductase, nitrate reductase A, is located in the cytoplasm, so nitrate reduction by these four strains is totally dependent upon a mechanism for importing nitrate. These strains were exploited to determine the roles of NarK and NarU in both nitrate and nitrite transport. Single mutants that lack either NarK or NarU were competent for nitrate-dependent anaerobic growth on a non-fermentable carbon source, glycerol. They transported and reduced nitrate almost as rapidly as the parental strain. In contrast, the narK-narU double mutant was defective in nitrate-dependent growth unless nitrate transport was facilitated by the nitrate ionophore, reduced benzyl viologen (BV). It was also unable to catalyse nitrate reduction in the presence of physiological electron donors. Synthesis of active nitrate reductase A and the cytoplasmic, NADH-dependent nitrite reductase were unaffected by the narK and narU mutations. The rate of nitrite reduction catalysed by the cytoplasmic, NADH-dependent nitrite reductase by the double mutant was almost as rapid as that of the NarK+-NarU+ strain, indicating that there is a mechanism for nitrite uptake by E. coli that is in-dependent of either NarK or NarU. The nir operon encodes a soluble, cytoplasmic nitrite reductase that catalyses NADH-dependent reduction of nitrite to ammonia. One additional component that contributes to nitrite uptake was shown to be NirC, the hydrophobic product of the third gene of the nir operon, which is predicted to be a polytopic membrane protein with six membrane-spanning helices. Deletion of both NarK and NirC decreased nitrite uptake and reduction to a basal rate that was fully restored by a single chromosomal copy of either narK or nirC. A multicopy plasmid encoding NarU complemented a narK mutation for nitrite excretion, but not for nitrite uptake. We conclude that, in contrast to NirC, which transports only nitrite, NarK and NarU provide alternative mechanisms for both nitrate and nitrite transport. However, NarU might selectively promote nitrite ex-cretion, not nitrite uptake.     

Characterization of Tn5 mutants deficient in dissimilatory nitrite reduction in Pseudomonas sp. strain G-179, which contains a copper nitrite reductase.

Tn5 was used to generate mutants that were deficient in the dissimilatory reduction of nitrite for Pseudomonas sp. strain G-179, which contains a copper nitrite reductase. Three types of mutants were isolated. The first type showed a lack of growth on nitrate, nitrite, and nitrous oxide. The second type grew on nitrate and nitrous oxide but not on nitrite (Nir-). The two mutants of this type accumulated nitrite, showed no nitrite reductase activity, and had no detectable nitrite reductase protein bands in a Western blot (immunoblot). Tn5 insertions in these two mutants were clustered in the same region and were within the structural gene for nitrite reductase. The third type of mutant grew on nitrate but not on nitrite or nitrous oxide (N2O). The mutant of this type accumulated significant amounts of nitrite, NO, and N2O during anaerobic growth on nitrate and showed a slower growth rate than the wild type. Diethyldithiocarbamic acid, which inhibited nitrite reductase activity in the wild type, did not affect NO reductase activity, indicating that nitrite reductase did not participate in NO reduction. NO reductase activity in Nir- mutants was lower than that in the wild type when the strains were grown on nitrate but was the same as that in the wild type when the strains were grown on nitrous oxide. These results suggest that the reduction of NO and N2O was carried out by two distinct processes and that mutations affecting nitrite reduction resulted in reduced NO reductase activity following anaerobic growth with nitrate.     

Nitric oxide homeostasis in Salmonella typhimurium: roles of respiratory nitrate reductase and flavohemoglobin

Nitric oxide (NO) is generated in biological systems primarily via the activity of NO synthases and nitrate and nitrite reductases. Here we show that Salmonella enterica serovar Typhimurium (S. typhimurium) grown anaerobically with nitrate is capable of generating polarographically detectable NO after nitrite (NO(2)(-)) addition. NO accumulation is sensitive to the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide. Neither an fnr mutant nor an fnr hmp double mutant produces NO, indicating the involvement in NO evolution from NO(2)(-) of protein(s) positively regulated by FNR. Contrary to previous findings in Escherichia coli, we demonstrate that neither the periplasmic nitrite reductase (NrfA) nor the cytoplasmic nitrite reductase (NirB) is involved in NO production in S. typhimurium. However, mutant cells lacking the membrane-bound nitrate reductase, NarGHI, and membranes derived from these cells are unable to produce NO, demonstrating that, in wild-type S. typhimurium, this enzyme is responsible for NO production. Membrane terminal oxidases cannot account for the NO levels measured. The nitrate reductase inhibitor, azide, abrogates NO evolution by Salmonella, and production of NO occurs only in the absence from the assays of nitrate; both features reveal a marked similarity between the NO-generating activities of this bacterium and plants. Unlike the situation in E. coli, an S. typhimurium hmp mutant produces NO both aerobically and anaerobically. Under aerobic conditions, when a functional flavohemoglobin is present, no NO is detectable. We propose a homeostatic mechanism in S. typhimurium, in which NO produced from NO(2)(-) by nitrate reductase derepresses Hmp expression (via FNR and NsrR) and NorV expression (via NorR) and thus limits NO toxicity.     

Nitric oxide metabolism in Neisseria meningitidis

Neisseria meningitidis, the causative agent of meningococcal disease in humans, is likely to be exposed to nitrosative stress during natural colonization and disease. The genome of N. meningitidis includes the genes aniA and norB, predicted to encode nitrite reductase and nitric oxide (NO) reductase, respectively. These gene products should allow the bacterium to denitrify nitrite to nitrous oxide. We show that N. meningitidis can support growth microaerobically by the denitrification of nitrite via NO and that norB is required for anaerobic growth with nitrite. NorB and, to a lesser extent, the cycP gene product cytochrome c' are able to counteract toxicity due to exogenously added NO. Expression of these genes by N. meningitidis during colonization and disease may confer protection against exogenous or endogenous nitrosative stress.     

Growth of Campylobacter jejuni on nitrate and nitrite: electron transport to NapA and NrfA via NrfH and distinct roles for NrfA and the globin Cgb in protection against nitrosative stress

Pathways of electron transport to periplasmic nitrate (NapA) and nitrite (NrfA) reductases have been investigated in Campylobacter jejuni, a microaerophilic food-borne pathogen. The nap operon is unusual in lacking napC (encoding a tetra-haem c-type cytochrome) and napF, but contains a novel gene of unknown function, napL. The iron-sulphur protein NapG has a major role in electron transfer to the NapAB complex, but we show that slow nitrate-dependent growth of a napG mutant can be sustained by electron transfer from NrfH, the electron donor to the nitrite reductase NrfA. A napL mutant possessed approximately 50% lower NapA activity than the wild type but showed normal growth with nitrate as the electron acceptor. NrfA was constitutive and was shown to play a role in protection against nitrosative stress in addition to the previously identified NO-inducible single domain globin, Cgb. However, nitrite also induced cgb expression in an NssR-dependent manner, suggesting that growth of C. jejuni with nitrite causes nitrosative stress. This was confirmed by lack of growth of cgb and nssR mutants, and slow growth of the nrfA mutant, in media containing nitrite. Thus, NrfA and Cgb together provide C. jejuni with constitutive and inducible components of a robust defence against nitrosative stress.     

New functions for the ancient globin family: bacterial responses to nitric oxide and nitrosative stress

Globin-like oxygen-binding proteins occur in bacteria, yeasts and other fungi, and protozoa. The simplest contain protohaem as sole prosthetic group, but show considerable variation in their similarity to the classical animal globins and plant globins. Flavohaemoglobins comprise a haem domain homologous to classical globins and a ferredoxin-NADP+ reductase (FNR)-like domain that converts the globin into an NAD(P)H-oxidizing protein with diverse reductase activities. In Escherichia coli, the prototype flavohaemoglobin (Hmp) is clearly involved in responses to nitric oxide (NO) and nitrosative stress: (i) the structural gene hmp is upregulated by NO and nitrosating agents; (ii) purified Hmp binds NO avidly, but also converts it to nitrate (aerobically) or nitrous oxide (anaerobically); (iii) hmp mutants are hypersensitive to NO and nitrosative stresses. Here, we review recent advances in E. coli and the growing number of microbes in which globins are known, draw particular attention to the essential chemistry of NO and related reactive species and their interactions with globins, and suggest that microbial globins have additional functions unrelated to 'NO' stresses.     

Metabolism of reactive nitrogen species in pea plants under abiotic stress conditions

Nitric oxide (*NO) is a key signaling molecule in different physiological processes of animals and plants. However, little is known about the metabolism of endogenous *NO and other reactive nitrogen species (RNS) in plants under abiotic stress conditions. Using pea plants exposed to six different abiotic stress conditions (high light intensity, low and high temperature, continuous light, continuous dark and mechanical wounding), several key components of the metabolism of RNS including the content of *NO, S-nitrosothiols (RSNOs) and nitrite plus nitrate, the enzyme activities of l-arginine-dependent nitric oxide synthase (NOS) and S-nitrosogluthathione reductase (GSNOR), and the profile of protein tyrosine nitration (NO(2)-Tyr) were analyzed in leaves. Low temperature was the stress that produced the highest increase of NOS and GSNOR activities, and this was accompanied by an increase in the content of total *NO and S-nitrosothiols, and an intensification of the immunoreactivity with an antibody against NO(2)-Tyr. Mechanical wounding, high temperature and light also had a clear activating effect on the different indicators of RNS metabolism in pea plants. However, the total content of nitrite and nitrate in leaves was not affected by any of these stresses. Considering that protein tyrosine nitration is a potential marker of nitrosative stress, the results obtained suggest that low and high temperature, continuous light and high light intensity are abiotic stress conditions that can induce nitrosative stress in pea plants.     

The emerging roles of nitric oxide (NO) in plant mitochondria

In recent years nitric oxide (NO) has been recognized as an important signal molecule in plants. Both, reductive and oxidative pathways and different subcellular compartments appear involved in NO production. The reductive pathway uses nitrite as substrate, which is exclusively generated by cytosolic nitrate reductase (NR) and can be converted to NO by the same enzyme. The mitochondrial electron transport chain is another site for nitrite to NO reduction, operating specifically when the normal electron acceptor, O₂, is low or absent. Under these conditions, the mitochondrial NO production contributes to hypoxic survival by maintaining a minimal ATP formation. In contrast, excessive NO production and concomitant nitrosative stress may be prevented by the operation of NO-scavenging mechanisms in mitochondria and cytosol. During pathogen attacks, mitochondrial NO serves as a nitrosylating agent promoting cell death; whereas in symbiotic interactions as in root nodules, the turnover of mitochondrial NO helps in improving the energy status similarly as under hypoxia/anoxia. The contribution of NO turnover during pathogen defense, symbiosis and hypoxic stress is discussed in detail.     

The oxidative and nitrosative stress defence network of Wolinella succinogenes: cytochrome c nitrite reductase mediates the stress response to nitrite, nitric oxide, hydroxylamine and hydrogen peroxide

Microorganisms employ diverse mechanisms to withstand physiological stress conditions exerted by reactive or toxic oxygen and nitrogen species such as hydrogen peroxide, organic hydroperoxides, superoxide anions, nitrite, hydroxylamine, nitric oxide or NO-generating compounds. This study identified components of the oxidative and nitrosative stress defence network of Wolinella succinogenes, an exceptional Epsilonproteobacterium that lacks both catalase and haemoglobins. Various gene deletion-insertion mutants were constructed, grown by either fumarate respiration or respiratory nitrate ammonification and subjected to disc diffusion, growth and viability assays under stress conditions. It was demonstrated that mainly two periplasmic multihaem c-type cytochromes, namely cytochrome c peroxidase and cytochrome c nitrite reductase (NrfA), mediated resistance to hydrogen peroxide. Two AhpC-type peroxiredoxin isoenzymes were shown to be involved in protection against different organic hydroperoxides. The phenotypes of two superoxide dismutase mutants lacking either SodB or SodB2 implied that both isoenzymes play important roles in oxygen and superoxide stress defence although they are predicted to reside in the cytoplasm and periplasm respectively. NrfA and a cytoplasmic flavodiiron protein (Fdp) were identified as key components of nitric oxide detoxification. In addition, NrfA (but not the hybrid cluster protein Hcp) was found to mediate resistance to hydroxylamine stress. The results indicate the presence of a robust oxidative and nitrosative stress defence network and identify NrfA as a multifunctional cytochrome c involved in both anaerobic respiration and stress protection.     

Neuroglobin expression and oxidant/antioxidant balance after graded traumatic brain injury in the rat

Neuroglobin is a neuron-specific hexacoordinated globin capable of binding various ligands, including O₂, NO, and CO, the biological function of which is still uncertain. Various studies seem to indicate that neuroglobin is a neuroprotective agent when overexpressed, acting as a potent inhibitor of oxidative and nitrosative stress. In this study, we evaluated the pathophysiological response of the neuroglobin gene and protein expression in the cerebral tissue of rats sustaining traumatic brain injury of differing severity, while simultaneously measuring the oxidant/antioxidant balance. Two levels of trauma (mild and severe) were induced in anesthetized animals using the weight-drop model of diffuse axonal injury. Rats were then sacrificed at 6, 12, 24, 48, and 120 h after traumatic brain injury, and the gene and protein expression of neuroglobin and the concentrations of malondialdehyde (as a parameter representative of reactive oxygen species-mediated damage), nitrite + nitrate (indicative of NO metabolism), ascorbate, and glutathione (GSH) were determined in the brain tissue. Results indicated that mild traumatic brain injury, although causing a reversible increase in oxidative/nitrosative stress (increase in malondialdehyde and nitrite + nitrate) and an imbalance in antioxidants (decrease in ascorbate and GSH), did not induce any change in neuroglobin. Conversely, severe traumatic brain injury caused an over nine- and a fivefold increase in neuroglobin gene and protein expression, respectively, as well as a remarkable increase in oxidative/nitrosative stress and depletion of antioxidants. The results of this study, showing a lack of effect in mild traumatic brain injury as well as asynchronous time course changes in neuroglobin expression, oxidative/nitrosative stress, and antioxidants in severe traumatic brain injury, do not seem to support the role of neuroglobin as an endogenous neuroprotective antioxidant agent, at least under pathophysiological conditions.     

The metabolic reprogramming evoked by nitrosative stress triggers the anaerobic utilization of citrate in Pseudomonas fluorescens

Nitrosative stress is an ongoing challenge that most organisms have to contend with. When nitric oxide (NO) that may be generated either exogenously or endogenously encounters reactive oxygen species (ROS), it produces a set of toxic moieties referred to as reactive nitrogen species (RNS). As these RNS can severely damage essential biomolecules, numerous organisms have evolved elaborate detoxification strategies to nullify RNS. However, the contribution of cellular metabolism in fending off nitrosative stress is poorly understood. Using a variety of functional proteomic and metabolomic analyses, we have identified how the soil microbe Pseudomonas fluorescens reprogrammed its metabolic networks to survive in an environment enriched by sodium nitroprusside (SNP), a generator of nitrosative stress. To combat the RNS-induced ineffective aconitase (ACN) and tricarboxylic acid (TCA) cycle, the microbe invoked the participation of citrate lyase (CL), phosphoenolpyruvate carboxylase (PEPC) and pyruvate phosphate dikinase (PPDK) to convert citrate, the sole source of carbon into pyruvate and ATP. These enzymes were not evident in the control conditions. This metabolic shift was coupled to the concomitant increase in the activities of such classical RNS detoxifiers as nitrate reductase (NR), nitrite reductase (NIR) and S-nitrosoglutathione reductase (GSNOR). Hence, metabolism may hold the clues to the survival of organisms subjected to nitrosative stress and may provide therapeutic cues against RNS-resistant microbes.     

Regulation of denitrification genes in Neisseria meningitidis by nitric oxide and the repressor NsrR

The human pathogen Neisseria meningitidis is capable of growth using the denitrification of nitrite to nitrous oxide under microaerobic conditions. This process is catalyzed by two reductases: nitrite reductase (encoded by aniA) and nitric oxide (NO) reductase (encoded by norB). Here, we show that in N. meningitidis MC58 norB is regulated by nitric oxide via the product of gene NMB0437 which encodes NsrR. NsrR is a repressor in the absence of NO, but norB expression is derepressed by NO in an NsrR-dependent manner. nsrR-deficient mutants grow by denitrification more rapidly than wild-type N. meningitidis, and this is coincident with the upregulation of both NO reductase and nitrite reductase even under aerobic conditions in the absence of nitrite or NO. The NsrR-dependent repression of aniA (unlike that of norB) is not lifted in the presence of NO. The role of NsrR in the control of expression of aniA is linked to the function of the anaerobic activator protein FNR: analysis of nsrR and fnr single and nsrR fnr double mutants carrying an aniA promoter lacZ fusion indicates that the role of NsrR is to prevent FNR-dependent aniA expression under aerobic conditions, indicating that FNR in N. meningitidis retains considerable activity aerobically.     

Induction of ferredoxin-NADP+ oxidoreductase and ferredoxin synthesis in pea root plastids during nitrate assimilation

A 14.5 kDa protein with antigenic components in common with pea leaf ferredoxin was detected on transblots of the soluble proteins of pea root plastids. The amount of this protein was found to increase during the induction of nitrate assimilation in pea roots, reaching a maximal level at 8–12 h. Concurrent with this, a fourfold increase in NADPH-dependent ferredoxin-NADP⁺ oxidoreductase (FNR) activity was observed corresponding to an increase in the amount of this protein detected immunologically on transblots using a leaf FNR antibody. These changes were not observed in plastids from roots of plants grown on ammonia or depleted of nitrogen. It is suggested that in addition to the already well reported induction by nitrate of nitrate reductase and nitrite reductase, there is a co-induction of a plastid located ferredoxin and FNR. Both these proteins are necessary for the transfer of reductant generated by the oxidative pentose phosphate pathway to nitrite reductase.