The sodium cycle: a novel type of bacterial energetics

Discovery and Study of Transmembrane Rotary Ion-Translocating Nano-Motors: F-ATPase/Synthase of Mitochondria/Bacteria and V-ATPase of Eukaryotic Cells

This review discusses the history of discovery and study of the operation of the two rotary ion-translocating ATPase nano-motors: (i) F-ATPase/synthase (holocomplex F₁F_O) of mitochondria/bacteria and (ii) eukaryotic V-ATPase (holocomplex V₁V_O). Vacuolar adenosine triphosphatase (V-ATPase) is a transmembrane multisubunit complex found in all eukaryotes from yeast to humans. It is structurally and functionally similar to the F-ATPase/synthase of mitochondria/bacteria and the A-ATPase/synthase of archaebacteria, which indicates a common evolutionary origin of the rotary ion-translocating nano-motors built into cell membranes and invented by Nature billions of years ago. Previously we have published several reviews on this topic with appropriate citations of our original research. This review is focused on the historical analysis of the discovery and study of transmembrane rotary ion-translocating ATPase nano-motors functioning in bacteria, eukaryotic cells and mitochondria of animals.

Na+ homeostasis in Acinetobacter baumannii is facilitated via the activity of the Mrp antiporter

The human opportunistic pathogen Acinetobacter baumannii is a global threat to healthcare institutions worldwide, since it developed very efficient strategies to evade host defense and to adapt to the different environmental conditions of the host. This worked focused on the importance of Na⁺ homeostasis in A. baumannii with regards to pathobiological aspects. In silico studies revealed a homologue of a multicomponent Na⁺ /H⁺ antiporter system. Inactivation of the Mrp antiporter through deletion of the first gene (mrpA') resulted in a mutant that was sensitive to increasing pH values. Furthermore, the strain was highly sensitive to increasing Na⁺ and Li⁺ concentrations. Increasing Na⁺ sensitivity is thought to be responsible for growth impairment in human fluids. Furthermore, deletion of mrpA' is associated with energetic defects, inhibition of motility and survival under anoxic and dry conditions.

Accumulation of ambient phosphate into the periplasm of marine bacteria is proton motive force dependent

Bacteria acquire phosphate (P_i) by maintaining a periplasmic concentration below environmental levels. We recently described an extracellular P_i buffer which appears to counteract the gradient required for P_i diffusion. Here, we demonstrate that various treatments to outer membrane (OM) constituents do not affect the buffered P_i because bacteria accumulate P_i in the periplasm, from which it can be removed hypo-osmotically. The periplasmic P_i can be gradually imported into the cytoplasm by ATP-powered transport, however, the proton motive force (PMF) is not required to keep P_i in the periplasm. In contrast, the accumulation of P_i into the periplasm across the OM is PMF-dependent and can be enhanced by light energy. Because the conventional mechanism of P_i-specific transport cannot explain P_i accumulation in the periplasm we propose that periplasmic P_i anions pair with chemiosmotic cations of the PMF and millions of accumulated P_i pairs could influence the periplasmic osmolarity of marine bacteria.

The ubiquitous oceanic bacteria harbour an external phosphate buffer for modulating phosphate (P_i) uptake. Here, using both oceanic SAR11, Prochlorococcus and Synechococcus strains as a model, the authors show that the P_i buffer accumulation in the periplasm is proton motive force-dependent and can be enhanced by light energy.

A simple strategy to differentiate between H+- and Na+-transporting NADH:quinone oxidoreductases

We describe here a simple strategy to characterize transport specificity of NADH:quinone oxidoreductases, using Na⁺-translocating (NQR) and H⁺-translocating (NDH-1) enzymes of the soil bacterium Azotobactervinelandii as the models. Submillimolar concentrations of Na⁺ and Li⁺ increased the rate of deaminoNADH oxidation by the inverted membrane vesicles prepared from the NDH-1-deficient strain. The vesicles generated carbonyl cyanide m-chlorophenyl hydrazone (CCCP)-resistant electric potential difference and CCCP-stimulated pH difference (alkalinization inside) in the presence of Na⁺. These findings testified a primary Na⁺-pump function of A. vinelandii NQR. Furthermore, ΔpH measurements with fluorescent probes (acridine orange and pyranine) demonstrated that A. vinelandii NQR cannot transport H⁺ under various conditions. The opposite results obtained in similar measurements with the vesicles prepared from the NQR-deficient strain indicated a primary H⁺-pump function of NDH-1. Based on our findings, we propose a package of simple experiments that are necessary and sufficient to unequivocally identify the pumping specificity of a bacterial Na⁺ or H⁺ transporter. The NQR-deficient strain, but not the NDH-1-deficient one, exhibited impaired growth characteristics under diazotrophic condition, suggesting a role for the Na⁺ transport in nitrogen fixation by A. vinelandii.

Na+-Translocating Ferredoxin:NAD+ Oxidoreductase Is a Component of Photosynthetic Electron Transport Chain in Green Sulfur Bacteria

Genomes of photoautotrophic organisms containing type I photosynthetic reaction center were searched for the rnf genes encoding Na+-translocating ferredoxin:NAD+ oxidoreductase (RNF). These genes were absent in heliobacteria, cyanobacteria, algae, and plants; however, genomes of many green sulfur bacteria (especially marine ones) were found to contain the full rnf operon. Analysis of RNA isolated from the marine green sulfur bacterium Chlorobium phaeovibrioides revealed a high level of rnf expression. It was found that the activity of Na+-dependent flavodoxin:NAD+ oxidoreductase detected in the membrane fraction of Chl. phaeovibrioides was absent in the membrane fraction of the freshwater green sulfur bacterium Chlorobaculum limnaeum, which is closely related to Chl. phaeovibrioides but whose genome lacks the rnf genes. Illumination of the membrane fraction of Chl. phaeovibrioides but not of Cba. limnaeum resulted in the light-induced NAD+ reduction. Based on the obtained data, we concluded that in some green sulfur bacteria, RNF may be involved in the NADH formation that should increase the efficiency of light energy conservation in these microorganisms and can serve as the first example of the use of Na+ energetics in photosynthetic electron transport chains.

Brevibacterium linens RS16 confers salt tolerance to Oryza sativa genotypes by regulating antioxidant defense and H+ ATPase activity

Soil salinity is one of the major limitations that affects both plant and its soil environment, leading to reduced agricultural production. Evaluation of stress severity by plant physical and biochemical characteristics is an established way to study plant-salt stress interaction, but the halotolerant properties of plant growth promoting bacteria (PGPB) along with plant growth promotion is less studied till date. The aim of the present study was to elucidate the strategy, used by ACC deaminase-containing halotolerant Brevibacterium linens RS16 to confer salt stress tolerance in moderately salt-tolerant (FL478) and salt-sensitive (IR29) rice (Oryza sativa L.) cultivars. The plants were exposed to salt stress using 0, 50, and 100 mM of NaCl with and without bacteria. Plant physiological and biochemical characteristics were estimated after 1, 5, 10 days of stress application. H⁺ ATPase activity and the presence of hydroxyectoine gene (ectD) that is responsible for compatible solute accumulation were also analyzed in bacteria. The height and dry mass of bacteria inoculated plants significantly increased compared to salt-stressed plants, and the differences increased in time dependent manner. Bacteria priming reduced the plant antioxidant enzyme activity, lipid peroxidation and it also regulated the salt accumulation by modulating vacuolar H⁺ ATPase activity. ATPase activity and presence of hydroxyectoine gene in RS16 might have played a vital role in providing salt tolerance in bacteria inoculated rice cultivars. We conclude that dual benefits provided by the halotolerant plant growth promoting bacteria (PGPB) can provide a major way to improve rice yields in saline soil.

Ancient Systems of Sodium/Potassium Homeostasis as Predecessors of Membrane Bioenergetics

Cell cytoplasm of archaea, bacteria, and eukaryotes contains substantially more potassium than sodium, and potassium cations are specifically required for many key cellular processes, including protein synthesis. This distinct ionic composition and requirements have been attributed to the emergence of the first cells in potassium-rich habitats. Different, albeit complementary, scenarios have been proposed for the primordial potassium-rich environments based on experimental data and theoretical considerations. Specifically, building on the observation that potassium prevails over sodium in the vapor of inland geothermal systems, we have argued that the first cells could emerge in the pools and puddles at the periphery of primordial anoxic geothermal fields, where the elementary composition of the condensed vapor would resemble the internal milieu of modern cells. Marine and freshwater environments generally contain more sodium than potassium. Therefore, to invade such environments, while maintaining excess of potassium over sodium in the cytoplasm, primordial cells needed means to extrude sodium ions. The foray into new, sodium-rich habitats was the likely driving force behind the evolution of diverse redox-, light-, chemically-, or osmotically-dependent sodium export pumps and the increase of membrane tightness. Here we present a scenario that details how the interplay between several, initially independent sodium pumps might have triggered the evolution of sodium-dependent membrane bioenergetics, followed by the separate emergence of the proton-dependent bioenergetics in archaea and bacteria. We also discuss the development of systems that utilize the sodium/potassium gradient across the cell membranes.

Respiratory complex I: A dual relation with H(+) and Na(+)?

Respiratory complex I couples NADH:quinone oxidoreduction to ion translocation across the membrane, contributing to the buildup of the transmembrane difference of electrochemical potential. H(+) is well recognized to be the coupling ion of this system but some studies suggested that this role could be also performed by Na(+). We have previously observed NADH-driven Na(+) transport opposite to H(+) translocation by menaquinone-reducing complexes I, which indicated a Na(+)/H(+) antiporter activity in these systems. Such activity was also observed for the ubiquinone-reducing mitochondrial complex I in its deactive form. The relation of Na(+) with complex I may not be surprising since the enzyme has three subunits structurally homologous to bona fide Na(+)/H(+) antiporters and translocation of H(+) and Na(+) ions has been described for members of most types of ion pumps and transporters. Moreover, no clearly distinguishable motifs for the binding of H(+) or Na(+) have been recognized yet. We noticed that in menaquinone-reducing complexes I, less energy is available for ion translocation, compared to ubiquinone-reducing complexes I. Therefore, we hypothesized that menaquinone-reducing complexes I perform Na(+)/H(+) antiporter activity in order to achieve the stoichiometry of 4H(+)/2e(-). In agreement, the organisms that use ubiquinone, a high potential quinone, would have kept such Na(+)/H(+) antiporter activity, only operative under determined conditions. This would imply a physiological role(s) of complex I besides a simple "coupling" of a redox reaction and ion transport, which could account for the sophistication of this enzyme. This article is part of a Special Issue entitled Respiratory complex I, edited by Volker Zickermann and Ulrich Brandt.

The Kinetic Reaction Mechanism of the Vibrio cholerae Sodium-dependent NADH Dehydrogenase

The sodium-dependent NADH dehydrogenase (Na(+)-NQR) is the main ion transporter in Vibrio cholerae. Its activity is linked to the operation of the respiratory chain and is essential for the development of the pathogenic phenotype. Previous studies have described different aspects of the enzyme, including the electron transfer pathways, sodium pumping structures, cofactor and subunit composition, among others. However, the mechanism of the enzyme remains to be completely elucidated. In this work, we have studied the kinetic mechanism of Na(+)-NQR with the use of steady state kinetics and stopped flow analysis. Na(+)-NQR follows a hexa-uni ping-pong mechanism, in which NADH acts as the first substrate, reacts with the enzyme, and the oxidized NAD leaves the catalytic site. In this conformation, the enzyme is able to capture two sodium ions and transport them to the external side of the membrane. In the last step, ubiquinone is bound and reduced, and ubiquinol is released. Our data also demonstrate that the catalytic cycle involves two redox states, the three- and five-electron reduced forms. A model that gathers all available information is proposed to explain the kinetic mechanism of Na(+)-NQR. This model provides a background to understand the current structural and functional information.

The CydDC Family of Transporters and Their Roles in Oxidase Assembly and Homeostasis

The CydDC complex of Escherichia coli is a heterodimeric ATP-binding cassette type transporter (ABC transporter) that exports the thiol-containing redox-active molecules cysteine and glutathione. These reductants are thought to aid redox homeostasis of the periplasm, permitting correct disulphide folding of periplasmic and secreted proteins. Loss of CydDC results in the periplasm becoming more oxidising and abolishes the assembly of functional bd-type respiratory oxidases that couple the oxidation of ubiquinol to the reduction of oxygen to water. In addition, CydDC-mediated redox control is important for haem ligation during cytochrome c assembly. Given the diverse roles for CydDC in redox homeostasis, respiratory metabolism and the maturation of virulence factors, this ABC transporter is an intriguing system for researchers interested in both the physiology of redox perturbations and the role of low-molecular-weight thiols during infection.

Origin and evolution of the sodium -pumping NADH: ubiquinone oxidoreductase

The sodium -pumping NADH: ubiquinone oxidoreductase (Na⁺-NQR) is the main ion pump and the primary entry site for electrons into the respiratory chain of many different types of pathogenic bacteria. This enzymatic complex creates a transmembrane gradient of sodium that is used by the cell to sustain ionic homeostasis, nutrient transport, ATP synthesis, flagellum rotation and other essential processes. Comparative genomics data demonstrate that the nqr operon, which encodes all Na⁺-NQR subunits, is found in a large variety of bacterial lineages with different habitats and metabolic strategies. Here we studied the distribution, origin and evolution of this enzymatic complex. The molecular phylogenetic analyses and the organizations of the nqr operon indicate that Na⁺-NQR evolved within the Chlorobi/Bacteroidetes group, after the duplication and subsequent neofunctionalization of the operon that encodes the homolog RNF complex. Subsequently, the nqr operon dispersed through multiple horizontal transfer events to other bacterial lineages such as Chlamydiae, Planctomyces and α, β, γ and δ -proteobacteria. Considering the biochemical properties of the Na⁺-NQR complex and its physiological role in different bacteria, we propose a detailed scenario to explain the molecular mechanisms that gave rise to its novel redox- dependent sodium -pumping activity. Our model postulates that the evolution of the Na⁺-NQR complex involved a functional divergence from its RNF homolog, following the duplication of the rnf operon, the loss of the rnfB gene and the recruitment of the reductase subunit of an aromatic monooxygenase.

Loss of NHE3 alters gut microbiota composition and influences Bacteroides thetaiotaomicron growth

Changes in the intestinal microbiota have been linked to diabetes, obesity, inflammatory bowel disease, and Clostridium difficile (C. difficile)-associated disease. Despite this, it remains unclear how the intestinal environment, set by ion transport, affects luminal and mucosa-associated bacterial composition. Na(+)/H(+)-exchanger isoform 3 (NHE3), a target of C. difficile toxin B, plays an integral role in intestinal Na(+) absorption. Thus the NHE3-deficient mouse model was chosen to examine the effect of pH and ion composition on bacterial growth. We hypothesized that ion transport-induced change in the intestinal environment would lead to alteration of the microbiota. Region-specific changes in ion composition and pH correlated with region-specific alteration of luminal and mucosal-associated bacteria with general decreases in Firmicutes and increases in Bacteroidetes members. Bacteroides thetaiotaomicron (B. thetaiotaomicron) increased in NHE3(-/-) terminal ileum and was examined in vitro to determine whether altered Na(+) was sufficient to affect growth. Increased in vitro growth of B. thetaiotaomicron occurred in 43 mM Na(+) correlating with the NHE3(-/-) mouse terminal ileum [Na(+)]. NHE3(-/-) terminal ileum displayed increased fut2 mRNA and fucosylation correlating with B. thetaiotaomicron growth. Inoculation of B. thetaiotaomicron in wild-type and NHE3(-/-) terminal ileum organoids displayed increased fut2 and fucosylation, indicating that B. thetaiotaomicron alone is sufficient for the increased fucosylation seen in vivo. These data demonstrate that loss of NHE3 alters the intestinal environment, leading to region-specific changes in bacteria, and shed light on the growth requirements of some gut microbiota members, which is vital for creating better treatments of complex diseases with an altered gut microbiota.

Sodium-translocating NADH:quinone oxidoreductase as a redox-driven ion pump

The Na+-translocating NADH:ubiquinone oxidoreductase (Na+-NQR) is a component of the respiratory chain of various bacteria. This enzyme is an analogous but not homologous counterpart of mitochondrial Complex I. Na+-NQR drives the same chemistry and also uses released energy to translocate ions across the membrane, but it pumps Na+ instead of H+. Most likely the mechanism of sodium pumping is quite different from that of proton pumping (for example, it could not accommodate the Grotthuss mechanism of ion movement); this is why the enzyme structure, subunits and prosthetic groups are completely special. This review summarizes modern knowledge on the structural and catalytic properties of bacterial Na+-translocating NADH:quinone oxidoreductases. The sequence of electron transfer through the enzyme cofactors and thermodynamic properties of those cofactors is discussed. The resolution of the intermediates of the catalytic cycle and localization of sodium-dependent steps are combined in a possible molecular mechanism of sodium transfer by the enzyme.

The Na+-translocating NADH : ubiquinone oxidoreductase of Azotobacter vinelandii negatively regulates alginate synthesis

Azotobacter vinelandii is a nitrogen-fixing soil bacterium that produces the exopolysaccharide alginate. In this report we describe the isolation and characterization of A. vinelandii strain GG4, which carries an nqrE : : Tn5 mutation resulting in alginate overproduction. The nqrE gene encodes a subunit of the Na+-translocating NADH : ubiquinone oxidoreductase (Na+-NQR). As expected, Na+-NQR activity was abolished in mutant GG4. When this strain was complemented with the nqrEF genes this activity was restored and alginate production was reduced to wild-type levels. Na+-NQR may be the main sodium pump of A. vinelandii under the conditions tested (∼2 mM Na+) since no Na+/H+-antiporter activity was detected. Collectively our results indicate that in A. vinelandii the lack of Na+-NQR activity caused the absence of a transmembrane Na+ gradient and an increase in alginate production.

The past and present of sodium energetics: may the sodium-motive force be with you

All living cells routinely expel Na(+) ions, maintaining lower concentration of Na(+) in the cytoplasm than in the surrounding milieu. In the vast majority of bacteria, as well as in mitochondria and chloroplasts, export of Na(+) occurs at the expense of the proton-motive force. Some bacteria, however, possess primary generators of the transmembrane electrochemical gradient of Na(+) (sodium-motive force). These primary Na(+) pumps have been traditionally seen as adaptations to high external pH or to high temperature. Subsequent studies revealed, however, the mechanisms for primary sodium pumping in a variety of non-extremophiles, such as marine bacteria and certain bacterial pathogens. Further, many alkaliphiles and hyperthermophiles were shown to rely on H(+), not Na(+), as the coupling ion. We review here the recent progress in understanding the role of sodium-motive force, including (i) the conclusion on evolutionary primacy of the sodium-motive force as energy intermediate, (ii) the mechanisms, evolutionary advantages and limitations of switching from Na(+) to H(+) as the coupling ion, and (iii) the possible reasons why certain pathogenic bacteria still rely on the sodium-motive force.

Regulation of expression of Na+ -translocating NADH:quinone oxidoreductase genes in Vibrio harveyi and Klebsiella pneumoniae

The expression of genes encoding sodium-translocating NADH:quinone oxidoreductase (Na(+)-NQR) was studied in the marine bacterium Vibrio harveyi and in the enterobacterium Klebsiella pneumoniae. It has been shown that such parameters as NaCl concentration, pH value, and presence of an uncoupler in the growth media do not influence significantly the level of nqr expression. However, nqr expression depends on the growth substrates used by these bacteria. Na(+)-NQR is highly repressed in V. harveyi during anaerobic growth, and nqr expression is modulated by electron acceptors and values of their redox potentials. The latter effect was shown to be independent of the ArcAB regulatory system.

Amiloride resistance in the methanoarcheon Methanothermobacter thermoautotrophicus: characterization of membrane-associated proteins

An amiloride-resistant mutant with diminished Na+/H+ antiporter activity was isolated from Methanothermobacter thermoautotrophicus. To define the protein basis of amiloride resistance, the composition of membrane-associated proteins was partially characterized and compared with that of the wild type strain. An abundant 670-kDa membrane-associated protein that was present only in the mutant strain was analyzed by MALDI-TOF MS and identified as a coenzyme F420-reducing hydrogenase. The amiloride resistance was not accompanied by changes in protein size or changes in the level of subunits A or B of the A1A0-type ATP synthase; on the other hand, the SDS-PAGE patterns of the chloroform-methanol extract of membranes from both strains were different. Two bands with calculated molecular mass 16 and 11 kDa were identified as MtrD and AtpK, respectively. The observed over-expression of a 22.7-kDa protein in the mutant cells may represent the multimeric form of the MtrD subunit. These results show that the impairment of the Na+/H+ antiporter system in the amiloride-resistant mutant of Methanothermobacter thermoautotrophicus is accompanied by only small changes in a few membrane-associated proteins.

Analysis of the topology of Vibrio cholerae NorM and identification of amino acid residues involved in norfloxacin resistance

NorM, a putative efflux pump of Vibrio cholerae, is a member of the multidrug and toxic compound extrusion family of transporters. We demonstrate that NorM confers resistance to norfloxacin, ciprofloxacin, and ethidium bromide. Inactivation of norM rendered V. cholerae hypersensitive towards these fluoroquinolones. Multiple sequence alignment of members of its family identified several regions of high sequence conservation. The topology of NorM was determined using beta-lactamase and chloramphenicol acetyltransferase fusions. The amino acid residues G(184), K(185), G(187), P(189), E(190), G(192), and G(195) in the periplasmic loops and L(381), R(382), G(383), Y(384), K(385), and D(386) in the cytoplasmic loops, as well as all the acidic and cysteine residues of NorM, were mutated. Mutants G184V, G184W, K185I, P189S, E190K, and E190A lost the norfloxacin resistance-imparting phenotype characteristic of NorM. Mutants E124V, D155V, G187V, G187R, C196S, Y384H, Y384S, and Y384F exhibited partial resistance to norfloxacin. Mutants with replacements of G(184) or G(187) by A, K(185) by R, and E(190) by D retained the norfloxacin resistance phenotype of NorM. Analysis of the accumulation of norfloxacin in intact cells of Escherichia coli expressing NorM or its mutants in the presence or absence of carbonyl cyanide m-chlorophenylhydrazone supported the results obtained through susceptibility testing and argued in favor of NorM-mediated efflux as the determining factor in norfloxacin susceptibility in the genetically manipulated strains. Taken together, these results suggested that E(124), D(155), G(184), K(185), G(187), P(189), E(190), C(196), and Y(384) are likely involved in NorM-dependent norfloxacin efflux. Except for D(155), C(196), and Y(384), all of these residues are located in periplasmic loops.

Na(+)-Translocating NADH:quinone oxidoreductase: progress achieved and prospects of investigations

Structural and catalytic properties of bacterial Na+-translocating NADH:quinone oxidoreductases are briefly described. Special attention is given to studies on kinetics of the enzyme interaction with NADH and the role of sodium ions in this process. Based on the existing data, possible model mechanisms of sodium transfer by Na+-translocating NADH:quinone oxidoreductase are proposed.

Salt in the wound: a possible role of Na+ gradient in chlamydial infection

Genome analysis has revealed the presence of key components of the Na(+) chemiosmotic cycle, including the primary Na(+) pump (Na(+)-translocating NADH:ubiquinone oxidoreductase), in the cytoplasmic membrane of two ubiquitous human pathogens, Chlamydia trachomatis and Chlamydiophyla pneumoniae. This observation seemed paradoxical in the case of obligatory intracellular parasites because the Na(+) cycle is thought to be primarily a mechanism that enhances the adaptive potential in free-living bacteria that are often facing drastic changes in the salinity and pH of the environment. We present a model suggesting that operation of the Na(+) cycle may play an important role in the course of chlamydial infection, when the Na(+) and H(+) homeostasis of the host cell become severely impaired. This introduces the intriguing possibility of the application of drugs targeting Na(+)-transporting enzymes to chlamydial infections, which are notoriously difficult to treat.

Ion motive force dependence of protease secretion and phage tranduction inVibrio choleraeandPseudomonas aeruginosa

Vibrio cholerae is known to secrete a large number of proteins into the extracellular milieu, including the important virulence factor cholera toxin (CT). However, one of the most abundant proteins found in V. cholerae supernatants is the zinc-metalloprotease HA/protease (HAP). Whereas efficient protein secretion in Escherichia coli requires ATP hydrolysis and the proton motive force (pmf), little is known about the energy requirements for protein secretion in V. cholerae. To analyze some of the energy requirements for protein secretion in V. cholerae, HAP accumulation in culture supernatants following growth in the presence of various ionophores was assayed. Extracellular production of HAP was strongly reduced in the presence of monensin, an artificial Na(+)/H(+) antiporter that collapses the DeltapNa(+) across the membrane without affecting Deltapsi, whereas the protonophore CCCP had no significant effect on the extracellular accumulation of HAP. In contrast, extracellular protease production in Pseudomonas aeruginosa was affected by CCCP, but not monensin. Furthermore, extracellular protease production of V. cholerae, but not P. aeruginosa, was increased in increasing amounts of NaCl in the culture medium. Together these results indicate that the V. cholerae HAP requires an intact sodium motive force (smf) for its efficient translocation across the membranes, whereas extracellular protease production by P. aeruginosa requires only pmf. As the entry of some bacteriophage genomes has been reported to require pmf, the effects of ionophores on the efficiency of tranduction of V. cholerae by the CTXPhi phage were analyzed. CTXPhi transduction was strongly affected by CCCP, but not monensin, suggesting that phage entry requires pmf but not smf. Understanding the energy requirements for these potentially important virulence aspects of pathogens might lead to novel intervention strategies.

Sodium ion cycle in bacterial pathogens: evidence from cross-genome comparisons

Analysis of the bacterial genome sequences shows that many human and animal pathogens encode primary membrane Na+ pumps, Na+-transporting dicarboxylate decarboxylases or Na+ translocating NADH:ubiquinone oxidoreductase, and a number of Na+ -dependent permeases. This indicates that these bacteria can utilize Na+ as a coupling ion instead of or in addition to the H+ cycle. This capability to use a Na+ cycle might be an important virulence factor for such pathogens as Vibrio cholerae, Neisseria meningitidis, Salmonella enterica serovar Typhi, and Yersinia pestis. In Treponema pallidum, Chlamydia trachomatis, and Chlamydia pneumoniae, the Na+ gradient may well be the only energy source for secondary transport. A survey of preliminary genome sequences of Porphyromonas gingivalis, Actinobacillus actinomycetemcomitans, and Treponema denticola indicates that these oral pathogens also rely on the Na+ cycle for at least part of their energy metabolism. The possible roles of the Na+ cycling in the energy metabolism and pathogenicity of these organisms are reviewed. The recent discovery of an effective natural antibiotic, korormicin, targeted against the Na+ -translocating NADH:ubiquinone oxidoreductase, suggests a potential use of Na+ pumps as drug targets and/or vaccine candidates. The antimicrobial potential of other inhibitors of the Na+ cycle, such as monensin, Li+ and Ag+ ions, and amiloride derivatives, is discussed.

Redundancy of aerobic respiratory chains in bacteria? Routes, reasons and regulation

Bacteria are the most remarkable organisms in the biosphere, surviving and growing in environments that support no other life forms. Underlying this ability is a flexible metabolism controlled by a multitude of environmental sensors and regulators of gene expression. It is not surprising, therefore, that bacterial respiration is complex and highly adaptable: virtually all bacteria have multiple, branched pathways for electron transfer from numerous low-potential reductants to several terminal electron acceptors. Such pathways, particularly those involved in anaerobic respiration, may involve periplasmic components, but the respiratory apparatus is largely membrane-bound and organized such that electron flow is coupled to proton (or sodium ion) transport, generating a protonmotive force. It has long been supposed that the multiplicity of pathways serves to provide flexibility in the face of environmental stresses, but the existence of apparently redundant pathways for electrons to a single acceptor, say dioxygen, is harder to explain. Clues have come from studying the expression of oxidases in response to growth conditions, the phenotypes of mutants lacking one or more oxidases, and biochemical characterization of individual oxidases. Terminal oxidases that share the essential properties of substrate (cytochrome c or quinol) oxidation, dioxygen reduction and, in some cases, proton translocation, differ in subunit architecture and complement of redox centres. Perhaps more significantly, they differ in their affinities for oxidant and reductant, mode of regulation, and inhibitor sensitivity; these differences to some extent rationalize the presence of multiple oxidases. However, intriguing requirements for particular functions in certain physiological functions remain unexplained. For example, a large body of evidence demonstrates that cytochrome bd is essential for growth and survival under certain conditions. In this review, the physiological basis of the many phenotypes of Cyd-mutants is explored, particularly the requirement for this oxidase in diazotrophy, growth at low protonmotive force, survival in the stationary phase, and resistance to oxidative stress and Fe(III) chelators.

Sequencing and preliminary characterization of the Na+-translocating NADH:ubiquinone oxidoreductase from Vibrio harveyi

The Na(+)-translocating NADH: ubiquinone oxidoreductase (Na(+)-NQR) generates an electrochemical Na(+) potential driven by aerobic respiration. Previous studies on the enzyme from Vibrio alginolyticus have shown that the Na(+)-NQR has six subunits, and it is known to contain FAD and an FeS center as redox cofactors. In the current work, the enzyme from the marine bacterium Vibrio harveyi has been purified and characterized. In addition to FAD, a second flavin, tentatively identified as FMN, was discovered to be covalently attached to the NqrC subunit. The purified V. harveyi Na(+)-NQR was reconstituted into proteoliposomes. The generation of a transmembrane electric potential by the enzyme upon NADH:Q(1) oxidoreduction was strictly dependent on Na(+), resistant to the protonophore CCCP, and sensitive to the sodium ionophore ETH-157, showing that the enzyme operates as a primary electrogenic sodium pump. Interior alkalinization of the inside-out proteoliposomes due to the operation of the Na(+)-NQR was accelerated by CCCP, inhibited by valinomycin, and completely arrested by ETH-157. Hence, the protons required for ubiquinol formation must be taken up from the outside of the liposomes, which corresponds to the bacterial cytoplasm. The Na(+)-NQR operon from this bacterium was sequenced, and the sequence shows strong homology to the previously reported Na(+)-NQR operons from V. alginolyticus and Haemophilus influenzae. Homology studies show that a number of other bacteria, including a number of pathogenic species, also have an Na(+)-NQR operon.

NorM, a putative multidrug efflux protein, of Vibrio parahaemolyticus and its homolog in Escherichia coli

We found that cells of Vibrio parahaemolyticus possess an energy-dependent efflux system for norfloxacin. We cloned a gene for a putative norfloxacin efflux protein from the chromosomal DNA of V. parahaemolyticus by using an Escherichia coli mutant lacking the major multidrug efflux system AcrAB as the host and sequenced the gene (norM). Cells of E. coli transformed with a plasmid carrying the norM gene showed elevated energy-dependent efflux of norfloxacin. The transformants showed elevated resistance not only to norfloxacin and ciprofloxacin but also to the structurally unrelated compounds ethidium, kanamycin, and streptomycin. These results suggest that this is a multidrug efflux system. The hydropathy pattern of the deduced amino acid sequence of NorM suggested the presence of 12 transmembrane domains. The deduced primary structure of NorM showed 57% identity and 88% similarity with that of a hypothetical E. coli membrane protein, YdhE. No reported drug efflux protein in the sequence databases showed significant sequence similarity with NorM. Thus, NorM seems to be a novel type of multidrug efflux protein. We cloned the ydhE gene from E. coli. Cells of E. coli transformed with the cloned ydhE gene showed elevated resistance to norfloxacin, ciprofloxacin, acriflavine, and tetraphenylphosphonium ion, but not to ethidium, when MICs were measured. Thus, it seems that NorM and YdhE differ somehow in substrate specificity.

Specific cation binding site in mammalian cytochrome oxidase

Calcium ion binds reversibly with cytochrome c oxidase from beef heart mitochondria (Kd approximately 2 microM) shifting alpha- and gamma-absorption bands of heme a to the red. Two sodium ions compete with one Ca2+ for the binding site with an average dissociation constant square root[K1(Na) x K2(Na)] approximately 3.6 mM. The Ca2+-induced spectral shift of heme a is specific for mammalian cytochrome c oxidase and is not observed in bacterial or yeast aa3 oxidases although the Ca2+-binding site has been revealed in the bacterial enzyme [Ostermeier, C., Harrenga, A., Ermler, U. and Michel, H. (1997) Proc. Natl. Acad. Sci. USA 94, 10547-10553]. As His-59 and Gln-63 involved in Ca2+ binding with Subunit I of P. denitrificans oxidase are not conserved in bovine oxidase, these residues have to be substituted by alternative ligands in mammalian enzyme, which is indeed the case as shown by refined structure of bovine heart cytochrome oxidase (S. Yoshikawa, personal communication). We propose that it is interaction of Ca2+ with the species-specific ligand(s) in bovine oxidase that accounts for perturbation of heme a. The Ca2+/Na2+-binding site may be functionally associated with the exit part of 'pore B' proton channel in subunit I of mammalian cytochrome c oxidase.

Bacterial resistance to uncouplers

Uncoupler resistance presents a potential challenge to the conventional chemiosmotic coupling mechanism. In E. coli, an adaptive response to uncouplers was found in cell growing under conditions requiring oxidative phosphorylation. It is suggested that uncoupler-resistant mutants described in the earlier literature might represent a constitutive state of expression of this "low energy shock" adaptive response. In the environment, bacteria are confronted by nonclassical uncoupling factors such as organic solvents, heat, and extremes of pH. It is suggested that the low energy shock response will aid the cell in coping with the effects of natural uncoupling factors. The genetic analysis of uncoupler resistance has only recently began, and is yielding interesting and largely unexpected results. In Bacillus subtilis, a mutation in fatty acid desaturase causes an increased content of saturated fatty acids in the membrane and increased uncoupler resistance. The protonophoric efficiency of uncouplers remains unchanged in the mutants, inviting nonorthodox interpretations of the mechanism of resistance. In E. coli, two loci conferring resistance to CCCP and TSA were cloned and were found to encode multidrug resistance pumps. Resistance to one of the uncouplers, TTFB, remained unchanged in strains mutated for the MDRs, suggesting a resistance mechanism different from uncoupler extrusion.

Na(+)-driven ATP synthesis in Methanobacterium thermoautotrophicum and its differentiation from H(+)-driven ATP synthesis by rhodamine 6G

Rhodamine 6G (3 microM) effectively inhibited delta pH-driven ATP synthesis in Methanobacterium thermoautotrophicum while delta pNA-driven ATP synthesis was not affected by it. Rhodamine 6G inhibited Mg(2+)-stimulated ATPase activity of membrane vesicles prepared from these cells but the ATPase catalytic sector detached from the membrane was insensitive to this inhibitor. Methanogenesis-driven ATP synthesis at pH 6.8 of the cells grown in the presence of 50 mM NaCl was inhibited by rhodamine 6G both in the presence of 5 mM and 50 mM NaCl. On the other hand, the methanogenesis-driven ATP synthesis at pH 8.0 of cells grown in the presence of 50 mM NaCl was slightly inhibited by rhodamine 6G in the presence of 5 mM NaCl and was not inhibited at all in the presence of 50 mM NaCl. The growth experiments have shown that cells of Methanobacterium thermoautotrophicum can grow under alkaline conditions even in the presence of rhodamine 6G and of high NaCl concentration when the growth media were inoculated with the cells which had been grown in the presence of 50 mM NaCl. These results indicate that sodium-motive force-driven ATP synthase in Methanobacterium thermoautotrophicum operates effectively at alkaline conditions and it might be the sole ATP synthesizing system when the proton motive force-supported ATP synthesis is inhibited by rhodamine 6G.

Na(+)-driven ATP synthesis in Methanobacterium thermoautotrophicum and its differentiation from H(+)-driven ATP synthesis by rhodamine 6G

Rhodamine 6G (3 microM) effectively inhibited delta pH-driven ATP synthesis in Methanobacterium thermoautotrophicum while delta pNa-driven ATP synthesis was not affected by it. Rhodamine 6G inhibited Mg(2+)-stimulated ATPase activity of membrane vesicles prepared from these cells but the ATPase catalytic sector detached from the membrane was insensitive to this inhibitor. Methanogenesis-driven ATP synthesis at pH 6.8 of cells grown in the presence of 50 mM NaCl was inhibited by rhodamine 6G both in the presence of 5 mM and 50 mM NaCl. On the other hand, the methanogenesis-driven ATP synthesis at pH 8.0 of cells grown in the presence of 50 mM NaCl was slightly inhibited by rhodamine 6G in the presence of 5 mM NaCl and was not inhibited at all in the presence of 50 mM NaCl. The growth experiments have shown that cells of Methanobacterium thermoautotrophicum can grow under alkaline conditions even in the presence of rhodamine 6G and of high NaCl concentration when the growth media were inoculated with the cells which had been grown in the presence of 50 mM NaCl. These results indicate that sodium-motive force-driven ATP synthase in Methanobacterium thermoautotrophicum operates effectively in alkaline conditions and it might be the sole ATP synthesizing system when the proton-motive force-supported ATP synthesis is inhibited by rhodamine 6G.

The role of protonic and sodium potentials in the motility of E. coli and Bacillus FTU

The motility of Escherichia coli and of alkalo- and halotolerant Bacillus FTU has been studied. It is found that Bac. FTU motility (i) requires Na+, (ii) is resistant to the protonophorous uncoupler pentachlorophenol (PCP) if cells grow at high pH, and is sensitive to the uncouplers at neutral pH, (iii) is sensitized to the uncouplers with the addition of monensin, (iv) sensitive to amiloride and (v) can be supported by an artificially imposed Na+ gradient in the presence of uncoupler, cyanide and arsenate. On the other hand, E. coli motility (a) does not require Na+, (b) is always uncoupler-sensitive, (c) is amiloride-resistant, and (d) can be supported by an artificially-imposed gradient of H+, not Na+. It is concluded that the motilities of Bac. FTU and E. coli are due to the operation of the Na+ and the H+ motors, respectively. In Bac. FTU growing at alkaline pH, the Na+ motors are assumed to be energized by delta mu Na+ produced by the Na(+)-motive respiratory chain, and therefore delta mu H+ is not involved in the motility process. As to Bac. FTU growing in a neutral medium, delta mu Na+ is produced secondarily, via the Na+/H(+)-antiporter, i.e., at the expense of delta mu H+ formed by the H(+)-motive respiratory chain.

Amino acid transport in the thermophilic anaerobe Clostridium fervidus is driven by an electrochemical sodium gradient

Amino acid transport was studied in membranes of the peptidolytic, thermophilic, anaerobic bacterium Clostridium fervidus. Uptake of the negatively charged amino acid L-glutamate, the neutral amino acid L-serine, and the positively charged amino acid L-arginine was examined in membrane vesicles fused with cytochrome c-containing liposomes. Artificial ion diffusion gradients were also applied to establish the specific driving forces for the individual amino acid transport systems. Each amino acid was driven by the delta psi and delta mu Na+/F and not by the Z delta pH. The Na+ stoichiometry was estimated from the amino acid-dependent 22Na+ efflux and Na(+)-dependent 3H-amino acid efflux. Serine and arginine were symported with 1 Na+ and glutamate with 2 Na+. C. fervidus membranes contain Na+/Na+ exchange activity, but Na+/H+ exchange activity could not be demonstrated.

Annual review prize lecture. 'All hands to the sodium pump'

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The laws of cell energetics

Recent progress in membrane bioenergetics studies has resulted in the important discovery that Na+ can effectively substitute for H+ as the energy coupling ion. This means that living cells can possess three convertible energy currencies, i.e. ATP, protonic and sodium potentials. Analysis of interrelations of these components in various types of living cells allows bioenergetic laws of universal applicability to be inferred.

Involvement of a d-type oxidase in the Na(+)-motive respiratory chain of Escherichia coli growing under low delta mu H+ conditions

An attempt has been made to find out which of the two terminal oxidases, the d-type or the o-type, operates as a Na+ pump in Escherichia coli grown at low delta mu H+ conditions. For this purpose, mutants lacking either d or o oxidase have been studied. It is shown that a d-,o+ mutant grows slowly or does not grow at all under low delta mu H+ conditions (alkaline or protonophore-containing growth media were used). Inside-out subcellular vesicles from the d-,o+ mutant cannot oxidize ascorbate and TMPD, and cannot transport Na+ when succinate is oxidized in the presence of a protonophore. The same vesicles are found to transport Na+ when NADH is oxidized as if the Na(+)-motive NADH-quinone oxidase were operative. On the other hand, a mutant lacking o oxidase (d+,o-) grows at low delta mu H+ conditions as fast as the maternal E. coli strain containing both d and o oxidases. Corresponding vesicles oxidize ascorbate and TMPD as well as succinate, the oxidations being coupled to the protonophore-stimulated Na+ transport. Growth in the presence of a protonophore is found to induce a strong increase in the d oxidase level in the maternal d+,o+ E.coli strain. It is concluded that oxidase of the d-type, rather than of the o-type, operates as a Na+ pump in E. coli grown under conditions unfavorable for the H+ cycle.

Mode of sodium ion action on methanogenesis and ATPase of the moderate halophilic methanogenis bacterium Methanohalophilus halophilus

Cells of Methanohalophilus halophilus swelled when exposed to hypotonic solutions of NaCl at pH 7.0. The swelling of the cells ceased in the presence of Mg2+. Methane formation by non-growing cells was strongly dependent on the NaCl concentration. Among other monovalent and divalent cations only Li+ and Mg2+ could partly substitute for a specific function of sodium ions. The artificial Na+/H+ antiporter, monensin, exerted a strong inhibitory effect on methane formation from methylamine. The membrane-bound Mg(2+)-stimulated ATPase of these cells was enhanced at low (40 mM) NaCl concentration while higher concentrations of this solute were inhibitory. The results obtained show that sodium ions are a prerequisite for optimal methane formation and ATPase activity in these cells. However, both of these processes required different sodium ion concentrations.

Chemiosmotic systems in bioenergetics: H(+)-cycles and Na(+)-cycles

The development of membrane bioenergetic studies during the last 25 years has clearly demonstrated the validity of the Mitchellian chemiosmotic H+ cycle concept. The circulation of H+ ions was shown to couple respiration-dependent or light-dependent energy-releasing reactions to ATP formation and performance of other types of membrane-linked work in mitochondria, chloroplasts, some bacteria, tonoplasts, secretory granules and plant and fungal outer cell membranes. A concrete version of the direct chemiosmotic mechanism, in which H+ potential formation is a simple consequence of the chemistry of the energy-releasing reaction, is already proved for the photosynthetic reaction centre complexes. Recent progress in the studies on chemiosmotic systems has made it possible to extend the coupling-ion principle to an ion other than H+. It was found that, in certain bacteria, as well as in the outer membrane of the animal cell, Na+ effectively substitutes for H+ as the coupling ion (the chemiosmotic Na+ cycle). A precedent is set when the Na+ cycle appears to be the only mechanism of energy production in the bacterial cell. In the more typical case, however, the H+ and Na+ cycles coexist in one and the same membrane (bacteria) or in two different membranes of one and the same cell (animals). The sets of delta mu H+ and delta mu Na+ generators as well as delta mu H+ and delta mu Na+ consumers found in different types of biomembranes, are listed and discussed.

The effect of F0 inhibitors on the Vibrio alginolyticus membrane ATPase

The inhibition of membrane ATPase from the marine alkalotolerant bacterium Vibrio alginolyticus by DCCD, triphenyltin and venturicidin was studied. DCCD proved to be an irreversible inhibitor, while venturicidin and triphenyltin produced a reversible inhibitory effect. The DCCD-binding proteolipid was identified in the membrane preparations. The effect of the inhibitors on ATPase activity and ATP-dependent Na(+)-transport in V. alginolyticus subcellular vesicles is discussed.

Release of the component of Streptococcus faecalis Na(+)-ATPase from the membranes

The Na(+)-stimulated ATPase activity of Streptococcus faecalis was lost by washing the membranes with ethylenediaminetetraacetic acid (EDTA). ATPase activities of both the EDTA extract and the stripped membranes did not show any stimulation by Na+ ions. However, the Na(+)-stimulated ATPase was readily reconstituted by an incubation of these fractions combined. It was only reconstituted from the fractions prepared under the condition that the Na(+)-ATPase is amplified, and not from those boiled or digested by trypsin. Thus, the component of Na(+)-ATPase of this organism is capable of being released from the membranes.

Light-dependent delta mu Na-generation and utilization in the marine cyanobacterium Oscillatoria brevis

Light-dependent Na+ and H+ transports, membrane potential (delta psi) and motility have been studied in the cells of the marine cyanobacterium Oscillatoria brevis. In the presence of a protonophorous uncoupler, carbonyl cyanide-m-chlorophenylhydrazone, the intracellular Na+ level is shown to increase in the dark and decrease in the light. The Na+/H+ antiporter, monensin, stimulates the dark CCCP-dependent [Na+]in increase and abolishes the light-dependent [Na+]in decrease. Na+ ions are necessary for the fast light-induced delta psi generation and H+ uptake by the cells. This uptake is inhibited by monensin being resistant to CCCP. Monensin sensitizes the delta psi level and the motility rate to low CCCP concentrations. The obtained data are consistent with the assumption that O. brevis possesses a primary Na+ pump which utilizes (directly or indirectly) the light energy.