The Na+-motive terminal oxidase activity in an alkalo- and halo-tolerant Bacillus

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.

Cytochrome cbb3 of Thioalkalivibrio is a Na+-pumping cytochrome oxidase

Cytochrome c oxidases (Coxs) are the basic energy transducers in the respiratory chain of the majority of aerobic organisms. Coxs studied to date are redox-driven proton-pumping enzymes belonging to one of three subfamilies: A-, B-, and C-type oxidases. The C-type oxidases (cbb3 cytochromes), which are widespread among pathogenic bacteria, are the least understood. In particular, the proton-pumping machinery of these Coxs has not yet been elucidated despite the availability of X-ray structure information. Here, we report the discovery of the first (to our knowledge) sodium-pumping Cox (Scox), a cbb3 cytochrome from the extremely alkaliphilic bacterium Thioalkalivibrio versutus. This finding offers clues to the previously unknown structure of the ion-pumping channel in the C-type Coxs and provides insight into the functional properties of this enzyme.

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.

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.

Bacterial energetics at high pH: what happens to the H+ cycle when the extracellular H+ concentration decreases?

A decrease in the extracellular H+ concentration creates difficulties for membrane-linked energetics in bacteria employing H+ as the coupling ion. At high extracellular pH (pHo), H+ ions pumped from the cell by, say, the respiratory chain, are immediately neutralized by the alkaline extracellular medium. Under such conditions, the only driving force that might compel outer H+ ions to return to cytosol and perform their function is the electric potential difference across the cytoplasmic membrane (delta psi). However, when delta pH in the opposite direction is equal to, e.g., 2 pH units (intracellular pH = 7.5 at pHo = 9.5), delta psi would be so high that the risk of membrane electric breakdown would increase. This is why some bacteria deal with high pH by, for example, replacing H+ by Na+ as the coupling ion rather than by increasing delta psi. It has been shown in several species of bacteria that the alkalinization of the growth medium induces primary Na+ pumps (e.g. Na(+)-motive respiratory chain enzymes and Na+ ATPase). Electrogenic Na+ efflux via these pumps produces an electrochemical Na+ potential difference (delta mu Na+) composed of delta psi and delta pNa+. delta mu Na+ can be used to perform various types of membrane-linked work. The delta psi constituent of delta mu Na+ may maintain electrophoretic influx of H+ such that the alkalinization of cytoplasm is prevented. The latter function may be supported by a mechanism based on the uphill influx of Cl- instead of Na+. This seems to be the case for alkaliphilic and halophilic Natronobacter pharaonis. There is an indication that not only Na+ but also Ca2+ may substitute for H+ in Gleobacter violaceus growing at high pH.

Chemiosmotic concept of the membrane bioenergetics: what is already clear and what is still waiting for elucidation?

The present state of the chemiosmotic concept is reviewed. Special attention is paid to (i) further progress in studies on the Na(+)-coupled energetics and (ii) paradoxical bioenergetic effects when protonic or sodium potentials are utilized outside the coupling membrane (TonB-mediated uphill transports across the outer bacterial membrane). A hypothesis is put forward assuming that the same principle is employed in the bacterial flagellar motor.

Kinetics of CO binding to H(+)-motive oxidases of the caa3-type from Bacillus FTU and of the o-type from Escherichia coli

The kinetics of CO rebinding with isolated Bacillus FTU caa3-type oxidase and with solubilized Escherichia coli membranes (GO103 strain) containing the o-type oxidase as the main O2-reducing enzyme were studied under reducing conditions by laser flash photolysis of the CO-oxidase complexes. The spectra of the optical absorbance changes upon photolysis were characteristic of CO-caa3- and CO-o-oxidase complexes in Bac. FTU and E. coli, respectively. Small quantities of d-type oxidase in E. coli GO103 membranes were detected. The kinetics of CO reassociation with reduced caa3- and o-type oxidases were monophasic with tau 25-30 ms in both cases.

Kinetics of CO binding to putative Na(+)-motive oxidases of the o-type from Bacillus FTU and of the d-type from Escherichia coli

The kinetics of CO reassociation with isolated Bacillus FTU o-type oxidase and with solubilized membranes of Escherichia coli (GO102 strain) containing the d-type oxidase only, upon laser flash photolysis under reducing conditions, were studied. In both cases, kinetics are shown to be composed of three phases (tau 35-70 microseconds, 0.25-0.5 ms and 2-5 ms). The spectra of the flash-induced absorbance changes of the first kinetic components proved to be characteristic of CO-o- and CO-b595 d-cytochrome complexes in Bac. FTU and E. coli, respectively. The spectra of the second and the third components appeared to be nearly the same in Bac. FTU and E. coli with peaks for the former at 436-437 and 590 nm and troughs at 419-420 and 569 nm; and for the latter with peaks at 436-437 and 558-560 nm and troughs at 419-420 and 575-578 nm. The similarity between the putative Na(+)-pumping Bac. FTU o- and E. coli d-type oxidases and their difference from the H(+)-motive Bac. FTU caa3- and E. coli o-type oxidases are discussed.

Na(+)-translocating NADH-quinone reductase of marine and halophilic bacteria

The respiratory chain of marine and moderately halophilic bacteria requires Na+ for maximum activity, and the site of Na(+)-dependent activation is located in the NADH-quinone reductase segment. The Na(+)-dependent NADH-quinone reductase purified from marine bacterium Vibrio alginolyticus is composed of three subunits, alpha, beta, and gamma, with apparent M(r) of 52, 46, and 32 kDa, respectively. The FAD-containing beta-subunit reacts with NADH and reduces ubiquinone-1 (Q-1) by a one-electron transfer pathway to produce ubisemiquinones. In the presence of the FMN-containing alpha-subunit and the gamma-subunit, Q-1 is converted to ubiquinol-1 without the accumulation of free radicals. The reaction catalyzed by the alpha-subunit is strictly dependent on Na+ and is strongly inhibited by 2-n-heptyl-4-hydroxyquinoline N-oxide (HQNO), which is tightly coupled to the electrogenic extrusion of Na+. A similar type of Na(+)-translocating NADH-quinone reductase is widely distributed among marine and moderately halophilic bacteria. The respiratory chain of V. alginolyticus contains another NADH-quinone reductase which is Na+ independent and has no energy-transducing capacity. These two types of NADH-quinone reductase are quite different with respect to their mode of quinone reduction and their sensitivity toward NADH preincubation.

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.

Proton-coupled bioenergetic processes in extremely alkaliphilic bacteria

Oxidative phosphorylation, which involves an exclusively proton-coupled ATP synthase, and pH homeostasis, which depends upon electrogenic antiport of cytoplasmic Na+ in exchange for H+, are the two known bioenergetic processes that require inward proton translocation in extremely alkaliphilic bacteria. Energy coupling to oxidative phosphorylation is particularly difficult to fit to a strictly chemiosmotic model because of the low bulk electrochemical proton gradient that follows from the maintenance of a cytoplasmic pH just above 8 during growth at pH 10.5 and higher. A large quantitative and variable discrepancy between the putative chemiosmotic driving force and the phosphorylation potential results. This is compounded by a nonequivalence between respiration-dependent bulk gradients and artificially imposed ones in energizing ATP synthesis, and by an apparent requirement for specific respiratory chain complexes that do not relate solely to their role in generation of bulk gradients. Special features of the synthase may contribute to the mode of energization, just as novel features of the Na+ cycle may relate to the extraordinary capacity of the extreme alkaliphiles to achieve pH homeostasis during growth at, or sudden shifts to, an external pH of 10.5 and above.

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.

Appearance of the Na(+)-motive terminal oxidase in Bacillus FTU grown under three different conditions lowering the delta mu H+ level

The terminal oxidases and coupled Na+ transport have been studied in intact cells and inside-out subcellucar vesicles of alkalo- and halotolerant Bacillus FTU grown under different conditions. Cells grown at pH 7.5 are shown to possess a system of respiration-dependent Na+ transport which is (i) inhibited by protonophorous uncoupler and (ii) activated by the delta psi-discharging agent valinomycin, suggesting that the Na+ transport is due to cooperation of the H(+)-motive oxidase and Na+/H+ antiporter. On the other hand, growth under conditions lowering the delta mu H+ level, namely (i) pH 8.6, (ii) pH 7.5 in the presence of protonophore, and (iii) pH 7.5 in the presence of low cyanide concentrations results in appearance of terminal oxidase-supported Na+ transport which is stimulated by protonophores (the Na(+)-motive oxidase). In all three cases, the appearance of ascorbate (+ TMPD) oxidation resistant to low and sensitive to high cyanide concentrations was found to occur. It is concluded that not only alkaline pH but also other conditions which lower delta mu H+ can cause substitution of Na+ for H+ as a coupling ion.

The H(+)-motive and Na(+)-motive respiratory chains in Bacillus FTU subcellular vesicles

Respiration-dependent pumping of Na+ and H+ into the inside-out subcellular vesicles of alkalotolerant and halotolerant Bacillus FTU grown at alkaline pH was studied. The vesicles were shown to be competent in Na+ and H+ transport coupled to ascorbate oxidation via N,N,N',N'-tetramethyl-p-phenylenediamine or diaminodurene. The uphill Na+ uptake is strongly stimulated by either protonophores or valinomycin, whereas H+ uptake is stimulated by valinomycin and completely inhibited by protonophores. The salt of a penetrating weak base and of the penetrating weak acid, diethylammonium acetate, potentiates the stimulating effect of protonophores on Na+ uptake and abolishes H+ uptake. Na+ transport, supported by ascorbate oxidation, is resistant to 2-heptyl-4-hydroxyquinoline N-oxide, but sensitive to Ag+ and Na+ ionophore, N,N'-dibenzyl-N,N'-diphenyl-1,2-phenylenediacetamide. Micromolar concentrations of cyanide specifically inhibit the H+ uptake but does not affect Na+ uptake. These cyanide concentrations are shown to cause 70% inhibition of respiration, complete reduction of alpha-type cytochromes and partial reduction of c/b-type cytochromes. To inhibit the remaining respiratory activity and Na/ uptake, approximately 100-fold higher cyanide concentrations are necessary. High cyanide concentrations cause some additional increase in absorbance in the region of cytochromes c and/or b. In the presence of a high cyanide concentration, Na+ uptake can be supported by NADH oxidation by fumarate. This Na+ transport is stimulated by protonophores and diethylammonium acetate, being sensitive to very low concentrations of 2-heptyl-4-hydroxyquinoline N-oxide and Ag+. The NADH-fumarate reductase reaction is also found to be competent in H+ uptake, which is inhibited by protonophores and by much higher 2-heptyl-4-hydroxyquinoline N-oxide concentrations, and is resistant to Ag+. It is inferred that Bacillus FTU possesses two respiratory chains: the H(+)-motive and the Na(+)-motive, which strongly differ in their inhibitor sensitivities. Each chain comprises at least two energy-coupling sites which are localized in their initial and terminal segments. It has been indicated that common redox carrier(s) are present in the two chains.

A cytochrome that can pump sodium ion

Previous studies have shown that the bacterium, Vitreoscilla, generates a respiratory-driven delta psi Na+. Two major respiratory electron transport proteins, NADH dehydrogenase (NADH:Quinone oxidoreductase), and cytochrome o terminal oxidase are candidates for the electrogenic Na+ pumping that mediates the delta psi Na+ formation. The NADH oxidase activity of the membranes was enhanced more by Na+ than by Li+. The NADH:Quinone oxidoreductase activity in the respiratory chain was enhanced by Na+ and Li+, whereas the quinol oxidase activity of cytochrome o was enhanced specifically by Na+, and not by Li+, K+, or choline. Purified cytochrome o, reconstituted into Na(+)-loaded liposomes in the right-side-out orientation, catalyzed a net Na+ extrusion when energized with Q1H2(1). In nonloaded inside-out proteoliposomes, this cytochrome catalyzed a net uptake of 22Na+ when energized with ascorbate/TMPD. Both Na(+)-pumping activities were inhibited by CN-. These results are consistent with the Vitreoscilla cytochrome o being a redox-driven Na+ pump.

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.

Submicromolar Ag+ increases passive Na+ permeability and inhibits the respiration-supported formation of Na+ gradient in Bacillus FTU vesicles

The effect of Ag+ on Na+ pumping by Na(+)-motive NADH-quinone reductase and terminal oxidase has been studied in Bacillus FTU inside-out vesicles. Very low concentrations of Ag+ (C1/2 = 1 x 10(-8) M or 2 x 10(-12) g ion.mg protein-1) are shown to inhibit the uphill Na+ uptake coupled to the oxidation of NADH by fumarate or of ascorbate + TMPD by oxygen but exert no effect on the H+ uptake by the H(+)-motive respiratory chain. Low Ag+ also induces a specific increase in the Na+ permeability of the vesicles. HQNO, added before and not after Ag+, prevents the Ag(+)-induced permeability increase, with effective HQNO concentrations being similar to those inhibiting the uphill Na(+)-uptake coupled to the NADH-fumarate oxidoreduction. Reduction of terminal oxidase by ascorbate + TMPD in the presence of cyanide sensitizes the Na+ permeability to Ag+. It is suggested that low [Ag+], known as a specific inhibitor of electron transport by the Na(+)-motive NADH-quinone reductase, uncouples the electron and Na+ transports so that the Ag(+)-modified NADH-quinone reductase operates as an Na+ channel rather than an Na+ pump. This effect is discussed in connection with the antibacterial action of Ag+.

Uncoupler-resistant mutants of bacteria

The chemiosmotic model of energy transduction offers a satisfying and widely confirmed understanding of the action of uncouplers on such processes as oxidative phosphorylation; the uncoupler, by facilitating the transmembrane movement of protons or other compensatory ions, reduces the electrochemical proton gradient that is posited as the energy intermediate for many kinds of bioenergetic work. In connection with this formulation, uncoupler-resistant mutants of bacteria that neither exclude nor inactivate these agents represent a bioenergetic puzzle. Uncoupler-resistant mutants of aerobic Bacillus species are, in fact, membrane lipid mutants with bioenergetic properties that are indeed challenging in connection with the chemiosmotic model. By contrast, uncoupler-resistant mutants of Escherichia coli probably exclude uncouplers, sometimes only under rather specific conditions. Related phenomena in eucaryotic and procaryotic systems, as well as various observations on uncouplers, decouplers, and certain other membrane-active agents, are also briefly considered.

The sodium cycle: a novel type of bacterial energetics

The progress of bioenergetic studies on the role of Na+ in bacteria is reviewed. Experiments performed over the past decade on several bacterial species of quite different taxonomic positions show that Na+ can, under certain conditions, substitute for H+ as the coupling ion. Various primary Na+ pumps (delta mu Na+ generators) are described, i.e., Na+ -motive decarboxylases, NADH-quinone reductase, terminal oxidase, and ATPase. The delta mu Na+ formed is shown to be consumed by Na+ driven ATP-synthase, Na+ flagellar motor, numerous Na+, solute symporters, and the methanogenesis-linked reverse electron transfer system. In Vibrio alginolyticus, it was found that delta mu Na+, generated by NADH-quinone reductase, can be utilized to support all three types of membrane-linked work, i.e., chemical (ATP synthesis), osmotic (Na+, solute symports), and mechanical (rotation of the flagellum). In Propionigenum modestum, circulation of Na+ proved to be the only mechanism of energy coupling. In other species studied, the Na+ cycle seems to coexist with the H+ cycle. For instance, in V. alginolyticus the initial and terminal steps of the respiratory chain are Na+ - and H+ -motive, respectively, whereas ATP hydrolysis is competent in the uphill transfer of Na+ as well as of H+. In the alkalo- and halotolerant Bacillus FTU, there are H+ - and Na+ -motive terminal oxidases. Sometimes, the Na+ -translocating enzyme strongly differs from its H+ -translocating homolog. So, the Na+ -motive and H+ -motive NADH-quinone reductases are composed of different subunits and prosthetic groups. The H+ -motive and Na+ -motive terminal oxidases differ in that the former is of aa3-type and sensitive to micromolar cyanide whereas the latter is of another type and sensitive to millimolar cyanide. At the same time, both Na+ and H+ can be translocated by one and the same P. modestum ATPase which is of the F0F1-type and sensitive to DCCD. The sodium cycle, i.e., a system composed of primary delta mu Na+ generator(s) and delta mu Na+ consumer(s), is already described in many species of marine aerobic and anaerobic eubacteria and archaebacteria belonging to the following genera: Vibrio, Bacillus, Alcaligenes, Alteromonas, Salmonella, Klebsiella, Propionigenum, Clostridium, Veilonella, Acidaminococcus, Streptococcus, Peptococcus, Exiguobacterium, Fusobacterium, Methanobacterium, Methanococcus, Methanosarcina, etc. Thus, the "sodium world" seems to occupy a rather extensive area in the biosphere.