Respiration-driven Na+ pump and Na+ circulation in Vibrio parahaemolyticus

Tripartite ATP-independent periplasmic (TRAP) transporters in bacteria and archaea

The tripartite ATP-independent periplasmic (TRAP) transporters are the best-studied family of substrate-binding protein (SBP)-dependent secondary transporters and are ubiquitous in prokaryotes, but absent from eukaryotes. They are comprised of an SBP of the DctP or TAXI families and two integral membrane proteins of unequal sizes that form the DctQ and DctM protein families, respectively. The SBP component has a structure comprised of two domains connected by a hinge that closes upon substrate binding. In DctP-TRAP transporters, substrate binding is mediated through a conserved and specific arginine/carboxylate interaction in the SBP. While the SBP component has now been relatively well characterized, the membrane components of TRAP transporters are still poorly understood both in terms of their structure and function. We review the expanding repertoire of substrates and physiological roles for experimentally characterized TRAP transporters in bacteria and discuss mechanistic aspects of these transporters using data primarily from the sialic acid-specific TRAP transporter SiaPQM from Haemophilus influenzae, which suggest that TRAP transporters are high-affinity, Na(+)-dependent unidirectional secondary transporters.

Multidrug efflux transporters in the MATE family

The MATE (Multidrug And Toxic Compound Extrusion) family is the most recently categorized one among five multidrug efflux transporter families. As far as we know, about twenty MATE transporters have been characterized so far. According to the information in sequence databases, huge numbers of MATE transporters seem to be present in various microorganisms. In this review, we would like to summarize the properties of the MATE-family transporters.

Ins and outs of glucose transport systems in eubacteria

Glucose is the classical carbon source that is used to investigate the transport, metabolism, and regulation of nutrients in bacteria. Many physiological phenomena like nutrient limitation, stress responses, production of antibiotics, and differentiation are inextricably linked to nutrition. Over the years glucose transport systems have been characterized at the molecular level in more than 20 bacterial species. This review aims to provide an overview of glucose uptake systems found in the eubacterial kingdom. In addition, it will highlight the diverse and sophisticated regulatory features of glucose transport systems.

A primary sodium pump gene of the moderate halophile Halobacillus dabanensis exhibits secondary antiporter properties

The primary sodium pump has been proved to be involved in Na(+) extrusion of bacteria. In our present study, a novel gene encoding a putative primary sodium pump was cloned from chromosomal DNA of moderate halophile Halobacillus dabanensis D-8 by functional complementation, which expression resulted in the growth of antiporter-deficient Escherichia coli strain KNabc in the presence of 0.2 M NaCl. The gene was sequenced and designated nap. The deduced amino acid sequence of Nap has 56% identity to NADH dehydrogenase of Bacillus cereus and 55% to NADH oxidase of Bacillus halodurans C-125. E. coli KNabc carrying nap exhibited resistance to uncoupler CCCP (carbonyl-cyanide m-chlorophenylhydrazone). Everted membrane vesicles prepared from E. coli KNabc carrying nap exhibited secondary Na(+)/H(+) antiporter activity, and nap also supported the growth of respiratory-deficient E. coli ANN0222 lacking NADH dehydrogenase. Based on these results, we proposed that Nap possessed both characteristics of secondary Na(+)/H(+) antiporter and primary sodium pump.

Physiological roles of three Na+/H+ antiporters in the halophilic bacterium Vibrio parahaemolyticus

Vibrio parahaemolyticus mutants lacking three Na+/H+ antiporters (NhaA, NhaB, NhaD) were constructed. The DeltanhaA strains showed significantly higher sensitivity to LiCl regarding their growth compared to the parental strain. The DeltanhaA and DeltanhaB strains exhibited higher sensitivities to LiCl. The mutant XACabd lacking all of the three antiporters could not grow in the presence of 500 mM LiCl at pH 7.0, or 50 mM at pH 8.5. The XACabd mutant was also sensitive to 1.0 M NaCl at pH 8.5. These results suggest that Na+/H+ antiporters, especially NhaA, are responsible for resistance to LiCl and to high concentrations of NaCl. Reduced Na+/H+ and Li+/H+ antiport activities were observed with everted membrane vesicles of DeltanhaB strains. However, Li+/H+ antiport activities of DeltanhaB strains were two times higher than those of DeltanhaA strains when cells were cultured at pH 8.5. It seems that expression of nhaA and nhaB is dependent on medium pH to some extent. In addition, HQNO (2-heptyl-4-hydroxyquinoline N-oxide), which is a potent inhibitor of the respiratory Na+ pump, inhibited growth of XACabd, but not of the wild type strain. Moreover, survival rate of XACabd under hypoosmotic stress was lower than that of wild type strain. It is likely that the Na+/H+ antiporters are involved in osmoregulation under hypoosmotic stress. Based on these findings, we propose that the Na+/H+ antiporters cooperate with the respiratory Na+ pump in ionic homeostasis in V. parahaemolyticus.

Flagellar motors of marine bacteriaHalomonasare driven by both protons and sodium ions

Bacterial cells in aquatic environments are able to reach or stay near nutrient patches by using motility. Motility is usually attained by rotating flagellar motors that are energized by electrochemical potential of H+or Na+. In this paper, the ion specificity for flagellar rotation of two marine isolates Halomonas spp. strains US172 and US201 was investigated. Both isolates require sodium for growth and possess a respiratory-driven primary sodium pump. They are motile because of lateral flagella regardless of the presence of sodium ions. Their swimming speed under various concentrations of sodium ions with and without carbonylcyanide m-chlorophenylhydrazone, a proton conductor, and with and without phenamil, a specific inhibitor for the sodium-driven flagellar motors, was examined. The effect of carbonylcyanide m-chlorophenylhydrazone on the transmembrane proton gradient was also determined. Our results showed that the flagellar motors of the Halomonas strains were energized by both H+and Na+in one cell. The bimodal nature of Halomonas spp. motility with respect to the driving energy source may reflect ecophysiological versatility to adapt to a wide range of salt conditions of the marine environment.Key words: marine bacteria, Halomonas, flagellar motor, sodium, proton.

VmrA, a member of a novel class of Na(+)-coupled multidrug efflux pumps from Vibrio parahaemolyticus

Gene vmrA, cloned from Vibrio parahaemolyticus, made Escherichia coli resistant to 4',6-diamino-2-phenylindol, tetraphenylphosphonium chloride, acriflavine, and ethidium bromide. VmrA belongs to the DinF branch of MATE family efflux transporters. VmrA catalyzed acriflavine efflux and showed Na(+)/drug transporter activity because the addition of tetraphenylphosphonium to Na(+)-loaded cells caused Na(+) efflux.

NorM of vibrio parahaemolyticus is an Na(+)-driven multidrug efflux pump

NorM of Vibrio parahaemolyticus apparently is a new type of multidrug efflux protein, with no significant sequence similarity to any known transport proteins. Based on the following experimental results, we conclude that NorM is an Na(+)-driven Na(+)/drug antiporter. (i) Energy-dependent ethidium efflux from cells possessing NorM was observed in the presence of Na(+) but not of K(+). (ii) An artificially imposed, inwardly directed Na(+) gradient elicited ethidium efflux from cells. (iii) The addition of ethidium to cells loaded with Na(+) elicited Na(+) efflux. Thus, NorM is an Na(+)/drug antiporting multidrug efflux pump, the first to be found in the biological world. Judging from the similarity of the NorM sequence to those of putative proteins in sequence databases, it seems that Na(+)/drug antiporters are present not only in V. parahaemolyticus but also in a wide range of other organisms.

Detection of the Na+-translocating NADH-quinone reductase in marine bacteria using a PCR technique

To examine the distribution of the Na+-translocating NADH-quinone reductase (Na+-NQR) among marine bacteria, we developed a simple screening method for the detection of this enzyme. By reference to the homologous sequences of the Na+-NQR operons from Vibrio alginolyticus and Haemophilus influenzae, a pair of primers was designed for amplification of a part of the sixth ORF (nqr6) of the Na+-NQR operon. When PCR was performed using genomic DNA from 13 marine bacteria, a 0.9-kbp fragment corresponding to nqr6 was amplified in 10 strains. Although there were three PCR-negative strains phylogenetically, based on the sequence of the 16S rRNA, these were placed far from the PCR-positive strains. No product was observed in the case of nonmarine bacteria. The nucleotide and predicted amino acid sequences of nqr6 were highly conserved among the PCR-positive marine bacteria. A phylogenetic analysis of marine bacteria, based on nqr6 sequencing, was performed.Key words: Na+-translocating, NADH-quinone reductase, marine bacteria, PCR.

A putative multisubunit Na+/H+ antiporter from Staphylococcus aureus

We cloned several genes encoding an Na+/H+ antiporter of Staphylococcus aureus from chromosomal DNA by using an Escherichia coli mutant, lacking all of the major Na+/H+ antiporters, as the host. E. coli cells harboring plasmids for the cloned genes were able to grow in medium containing 0.2 M NaCl (or 10 mM LiCl). Host cells without the plasmids were unable to grow under the same conditions. Na+/H+ antiport activity was detected in membrane vesicles prepared from transformants. We determined the nucleotide sequence of the cloned 7-kbp region. We found that seven open reading frames (ORFs) were necessary for antiporter function. A promoter-like sequence was found in the upstream region from the first ORF. One inverted repeat followed by a T-cluster, which may function as a terminator, was found in the downstream region from the seventh ORF. Neither terminator-like nor promoter-like sequences were found between the ORFs. Thus, it seems that the seven ORFs comprise an operon and that the Na+/H+ antiporter consists of seven kinds of subunits, suggesting that this is a novel type of multisubunit Na+/H+ antiporter. Hydropathy analysis of the deduced amino acid sequences of the seven ORFs suggested that all of the proteins are hydrophobic. As a result of a homology search, we found that components of the respiratory chain showed sequence similarity with putative subunits of the Na+/H+ antiporter. We observed a large Na+ extrusion activity, driven by respiration in E. coli cells harboring the plasmid carrying the genes. The Na+ extrusion was sensitive to an H+ conductor, supporting the idea that the system is not a respiratory Na+ pump but an Na+/H+ antiporter. Introduction of the plasmid into E. coli mutant cells, which were unable to grow under alkaline conditions, enabled the cells to grow under such conditions.

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.

Bioenergetics of marine bacteria

Some marine bacteria have a special energy-transducing mechanism that is different from those found in Escherichia coli or most of the freshwater and terrestrial bacteria. These marine bacteria specifically require Na+ for their growth and utilize a Na+ circuit for various cellular functions. So far, three types of primary Na+ pump have been identified (i.e. respiration-dependent, decarboxylase-driven and Na+ ATP synthase). Among them, the first type of Na+ pump plays the major role in the marine environment. Recently, the gene sequence and distribution of this Na+ pump have been clarified. In addition, information on genetics and the ecological significance of Na+ driven flagellar motors has also been accumulating. This recent progress in the research of the 'Na+ world' is revealing an interesting way of life that is unique to marine microorganisms.

A new Na+/H+ antiporter, NhaD, of Vibrio parahaemolyticus

A gene encoding an Na+/H+ antiporter was cloned from chromosomal DNA of Vibrio parahaemolyticus, a slightly halophilic bacterium, and expressed in Escherichia coli cells. The gene enabled mutant E. coli cells, which were unable to grow in the presence of 10 mM LiCl (or 0.2 M NaCl) because of the lack of major Na+(Li+)/H+ antiporters, to grow under such conditions. We detected Na+/H+ antiport activity due to the gene in membrane vesicles prepared from E. coli cells that harbored the plasmid carrying the gene. Li+ was also a substrate for this antiporter. Activity of this antiporter was pH-dependent with highest activity at pH 8.5 to 9 and no activity at 7.0 to 7.5. Restriction mapping and a Southern blot analysis revealed that the cloned gene was different from the nhaA and the nhaB of V. parahaemolyticus. We designated the gene nhaD. The gene was sequenced, and the amino acid sequence of the NhaD protein was deduced. The NhaD is a unique Na+/H+ antiporter with respect to the primary structure compared with known Na+/H+ antiporters.

Mutational analysis of amiloride sensitivity of the NhaA Na+/H+ antiporter from Vibrio parahaemolyticus

The activity of the NhaA Na+/H+ antiporter of Vibrio parahaemolyticus is inhibited by amiloride. We found an amino acid sequence in the NhaA that was identical to a putative amiloride binding domain of the Na+/H+ exchanger in mammalian cells. We constructed mutant NhaAs that had amino acid substitutions in the putative amiloride binding domain by site-directed mutagenesis. These include V62L (Val62 replaced by Leu), F63Y, F64Y, and L65F. Most mutant NhaAs showed decreased sensitivity for amiloride. Among these, the F64Y mutant NhaA showed the least amiloride sensitivity, with a Ki value 7 to 10 times greater than that in the wild type. Thus, the sequence between residues V62 and L65 in NhaA, especially F64, is very important for the inhibitory effect of amiloride on the antiporter.

Primary structure and properties of the Na+/glucose symporter (Sg1S) of Vibrio parahaemolyticus

Previously, we cloned and sequenced a DNA fragment from Vibrio parahaemolyticus and found four open reading frames (ORFs). Here, we clearly demonstrate that one of the ORFs, ORF1, is the gene (sglS) encoding a Na+/glucose symporter (SglS). We characterize the Na+/glucose symporter produced in Escherichia coli mutant (JM1100) cells which lack original glucose transport activity and galactose transport activity. We also show that phlorizin, a potent inhibitor of the SGLT1 Na+/glucose symporter of animal cells, inhibited glucose transport, but not galactose transport, via the SglS system.

Sequence of a Na+/glucose symporter gene and its flanking regions of Vibrio parahaemolyticus

The nucleotide sequence of an approximately 6 kbp segment of chromosomal DNA of Vibrio parahaemolyticus was determined. The nucleotide sequence revealed four open reading frames (ORFs) in this region. Hydropathy profiles of the deduced amino acid sequence of the ORFs indicate that ORF1 encodes a hydrophobic polypeptide with typical characteristics of a membrane transport protein. All other ORFs encode hydrophilic polypeptides. ORF1 showed significant amino acid sequence similarity to proteins of the SGLT (Na+/glucose symporter) family, and the amino acid sequence of ORF4 showed very high similarity to several bacterial transcriptional repressor proteins (GalR-LacI family). We observed elevated glucose transport activity in cells harboring a plasmid carrying the DNA region corresponding to ORF1, and the glucose transport was greatly stimulated by Na+. Thus, we believe that ORF1 encodes a Na+/glucose symporter.

Properties of a Na+/galactose (glucose) symport system in Vibrio parahaemolyticus

We have investigated galactose transport in a mutant strain of Vibrio parahaemolyticus that lacks a glucose-PTS (phosphoenolpyruvate:carbohydrate phosphotransferase system) and a trehalose-PTS. Cells of the V. parahaemolyticus actively transported D-galactose and Na+ greatly stimulated the transport. Maximum stimulation of D-galactose transport activity was observed at 10mM NaCl, and Na+ could be replaced with Li+. Addition of galactose to the cell suspension under anaerobic conditions elicited Na+ uptake. Therefore, we conclude that this organism accomplishes galactose transport by a Na+/solute symport mechanism. Judging from inhibition results, D-galactose, D-glucose and to a lesser extent alpha-D-fucose are substrates of this transport system. The Na+/galactose symport system exhibited a high affinity for D-galactose (Km: 40 microM) and showed a relatively lower affinity for D-glucose (Km: 420 microM), but the maximum velocities for galactose and glucose transport were almost same (about 52 nmol/min per mg protein). The Na+/D-galactose symport system was induced by either D-galactose or alpha-D-fucose, and repressed by D-glucose.

Characterization of a glucose transport system in Vibrio parahaemolyticus

Cells of a glucose-PTS (phosphoenolpyruvate:carbohydrate phosphotransferase system)-negative mutant of Vibrio parahaemolyticus transport D-glucose in the presence of Na+. Maximum stimulation of D-glucose transport was observed at 40 mM NaCl, and Na+ could be replaced partially with Li+. Addition of D-glucose to the cell suspension under anaerobic conditions elicited Na+ uptake. Thus, we conclude that glucose is transported by a Na+/glucose symport mechanism. Calculated Vmax and Km values for the Na(+)-dependent D-glucose transport were 15 nmol/min/mg of protein and 0.57 mM, respectively, when NaCl was added at 40 mM. Na+ lowered the Km value without affecting the Vmax value. D-Glucose was the best substrate for this transport system, followed by galactose, alpha-D-fucose, and methyl-alpha-glucoside, judging from the inhibition pattern of the glucose transport. D-Glucose itself partly repressed the transport system when cells were grown in its presence.

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.

Catabolite repression of the H(+)-translocating ATPase in Vibrio parahaemolyticus

Cells of Vibrio parahaemolyticus grown in the presence of glucose showed reduced (by about 40%) oxidative phosphorylation. With this observation as a basis, we examined the effect of glucose on the level of H(+)-translocating ATPase. The addition of glucose to the growth medium reduced the specific activity and the amount of the H(+)-translocating ATPase in membrane vesicles of V. parahaemolyticus. These reductions were reversed by adding cyclic AMP (cAMP) to the growth medium. We cloned some parts of the unc genes encoding subunits of the H(+)-translocating ATPase of V. parahaemolyticus by means of the polymerase chain reaction. Using an amplified DNA fragment, we carried out Northern (RNA) blot analysis and found that glucose reduced the mRNA level of the H(+)-translocating ATPase gene by about 40% and that cAMP restored it. We determined the DNA sequence of the unc promoter region of V. parahaemolyticus and found a consensus sequence for the cAMP receptor protein-cAMP-binding site. Such a sequence was also found in the promoter region of the unc operon of Vibrio alginolyticus but not in its counterpart in Escherichia coli. We observed a similar reduction in the level of ATPase due to glucose in V. alginolyticus. In E. coli, however, reductions in the ATPase and the unc mRNA levels were not observed. Thus, the unc operon is controlled by cAMP-regulated catabolite repression in V. parahaemolyticus and V. alginolyticus but not in E. coli. Catabolite repression of the unc operon in V. parahaemolyticus is not severe.

Energy transduction in the methanogen Methanococcus voltae is based on a sodium current

We provide experimental support for the proposal that ATP production in Methanococcus voltae, a methanogenic member of the archaea, is based on an energetic system in which sodium ions, not protons, are the coupling ions. We show that when grown at a pH of 6.0, 7.1, or 8.2, M. voltae cells maintain a membrane potential of approximately -150 mV. The cells maintain a transmembrane pH gradient (pH(in) - pH(out)) of -0.1, -0.2, and -0.2, respectively, values not favorable to the inward movement of protons. The cells maintain a transmembrane sodium concentration gradient (sodium(out)/sodium(in)) of 1.2, 3.4, and 11.6, respectively. While the protonophore 3,3',4',5-tetrachlorosalicylanilide inhibits ATP formation in cells grown at pH 6.5, neither ATP formation nor growth is inhibited in cells grown in medium at pH 8.2. We show that when grown at pH 8.2, cells synthesize ATP in the absence of a favorably oriented proton motive force. Whether grown at pH 6.5 or pH 8.2, M. voltae extrudes Na+ via a primary pump whose activity does not depend on a proton motive force. The addition of protons to the cells leads to a harmaline-sensitive efflux of Na+ and vice versa, indicating the presence of Na+/H+ antiporter activity and, thus, a second mechanism for the translocation of Na+ across the cell membrane. M. voltae contains a membrane component that is immunologically related to the H(+)-translocating ATP synthase of the archaeabacterium Sulfolobus acidocaldarius. Since we demonstrated that ATP production can be driven by an artificially imposed membrane potential only in the presence of sodium ions, we propose that ATP production in M. voltae is mediated by an Na+-translocating ATP synthase whose function is coupled to a sodium motive force that is generated through a primary Na+ pump.

Polar and lateral flagellar motors of marine Vibrio are driven by different ion-motive forces

Various species of marine Vibrio produce two distinct types of flagella, each adapted for a different type of motility. A single, sheathed polar flagellum is suited for swimming in liquid medium, and numerous unsheathed lateral flagella, which are produced only under viscous conditions, are suited for swarming over viscous surfaces. Both types of flagella are driven by reversible motors embedded in the cytoplasmic membrane. Here we report that the energy source for the polar flagellar motor of Vibrio parahaemolyticus is the sodium-motive force, whereas the lateral flagellar motors are driven by the proton-motive force. This is evidence that two distinct types of flagella powered by different energy sources are functionally active in one cell.

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.

A novel mechanism of cation/substrate cotransport: Na+/H+/adenosine cotransport in Vibrio parahaemolyticus

Adenosine is actively transported with Na+ in Vibrio parahaemolyticus (Sakai, Y., Tsuda, M., Tsuchiya, T. (1987) Biochim, Biophys. Acta 893, 43-48). The proton conductor carbonylcyanide m-chlorophenylhydrazone, CCCP, strongly inhibited active transport of adenosine at pH 8.5 as well as at pH 7.0. This seemed peculiar because the driving force, an electrochemical potential of Na+, is established by the Na(+)-extruding respiratory chain at pH 8.5 in this organism, although it is established by the function of the Na+/H+ antiporter at pH 7.0. This suggested that H+ might be involved in the adenosine transport. We detected H+ uptake induced by adenosine influx in V. parahaemolyticus cells in the presence of Na+, but not in its absence, suggesting the occurrence of Na+/H+/adenosine cotransport. We isolated formycin A-resistant mutants which showed defective adenosine transport. The mutation resulted in simultaneous losses of Na+ uptake and H+ uptake induced by adenosine. In revertants from these mutants the Na+ uptake and H+ uptake were restored simultaneously. The frequencies of reversion were in the order of 10(-7), indicating that the mutations were single mutations; namely that Na+/adenosine cotransport and H+/adenosine cotransport took place via the same carrier. Thus, we conclude that adenosine is transported by the novel mechanism of Na+/H+/adenosine cotransport in V. parahaemolyticus.

Na(+)-coupled ATP synthesis in a mutant of Vibrio parahaemolyticus lacking H(+)-translocating ATPase activity

An H(+)-translocating ATPase-defective mutant of Vibrio parahaemolyticus YS-1 grew well on lactate as a sole source of carbon at pH 8.5 under aerobic conditions, but not under anaerobic conditions. Both wild type cells and the mutant cells could grow on lactate at pH 8.5 even in the presence of an H+ conductor, carbonylcyanide m-chlorophenylhydrazone (CCCP), but not at pH 7.5. Oxidative phosphorylation resistant to CCCP in the mutant occurred at pH 8.5. These findings suggest the existence of Na(+)-coupled oxidative phosphorylation which is functional at alkaline pHs in V. parahaemolyticus. In fact, we observed ATP synthesis driven by an artificially imposed Na+ gradient in YS-1 cells, which was resistant to CCCP.

Correlation between Possession of a Respiration-Dependent Na Pump and Na Requirement for Growth of Marine Bacteria

The possession of a respiration-dependent primary sodium pump and the requirement of Na for growth were investigated in bacterial isolates from marine environments. The bacteria in which NADH oxidase specifically required Na for maximum activity were believed to possess a primary sodium pump. All bacteria that failed to grow without the addition of NaCl possessed a primary Na pump. All bacteria that had no primary Na pump grew without additional NaCl. The primary Na pump seems to be involved in the Na requirement of marine bacteria, and this can be regarded as a criterion for the definition of marine bacteria.

Rapid purification and characterization of F1-ATPase of Vibrio parahaemolyticus

The F1 portion of H(+)-translocating ATPase as purified from membrane vesicles of Vibrio parahaemolyticus by a rapid procedure. The whole purification process (from culture of cells to purification of the enzyme) could be completed in 1 day. The F1-ATPase consists of five subunits (alpha, beta, gamma, delta and epsilon) like F1 of Escherichia coli and other microorganisms. The F1-ATPase of V. parahaemolyticus showed some interesting properties. Its activity was greatly stimulated by high concentrations (about 0.5 M) of SO4(2-), SO3(2-) and CH3COO-, their effects decreasing in this order. Among the anions tested, Cl- and NO3- were ineffective, or rather inhibitory, and cations had no significant effects. Ethanol (or methanol) stimulated the activity 2- to 3-fold. The activity was inhibited by 4-acetamido-4'-isothiocyanostilbene 2,2'-disulfonate (SITS) (an anion exchanger inhibitor), tetrachlorosalicylanilide (TCS) (an H+ conductor), azide and N-ethylmaleimide. Zinc inhibited the activity only slightly, although it strongly inhibited the ATPase activity in membrane vesicles.

Respiratory Na+ pump and Na+-dependent energetics in Vibrio alginolyticus

The marine bacterium Vibrio alginolyticus was found to possess the respiratory Na+ pump that generates an electrochemical potential of Na+, which plays a central role in bioenergetics of V. alginolyticus, as a direct result of respiration. Mutants defective in the Na+ pump revealed that one of the two kinds of NADH: quinone oxidoreductase requires Na+ for activity and functions as the Na+ pump. The Na+ pump composed of three subunits was purified and reconstituted into liposomes. Generation of membrane potential by the reconstituted proteoliposomes required Na+. The respiratory Na+ pump coupled to the NADH: quinone oxidoreductase was found in wide varieties of Gram-negative marine bacteria belonging to the genera Alcaligenes, Alteromonas, and Vibrio, and showed a striking similarity in the mode of electron transfer and enzymic properties. Na+ extrusion seemed to be coupled to a dismutation reaction, which leads to the formation of quinol and quinone from semiquinone radical.

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.

Sodium-transport NADH-quinone reductase of a marine Vibrio alginolyticus

The respiratory chain of a marine bacterium, Vibrio alginolyticus, required Na+ for maximum activity, and the site of Na+ -dependent activation was localized on the NADH-quinone reductase segment. The Na+ -dependent NADH-quinone reductase extruded Na+ as a direct result of redox reaction. It was composed of three subunits, alpha, beta, and gamma, with apparent Mr of 52, 46, and 32 KDa, respectively. The reduction of ubiquinone-1 to ubiquinol proceeded via ubisemiquinone radicals. The former reaction was catalyzed by the FAD-containing beta subunit. This reaction showed no specific requirement for Na+. For the formation of ubiquinol, the presence of the gamma subunit and the FMN-containing alpha subunit was essential. The latter reaction specifically required Na+ for activity and was strongly inhibited by 2-n-heptyl-4-hydroxyquinoline N-oxide. It was assigned to the coupling site for Na+ transport. The mode of energy coupling of redox-driven Na+ pump was compared with those of decarboxylase- and ATP-driven Na+ pumps found in other bacteria.

The role of sodium ions in methanogenesis. Formaldehyde oxidation to CO2 and 2H2 in methanogenic bacteria is coupled with primary electrogenic Na+ translocation at a stoichiometry of 2-3 Na+/CO2

Cell suspensions of Methanosarcina barkeri were found to oxidize formaldehyde to CO2 and 2H2 (delta G0' = -27 kJ/mol CO2), when methanogenesis was inhibited by 2-bromoethanesulfonate. We report here that this reaction is coupled with (a) primary electrogenic Na+ translocation at a stoichiometry of 2-3 Na+/CO2, (b) with secondary H+ translocation via a Na+/H+ antiporter and (c) with ATP synthesis driven by an electrochemical proton potential. This is concluded from the following findings. Formaldehyde oxidation to CO2 and 2H2 was dependent on Na+ ions, 2-3 mol Na+/mol formaldehyde oxidized were extruded. Na+ translocation was inhibited by Na+ ionophores, but not affected by protonophores of Na+/H+ antiport inhibitors. Formaldehyde oxidation was associated with the build up of a membrane potential in the order of 100 mV (inside negative), which could be dissipated by sodium ionophores rather than by protonophores. Formaldehyde oxidation was coupled with ATP synthesis, which could be inhibited by Na+ ionophores, Na+/H+ antiport inhibitors, by protonophores and by the H+-translocating-ATP-synthase inhibitor, dicyclohexylcarbodiimide. With cell suspensions of Methanobacterium thermoautotrophicum similar results were obtained.

Bacterial Na+ energetics

Novel observations related to the Na+-linked energy transduction in bacterial membranes are considered. It is concluded that besides the well-known systems based on the circulation of protons, there are those based on the circulation of Na+. In some cases, H+ and Na+ cycles co-exist in one and the same membrane. Representatives of the 'sodium world', i.e. cells possessing primary Na+ pumps (delta mu Na generators and consumers) are found in many genera of bacteria. Among the delta mu Na generators, one should mention Na+-NADH-quinone reductase and Na+-terminal oxidase of the respiratory chain, Na+-decarboxylases and Na+-ATPases. For delta mu Na consumers, there are Na+-ATP-synthases, Na+-metabolite symporters and Na+ motors. Sometimes, one and the same enzyme can transport H+ or, alternatively, Na+. For instance, an Na+-ATP-synthase of the F0F1 type translocates H+ when Na+ is absent. Employment of the Na+ cycle, apart from or instead of the H+ cycle, increases the resistance of bacteria to alkaline or protonophore-containing media and, apparently, to some other unfavourable conditions.

Effect of carbonyl cyanide m-chlorophenylhydrazone on Escherichia coli halotolerance

The growth-inhibitory effect of carbonyl cyanide m-chlorophenylhydrazone (CCCP) was less on members of the family Enterobacteriaceae (halotolerant organisms) than it was on species of Vibrio (moderately halophilic organisms). When sodium chloride concentration increased from 0.5 to 0.85 M, this effect was more pronounced for Escherichia coli; it remained relatively stable for Vibrio spp. The effect of carbonyl cyanide m-chlorophenylhydrazone was antagonized by the addition of glycine betaine or proline or by growth in a rich medium.

A primary respiratory Na+ pump of an anaerobic bacterium: the Na+-dependent NADH:quinone oxidoreductase of Klebsiella pneumoniae

Membranes of Klebsiella pneumoniae, grown anaerobically on citrate, contain a NADH oxidase activity that is activated specifically by Na+ or Li+ ions and effectively inhibited by 2-heptyl-4-hydroxyquinoline-N-oxide (HQNO). Cytochromes b and d were present in the membranes, and the steady state reduction level of cytochrome b increased on NaCl addition. Inverted bacterial membrane vesicles accumulated Na+ ions upon NADH oxidation. Na+ uptake was completely inhibited by monensin and by HQNO and slightly stimulated by carbonylcyanide-p-trifluoromethoxy phenylhydrazone (FCCP), thus indicating the operation of a primary Na+ pump. A Triton extract of the bacterial membranes did not catalyze NADH oxidation by O2, but by ferricyanide or menadione in a Na+-independent manner. The Na+-dependent NADH oxidation by O2 was restored by adding ubiquinone-1 in micromolar concentrations. After inhibition of the terminal oxidase with KCN, ubiquinol was formed from ubiquinone-1 and NADH. The reaction was stimulated about 6-fold by 10 mM NaCl and was severely inhibited by low amounts of HQNO. Superoxide radicals were formed during electron transfer from NADH to ubiquinone-1. These radicals disappeared by adding NaCl, but not with NaCl and HQNO. It is suggested that the superoxide radicals arise from semiquinone radicals which are formed by one electron reduction of quinone in a Na+-independent reaction sequence and then dismutase in a Na+ and HQNO sensitive reaction to quinone and quinol. The mechanism of the respiratory Na+ pump of K. pneumoniae appears to be quite similar to that of Vibrio alginolyticus.

A respiratory-driven and an artificially driven ATP synthesis in mutants of Vibrio parahaemolyticus lacking H+-translocating ATPase

Mutants of Vibrio parahaemolyticus lacking the H+-translocating ATPase were isolated to evaluate both the role of this enzyme and the possibility of the involvement of other cation-translocating ATPase in the energy transduction in this organism. Dicyclohexylcarbodiimide-sensitive ATPase activity which represents the H+-translocating ATPase was not detected either in the membrane vesicles or in the cytosol of the mutants. Three major subunits, alpha, beta and gamma, of the H+-translocating ATPase were missing in the membranes of the mutants. Although ATP was synthesized in wild type cells when an artificial H+ gradient was imposed, little ATP was synthesized in the mutants. However, we observed a large ATP synthesis driven by the respiration not only in the wild type but also in the mutants. The respiratory-driven ATP synthesis in wild type was inhibited by an H+ conductor, carbonylcyanide m-chlorophenylhydrazone, by about 50%. On the other hand, the ATP synthesis in the mutants was not affected by the H+ conductor. Since this organism possesses a respiratory Na+ pump, Na+-coupled ATP synthesis might take place. In fact, we observed some ATP synthesis driven by an artificially imposed Na+ gradient both in the wild type and the mutant.

Electron-transport-driven sodium extrusion during methanogenesis from formaldehyde and molecular hydrogen by Methanosarcina barkeri

Methanogenesis from formaldehyde or formaldehyde + H2, as carried out by Methanosarcina barkeri, was strictly dependent on sodium ions whereas methane formation from methanol + H2 or methanol + formaldehyde was Na+-independent. This indicates that the reduction of formaldehyde to the formal redox level of methanol exhibits a Na+ requirement. During methanogenesis from formaldehyde, a delta pNa in the range of -62 mV to -80 mV was generated by means of a primary, electron-transport-driven sodium pump. This could be concluded from the following results obtained on cell suspensions of M. barkeri. 1. The addition of proton conductors or inhibitors of the Na+/H+ antiporter had no effect on sodium extrusion. 2. During methanogenesis from formaldehyde + H2 a delta psi of -60 mV to -70 mV was generated even in the presence of proton conductors. 3. ATPase inhibitors, applied in the presence of proton conductors, had no effect on primary sodium extrusion or generation of a delta psi. Evidence for a Na+-translocating ATPase could not be obtained.

Sensitivity of some marine bacteria, a moderate halophile, and Escherichia coli to uncouplers at alkaline pH

The inhibitory effects of uncouplers on amino acid transport into three marine bacteria, Vibrio alginolyticus 118, Vibrio parahaemolyticus 113, and Alteromonas haloplanktis 214, into a moderate halophile, Vibrio costicola NRC 37001, and into Escherichia coli K-12 were found to vary depending upon the uncoupler tested, its concentration, and the pH. Higher concentrations of all of the uncouplers were required to inhibit transport at pH 8.5 than at pH 7.0. The protonophore carbonyl cyanide m-chlorophenylhydrazone showed the greatest reduction in inhibitory capacity as the pH was increased, carbonyl cyanide p-trifluoromethoxyphenylhydrazone showed less reduction, and 3,3',4',5-tetrachlorosalicylanilide was almost as effective as an inhibitor of amino acid transport at pH 8.5 as at pH 7.0 for all of the organisms except A. haloplanktis 214. Differences between the protonophores in their relative activities at pHs 7.0 and 8.5 were attributed to differences in their pK values. 3,3',4',5-Tetrachlorosalicylanilide, carbonyl cyanide m-chlorophenylhydrazone, 2-heptyl-4-hydroxyquinoline-N-oxide, and NaCN all inhibited Na+ extrusion from Na+-loaded cells of V. alginolyticus 118 at pH 8.5. The results support the conclusion that Na+ extrusion from this organism at pH 8.5 occurs as a result of Na+/H+ antiport activity. Data are presented indicating the presence in V. alginolyticus 118 of an NADH oxidase which is stimulated by Na+ at pH 8.5.

Some general characteristics of the transport of nutrients and toxic compounds in yeasts and fungi

This essay deals in turn with the six main mechanisms that are believed to be involved in the passage of small molecules across the cell membrane of yeasts and fungi. Such solutes include amino acids, carbohydrates, purines and pyrimidines and other nutrients, among which are the ions K+,,or. The subject has been reviewed extensively (see, for example, Eddy, 1982; Cooper, 1982; Serrano, 1984; Harold, 1986). Transport systems relate to chemotherapy in two ways. First, they may themselves be potential targets for drug action. Second, they may deliver a drug to its intracellular target, the drug being an illicit passenger in a transport system with a different primary role. Most of our knowledge about yeast and fungal transport is currently based on studies with relatively few species selected because they are known to be amenable to genetic and biochemical analysis. The organisms include representative strains ofNeurospora crassaor ofSaccharomyces cerevisiaeand selected species ofRhodotorulaandSchizosaccharomyces. Fungal organisms pathogenic to man, such asCandida albicans, those causing either systemic fungal infections (see Kerridge, 1986; Shepherd, Poulter & Sullivan, 1985), or the numerous fungal diseases of plants have been little used as models of nutrient transport, despite their clinical or economic importance. Whether all these ecologically diverse organisms, which in evolutionary terms are only distantly related, employ only the transport mechanisms so far discovered inSaccharomyces cerevisiaeis an open question.

Sodium ion transport decarboxylases and other aspects of sodium ion cycling in bacteria

Images

Na+/adenosine co-transport in Vibrio parahaemolyticus

Adenosine transport in Vibrio parahaemolyticus was studied. Na+ greatly stimulated adenosine uptake. Addition of adenosine to a cell suspension under anaerobic conditions elicited Na+ uptake, and the Na+ uptake was inhibited by monensin, an Na+ ionophore. Imposition of an electrochemical potential of Na+ or a membrane potential in energy-depleted cells elicited adenosine uptake. Therefore, adenosine transport in this organism was concluded to proceed by an Na+/adenosine co-transport mechanism. The Na+/adenosine co-transport system was induced when cells were grown in the presence of adenosine, and repressed by glucose. Although Na+ uptake elicited by adenosine was reduced by glucose, it was enhanced by methyl alpha-glucoside, which reduced the intracellular ATP level. Thus, the effects of glucose and the glucoside on the Na+/adenosine co-transport system did not seem to be due to inducer exclusion, but to be related to the intracellular ATP level.