Sodium ion-dependent amino acid transport in membrane vesicles of Bacillus stearothermophilus

Gorse (Ulex europeaus) wastes with 5,6-dimethyl benzimidazole supplementation can support growth of vitamin B12 producing commensal gut microbes

Many commensal gut microbes are recognized for their potential to synthesize vitamin B12, offering a promising avenue to address deficiencies through probiotic supplementation. While bioinformatics tools aid in predicting B12 biosynthetic potential, empirical validation remains crucial to confirm production, identify cobalamin vitamers, and establish biosynthetic yields. This study investigates vitamin B12 production in three human colonic bacterial species: Anaerobutyricum hallii DSM 3353, Roseburia faecis DSM 16840, and Anaerostipes caccae DSM 14662, along with Propionibacterium freudenreichii DSM 4902 as a positive control. These strains were selected for their potential use as probiotics, based on speculated B12 production from prior bioinformatic analyses. Cultures were grown in M2GSC, chemically defined media (CDM), and Gorse extract medium (GEM). The composition of GEM was similar to CDM, except that the carbon and nitrogen sources were replaced with the protein-depleted liquid waste obtained after subjecting Gorse to a leaf protein extraction process. B12 yields were quantified using liquid chromatography with tandem mass spectrometry. The results suggested that the three butyrate-producing strains could indeed produce B12, although the yields were notably low and were detected only in the cell lysates. Furthermore, B12 production was higher in GEM compared to M2GSC medium. The positive control, P. freudenreichii DSM 4902 produced B12 at concentrations ranging from 7 ng mL-1 to 12 ng mL-1. Univariate-scaled Principal Component Analysis (PCA) of data from previous publications investigating B12 production in P. freudenreichii revealed that B12 yields diminished when the carbon source concentration was ≤30 g L-1. In conclusion, the protein-depleted wastes from the leaf protein extraction process from Gorse can be valorised as a viable substrate for culturing B12-producing colonic gut microbes. Furthermore, this is the first report attesting to the ability of A. hallii, R. faecis, and A. caccae to produce B12. However, these microbes seem unsuitable for industrial applications owing to low B12 yields.

Differential gene expression for investigation of the effect of germinants and heat activation to induce germination in Bacillus cereus spores

Differential gene expression was used to explore the mechanisms underpinning the differences in the impact of heat activation (70 °C for 30 min) on the germination of Bacillus cereus spores in the presence and absence of a germinant (L-alanine). The number of germinated cells, after heat activation plus L-alanine (3.5 ± 0.02 log CFU/ml) in the spore only initial population was found to be higher than that in only heat activated spores (2.01 ± 0.02 log CFU/ml). The concentration of DPA released by heat activated spores in the presence of L-alanine was 68.3 ± 0.1 and 112.1 ± 0.02 μg/ml after 30 and 60 min, compared to 96.5 and 166.2 ± 0.01 μg/ml after 30 and 90 min, respectively released by spores subjected only to heat activation. Gene (BC0784) encoding for the spore germination protein, gerA operon was up-regulated with a log₂-transformed fold change value of 1.2 due to heat activation in the presence of L-alanine. The GerA operon located in the inner membrane is known to be involved in the uptake of L-alanine by B. cereus and has been reported to be involved in L-alanine mediated germination. In addition the up-regulation of genes involved in the uptake of L-alanine is proposed to provide the answer to the synergistic effect of heat and L-alanine in inducing germination in B. cereus spores. In short, heat activation increases the ability of L-alanine to penetrate into the spore's inner membrane, where it can be recognized by the receptors for initiation of the germination pathway. In the current study, the majority of the ribosomal proteins were down-regulated (when spores were heat treated in presence of germinants) this process also appeared to slow down protein synthesis by restricting the protein translation machinery. Differential gene expression revealed the genes responsible for the pathways related to transport and recognition of L-alanine into the spore that could have led to the accelerated germination process along with partial shutting down of protein synthesis pathway and ABC transporters. Knowledge of gene regulation in spores during heat activation will help in the development of approaches to prevent spore germination, which could provide an additional safeguard against bacterial growth and toxin production in improperly cooled heat treated foods.

A novel glutamate transport system in poly(γ-glutamic acid)-producing strain Bacillus subtilis CGMCC 0833

Bacillus subtilis CGMCC 0833 is a poly(γ-glutamic acid) (γ-PGA)-producing strain. It has the capacity to tolerate high concentration of extracellular glutamate and to utilize glutamate actively. Such a high uptake capacity was owing to an active transport system for glutamate. Therefore, a specific transport system for L-glutamate has been observed in this strain. It was a novel transport process in which glutamate was symported with at least two protons, and an inward-directed sodium gradient had no stimulatory effect on it. K(m) and V(m) for glutamate transport were estimated to be 67 μM and 152 nmol⁻¹ min⁻¹ mg⁻¹ of protein, respectively. The transport system showed structural specificity and stereospecificity and was strongly dependent on extracellular pH. Moreover, it could be stimulated by Mg²⁺, NH₄⁺, and Ca²⁺. In addition, the glutamate transporter in this strain was studied at the molecular level. As there was no important mutation of the transporter protein, it appeared that the differences of glutamate transporter properties between this strain and other B. subtilis strains were not due to the differences of the amino acid sequence and the structure of transporter protein. This is the first extensive report on the properties of glutamate transport system in γ-PGA-producing strain.

Uncoupler-Resistant Glucose Uptake by the Thermophilic Glycolytic Anaerobe Thermoanaerobacter thermosulfuricus (Clostridium thermohydrosulfuricum)

The transport of glucose across the bacterial cell membrane of Thermoanaerobacter thermosulfuricus (Clostridium thermohydrosulfuricum) Rt8.B1 was governed by a permease which did not catalyze concomitant substrate transport and phosphorylation and thus was not a phosphoenolpyruvate-dependent phosphotransferase. Glucose uptake was carrier mediated, could not be driven by an artificial membrane potential (Deltapsi) in the presence or absence of sodium, and was not sensitive to inhibitors which dissipate the proton motive force (Deltap; tetrachlorosalicylanilide, N,N-dicyclohexylcarboiimide, and 2,4-dinitrophenol), and no uptake of the nonmetabolizable analog 2-deoxyglucose could be demonstrated. The glucokinase apparent K(m) for glucose (0.21 mM) was similar to the K(t) (affinity constant) for glucose uptake (0.15 mM), suggesting that glucokinase controls the rate of glucose uptake. Inhibitors of ATP synthesis (iodoacetate and sodium fluoride) also inhibited glucose uptake, and this effect was due to a reduction in the level of ATP available to glucokinase for glucose phosphorylation. These results indicated that T. thermosulfuricus Rt8.B1 lacks a concentrative uptake system for glucose and that uptake is via facilitated diffusion, followed by ATP-dependent phosphorylation by glucokinase. In T. thermosulfuricus Rt8.B1, glucose is metabolized by the Embden-Meyerhof-Parnas pathway, which yields 2 mol of ATP (G. M. Cook, unpublished data). Since only 1 mol of ATP is used to transport 1 mol of glucose, the energetics of this system are therefore similar to those found in bacteria which possess a phosphotransferase.

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.

Electron spin resonance of copper(II) as a tool for the determination of asparagine concentration in Bacillus subtilis cultures

A procedure is presented that is based on the detection of Cu(II)-asparagine complexes by quantitative ESR, and allows in a very simple and rapid manner to evaluate the changes of asparagine concentration during the entire time range of the growth of Bacillus subtilis in a typical growth medium. The analysis is carried out in terms of the decrease of the intensity of the ESR-active mono- and di-asparagine copper(II) complexes. It is resulted that at the end of the exponential growth the asparagine concentration was reduced to values as low as 2% of the initial value. The procedure here reported may be the basis of similar methods to be used for other amino acids and prokaryote systems.

Differential transport properties of D-leucine and L-leucine in the archaeon,Halobacterium salinarum

The transport of D-leucine was compared with that of L-leucine in Halobacterium salinarum. When a high-outside/low-inside Na+gradient was imposed, D-leucine as well as L-leucine accumulated in envelope vesicles, supporting the hypothesis that D-leucine is transported via a symport system along with Na+. Kinetic analyses, including inhibition experiments, indicated that both enantiomers are transported via a common carrier. However, a Hill plot indicated a single binding site for Na+during L-leucine transport, but dual binding sites for Na+during D-leucine transport. Furthermore, D-leucine transport was dependent on electrical membrane potential, suggesting that a transporter bound with D-leucine is positively charged. L-leucine transport was slightly, if at all, dependent on membrane potential, suggesting that a transporter bound with L-leucine is electrically neutral. These results indicate that the leucine carrier in Halobacterium salinarum translocates two moles of Na+per mole of D-leucine, and one mole of Na+per mole of L-leucine.Key words: D-leucine, sodium ion-dependent transport, stoichiometry, stereospecific recognition, halophilic archaea.

Structural features of the glutamate transporter family

Neuronal and glial glutamate transporters remove the excitatory neurotransmitter glutamate from the synaptic cleft and thus prevent neurotoxicity. The proteins belong to a large and widespread family of secondary transporters, including bacterial glutamate, serine, and C4-dicarboxylate transporters; mammalian neutral-amino-acid transporters; and an increasing number of bacterial, archaeal, and eukaryotic proteins that have not yet been functionally characterized. Sixty members of the glutamate transporter family were found in the databases on the basis of sequence homology. The amino acid sequences of the carriers have diverged enormously. Homology between the members of the family is most apparent in a stretch of approximately 150 residues in the C-terminal part of the proteins. This region contains four reasonably well-conserved sequence motifs, all of which have been suggested to be part of the translocation pore or substrate binding site. Phylogenetic analysis of the C-terminal stretch revealed the presence of five subfamilies with characterized members: (i) the eukaryotic glutamate transporters, (ii) the bacterial glutamate transporters, (iii) the eukaryotic neutral-amino-acid transporters, (iv) the bacterial C4-dicarboxylate transporters, and (v) the bacterial serine transporters. A number of other subfamilies that do not contain characterized members have been defined. In contrast to their amino acid sequences, the hydropathy profiles of the members of the family are extremely well conserved. Analysis of the hydropathy profiles has suggested that the glutamate transporters have a global structure that is unique among secondary transporters. Experimentally, the unique structure of the transporters was recently confirmed by membrane topology studies. Although there is still controversy about part of the topology, the most likely model predicts the presence of eight membrane-spanning alpha-helices and a loop-pore structure which is unique among secondary transporters but may resemble loop-pores found in ion channels. A second distinctive structural feature is the presence of a highly amphipathic membrane-spanning helix that provides a hydrophilic path through the membrane. Recent data from analysis of site-directed mutants and studies on the mechanism and pharmacology of the transporters are discussed in relation to the structural model.

Sodium-dependent glutamate uptake by an alkaliphilic, thermophilic Bacillus strain, TA2.A1

A strain of Bacillus designated TA2.A1, isolated from a thermal spring in Te Aroha, New Zealand, grew optimally at pH 9.2 and 70 degrees C. Bacillus strain TA2.A1 utilized glutamate as a sole carbon and energy source for growth, and sodium chloride (>5 mM) was an obligate requirement for growth. Growth on glutamate was inhibited by monensin and amiloride, both inhibitors that collapse the sodium gradient (DeltapNa) across the cell membrane. N, N-Dicyclohexylcarbodiimide inhibited the growth of Bacillus strain TA2.A1, suggesting that an F1F0-ATPase (H type) was being used to generate cellular ATP needed for anabolic reactions. Vanadate, an inhibitor of V-type ATPases, did not affect the growth of Bacillus strain TA2.A1. Glutamate transport by Bacillus strain TA2.A1 could be driven by an artificial membrane potential (DeltaPsi), but only when sodium was present. In the absence of sodium, the rate of DeltaPsi-driven glutamate uptake was fourfold lower. No glutamate transport was observed in the presence of DeltapNa alone (i.e., no DeltaPsi). Glutamate uptake was specifically inhibited by monensin, and the Km for sodium was 5.6 mM. The Hill plot had a slope of approximately 1, suggesting that sodium binding was noncooperative and that the glutamate transporter had a single binding site for sodium. Glutamate transport was not affected by the protonophore carbonyl cyanide m-chlorophenylhydrazone, suggesting that the transmembrane pH gradient was not required for glutamate transport. The rate of glutamate transport increased with increasing glutamate concentration; the Km for glutamate was 2.90 microM, and the Vmax was 0.7 nmol. min-1 mg of protein. Glutamate transport was specifically inhibited by glutamate analogues.

Mechanism of glucose and maltose transport in plasma-membrane vesicles from the yeast Candida utilis

Transport of glucose and maltose was studied in plasma-membrane vesicles from Candida utilis. The yeast was grown on a mixture of glucose and maltose in aerobic carbon-limited continuous cultures which enabled transport to be studied for both sugars with the same vesicles. Vesicles were prepared by fusion of isolated plasma membranes with proteoliposomes containing bovine heart cytochrome c oxidase as a proton-motive-force-generating system. Addition of reduced cytochrome c generated a proton-motive force, consisting of a membrane potential, negative inside, and a pH gradient, alkaline inside. Energization led to accumulation of glucose and maltose in these vesicles, reaching accumulation ratios of about 40-50. Accumulation also occurred in the presence of valinomycin or nigericin, but was prevented by a combination of the two ionophores or by uncoupler, showing that glucose and maltose transport are dependent on the proton-motive force. Comparison of sugar accumulation with quantitative data on the proton-motive force indicated a 1:1 H+/sugar stoichiometry for both transport systems. Efflux of accumulated glucose was observed on dissipation of the proton-motive force. Exchange and counterflow experiments confirmed the reversible character of the H+-glucose symporter. In contrast, uncoupler or a mixture of valinomycin plus nigericin induced only a slow efflux of accumulated maltose. Moreover under counterflow conditions, the expected transient accumulation was small. Thus the H+-maltose symporter has some characteristics of a carrier that is not readily reversible. It is concluded that in C. utilis the transport systems for glucose and maltose are both driven by the proton-motive force, but the mechanisms are different.

Utilization ofD-leucine byHalobacterium halobiumVO107

Using the extreme halophile Halobacterium halobium VO107, the utilization of D-leucine was studied in comparison with that of L-leucine. The initial uptake of D-leucine into cells was found to be less efficient than that of L-leucine, but like L-leucine, the D-leucine uptake was inhibited by uncouplers and an ATPase inhibitor, suggesting that an energy-dependent process was involved in its uptake. Following uptake, D-leucine was isomerized to L-leucine for further metabolism by an enzyme(s) that was induced in the presence of D-leucine in medium.Key words: Halobacterium halobium, archaebacteria, D-amino acid utilization, isomerization.

The glucose transport system of the hyperthermophilic anaerobic bacterium Thermotoga neapolitana

The glucose transport system of the extremely thermophilic anaerobic bacterium Thermotoga neapolitana was studied with the nonmetabolizable glucose analog 2-deoxy-D-glucose (2-DOG). T. neapolitana accumulated 2-DOG against a concentration gradient in an intracellular free sugar pool that was exchangeable with external source of energy, such as pyruvate, and was inhibited by arsenate and gramicidin D. There was no phosphoenolpyruvate-dependent phosphorylation of glucose, 2-DOG, or fructose by cell extracts or toluene-treated cells, indicating the absence of a phosphoenolpyruvate:sugar phosphotransferase system. These data indicate that D-glucose is taken up by T. neapolitana via an active transport system that is energized by an ion gradient generated by ATP, derived from substrate-level phosphorylation.

Characterization of the proton/glutamate symport protein of Bacillus subtilis and its functional expression in Escherichia coli

Transport of acidic amino acids in Bacillus subtilis is an electrogenic process in which L-glutamate or L-aspartate is symported with at least two protons. This is shown by studies of transport in membrane vesicles in which a proton motive force is generated by oxidation of ascorbate-phenazine methosulfate or by artificial ion gradients. An inwards-directed sodium gradient had no (stimulatory) effect on proton motive force-driven L-glutamate uptake. The transporter is specific for L-glutamate and L-aspartate. L-Glutamate transport is inhibited by beta-hydroxyaspartate and cysteic acid but not by alpha-methyl-glutamate. The gene encoding the L-glutamate transport protein of B. subtilis (gltPBsu) was cloned by complementation of Escherichia coli JC5412 for growth on glutamate as the sole source of carbon, energy, and nitrogen, and its nucleotide sequence was determined. Putative promoter, terminator, and ribosome binding site sequences were found in the flanking regions. UUG is most likely the start codon. gltPBsu encodes a polypeptide of 414 amino acid residues and is homologous to several proteins that transport glutamate and/or structurally related compounds such as aspartate, fumarate, malate, and succinate. Both sodium- and proton-coupled transporters belong to this family of dicarboxylate transporters. Hydropathy profiling and multiple alignment of the family of carboxylate transporters suggest that each of the proteins spans the cytoplasmic membrane 12 times with both the amino and carboxy termini on the inside.

Stability and proton-permeability of liposomes composed of archaeal tetraether lipids

Liposomes composed of tetraether lipids originating from the thermoacidophilic archaeon Sulfolobus acidocaldarius were analyzed for their stability and proton permeability from 20 degrees C up to 80 degrees C. At room temperature, these liposomes are considerably more stable and have a much lower proton permeability than liposomes composed of diester lipids originating from the mesophilic bacterium Escherichia coli or the thermophilic bacterium Bacillus stearothermophilus. With increasing temperature, the stability decreased and the proton permeability increased for all liposomes. Liposomes composed from tetraether lipids, however, remain the most stable. These data suggest these liposomes retain the rigidity of the cytoplasmic membrane of S. acidocaldarius needed to endure extreme environmental growth conditions.

Transport of C4-dicarboxylates by anaerobically grown Escherichia coli. Energetics and mechanism of exchange, uptake and efflux

Transport activities for uptake, efflux and exchange of C4-dicarboxylates were observed in anaerobically grown Escherichia coli. All three transport modes were only present in strains containing the transcriptional activator FNR of anaerobic respiration, and were repressed by nitrate and O2. The kinetic and energetic parameters of C4-dicarboxylate transport and the mechanism of the uptake, efflux and exchange reactions were analyzed in whole cells and in membrane vesicles. Fumarate/succinate exchange could be characterized as homologous or heterologous 1:1 counter-exchange. The external substrate was determined as divalent fumarate2- (or succinate2-) at pH 6-9, whereas monovalent H-fumarate dominated as the substrate at pH 3-4. The exchange was not inhibited by dissipation of delta p or constituents of it (delta psi or delta pH). We conclude that this transport mode functions as an electroneutral exchange of C4-dicarboxylates. The uptake of C4-dicarboxylates did not depend on internal counter-substrate and resulted in an accumulation of the substrate. Similar to antiport, fumarate was accepted in the divalent form at pH values greater than or equal to 6 and in the monovalent form at pH 3.5-6. The uptake was inhibited by dissipation of delta p or delta psi. Artificially imposed delta pH, delta psi or fumarate gradients were able to drive fumarate uptake. An involvement of Na+ could not be detected. Thus the uptake is likely to operate as an electrophoretic H+/fumarate symport. Independent of the presence of an external counter-substrate, the substrates were secreted from cells or membrane vesicles loaded with succinate or fumarate. The efflux was electrogenic. Energizing the cells or membrane vesicles inhibited efflux, maximal efflux rates were obtained only after dissipation of delta p or delta psi. An imposed K(+)-diffusion potential (outside positive) inhibited succinate excretion. The efflux of succinate from de-energized membrane vesicles generated a delta psi of -70 mV. It is thus suggested that succinate efflux functions as a H+/succinate symport.

Energy transduction in the thermophilic anaerobic bacterium Clostridium fervidus is exclusively coupled to sodium ions

The thermophilic, peptidolytic, anaerobic bacterium Clostridium fervidus is unable to generate a pH gradient in the range of 5.5-8.0, which limits growth of the organism to a narrow pH range (6.3-7.7). A significant membrane potential (delta psi approximately -60 mV) and chemical gradient of Na+ (-Z delta pNa approximately -60 mV) are formed in the presence of metabolizable substrates. Energy-dependent Na+ efflux is inhibited by the Na+/H+ ionophore monensin but is stimulated by uncouplers, suggesting that the Na+ gradient is formed by a primary pumping mechanism rather than by secondary Na+/H+ antiport. This primary sodium pump was found to be an ATPase that has been characterized in inside-out membrane vesicles and in proteoliposomes in which solubilized ATPase was reconstituted. The enzyme is stimulated by Na+, resistant to vanadate, and sensitive to nitrate, which is indicative of an F/V-type Na(+)-ATPase. In the proteoliposomes Na+ accumulation depends on the presence of ATP, is inhibited by the ATPase inhibitor nitrate, and is completely prevented by the ionophore monensin but is stimulated by protonophores and valinomycin. These and previous observations, which indicated that secondary amino acid transport uses solely Na+ as coupling ion, demonstrate that energy transduction at the membrane in C. fervidus is exclusively dependent on a Na+ cycle.

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.

Energy transduction and transport processes in thermophilic bacteria

Bacterial growth at the extremes of temperature has remained a fascinating aspect in the study of membrane function and structure. The stability of the integral membrane proteins of thermophiles make them particularly amenable to study. Respiratory enzymes of thermophiles appear to be functionally similar to the mesophilic enzymes but differ in their thermostability and unusual high turnover rates. Energy coupling at extreme temperatures seems inefficient as suggested by the high maintenance coefficients and the high permeability of the cell membrane to protons. Nevertheless, membranes maintain their structure at these extremes through changes in fatty acid acyl chain composition. Archaebacteria synthesize novel membrane-spanning lipids with unique physical characteristics. Thermophiles have adapted to life at extreme temperatures by using sodium ions rather than protons as coupling ions in solute transport. Genetic and biochemical studies of these systems now reveal fundamental principles of such adaptations. The recent development of reconstitution techniques using membrane-spanning lipids allows a rigorous biochemical characterization of membrane proteins of extreme thermophiles in their natural environment.

Characterization and functional expression in Escherichia coli of the sodium/proton/glutamate symport proteins of Bacillus stearothermophilus and Bacillus caldotenax

The genes encoding the Na+/H+/L-glutamate symport proteins of the thermophilic organisms Bacillus stearothermophilus (gltTBs) and Bacillus caldotenax (gltTBc) were cloned by complementation of Escherichia coli JC5412 for growth on glutamate as sole source of carbon, energy and nitrogen. The nucleotide sequences of the gltTBs and gltTBc genes were determined. In both cases the translated sequences corresponded with proteins of 421 amino acid residues (96.7% amino acid identity between GltTBs and GltTBc). Putative promoter, terminator and ribosome-binding-site sequences were found in the flanking regions. These expression signals were functional in E. coli. The hydropathy profiles indicate that the proteins are hydrophobic and could form 12 membrane-spanning regions. The Na+/H+ coupled L-glutamate symport proteins GltTBs and GltTBc are homologous to the strictly H+ coupled L-glutamate transport protein of E. coli K-12 (overall 57.2% identity). Functional expression of glutamate transport activity was demonstrated by uptake of glutamate in whole cells and membrane vesicles. In accordance with previous observations (de Vrij et al., 1989; Heyne et al., 1991), glutamate uptake was driven by the electrochemical gradients of sodium ions and protons.

Revised nucleotide sequence of the gltP gene, which encodes the proton-glutamate-aspartate transport protein of Escherichia coli K-12

The gene encoding the proton-glutamate carrier (GltP) of Escherichia coli K-12 was sequenced, and the primary structure of the protein was analyzed. The nucleotide sequence was found to differ in several aspects from the previously published sequence (B. Wallace, Y. Yang, J. Hong, and D. Lum, J. Bacteriol. 172:3214-3220, 1990). The corrected open reading frame encodes a protein of 437 (instead of 395) amino acids. Hydropathy analysis predicts 12 membrane-spanning alpha-helical regions. The complementary strand does contain an open reading frame possibly encoding a highly hydrophilic polypeptide of 272 amino acids.