Proline uptake through the major transport system of Salmonella typhimurium is coupled to sodium ions

The Effect of Proline on the Freeze-Drying Survival Rate of Bifidobacterium longum CCFM 1029 and Its Inherent Mechanism

Amino acids, which are important compatible solutes, play a significant role in probiotic lyophilization. However, studies on the functions of Bifidobacterium during freeze-drying are limited. Therefore, in this study, we compared the freeze-drying survival rate of Bifidobacterium longum CCFM 1029 cultivated in different media containing different kinds of compatible solutes. We found that the addition of 21 g/L proline to the culture media substantially improved the freeze-drying survival rate of B. longum CCFM 1029 from 18.61 ± 0.42% to 38.74 ± 1.58%. Interestingly, this change has only been observed when the osmotic pressure of the external culture environment is increased. Under these conditions, we found that proline accumulation in this strain increased significantly. This change also helped the strain to maintain its membrane integrity and the activity of some key enzymes during freeze-drying. Overall, these results show that the addition of proline can help the strain resist a tough environment during lyophilization. The findings of this study provide preliminary data for producers of probiotics who wish to achieve higher freeze-drying survival rates during industrial production.

The Regulatory Hierarchy Following Signal Integration by the CbrAB Two-Component System: Diversity of Responses and Functions

CbrAB is a two-component system, unique to bacteria of the family Pseudomonaceae, capable of integrating signals and involved in a multitude of physiological processes that allow bacterial adaptation to a wide variety of varying environmental conditions. This regulatory system provides a great metabolic versatility that results in excellent adaptability and metabolic optimization. The two-component system (TCS) CbrA-CbrB is on top of a hierarchical regulatory cascade and interacts with other regulatory systems at different levels, resulting in a robust output. Among the regulatory systems found at the same or lower levels of CbrAB are the NtrBC nitrogen availability adaptation system, the Crc/Hfq carbon catabolite repression cascade in Pseudomonas, or interactions with the GacSA TCS or alternative sigma ECF factor, such as SigX. The interplay between regulatory mechanisms controls a number of physiological processes that intervene in important aspects of bacterial adaptation and survival. These include the hierarchy in the use of carbon sources, virulence or resistance to antibiotics, stress response or definition of the bacterial lifestyle. The multiple actions of the CbrAB TCS result in an important competitive advantage.

Catabolism of Amino Acids and Related Compounds

This review considers the pathways for the degradation of amino acids and a few related compounds (agmatine, putrescine, ornithine, and aminobutyrate), along with their functions and regulation. Nitrogen limitation and an acidic environment are two physiological cues that regulate expression of several amino acid catabolic genes. The review considers Escherichia coli, Salmonella enterica serovar Typhimurium, and Klebsiella species. The latter is included because the pathways in Klebsiella species have often been thoroughly characterized and also because of interesting differences in pathway regulation. These organisms can essentially degrade all the protein amino acids, except for the three branched-chain amino acids. E. coli, Salmonella enterica serovar Typhimurium, and Klebsiella aerogenes can assimilate nitrogen from D- and L-alanine, arginine, asparagine, aspartate, glutamate, glutamine, glycine, proline, and D- and L-serine. There are species differences in the utilization of agmatine, citrulline, cysteine, histidine, the aromatic amino acids, and polyamines (putrescine and spermidine). Regardless of the pathway of glutamate synthesis, nitrogen source catabolism must generate ammonia for glutamine synthesis. Loss of glutamate synthase (glutamineoxoglutarate amidotransferase, or GOGAT) prevents utilization of many organic nitrogen sources. Mutations that create or increase a requirement for ammonia also prevent utilization of most organic nitrogen sources.

The γ-aminobutyrate permease GabP serves as the third proline transporter of Bacillus subtilis

PutP and OpuE serve as proline transporters when this imino acid is used by Bacillus subtilis as a nutrient or as an osmostress protectant, respectively. The simultaneous inactivation of the PutP and OpuE systems still allows the utilization of proline as a nutrient. This growth phenotype pointed to the presence of a third proline transport system in B. subtilis. We took advantage of the sensitivity of a putP opuE double mutant to the toxic proline analog 3,4-dehydro-dl-proline (DHP) to identify this additional proline uptake system. DHP-resistant mutants were selected and found to be defective in the use of proline as a nutrient. Whole-genome resequencing of one of these strains provided the lead that the inactivation of the γ-aminobutyrate (GABA) transporter GabP was responsible for these phenotypes. DNA sequencing of the gabP gene in 14 additionally analyzed DHP-resistant strains confirmed this finding. Consistently, each of the DHP-resistant mutants was defective not only in the use of proline as a nutrient but also in the use of GABA as a nitrogen source. The same phenotype resulted from the targeted deletion of the gabP gene in a putP opuE mutant strain. Hence, the GabP carrier not only serves as an uptake system for GABA but also functions as the third proline transporter of B. subtilis. Uptake studies with radiolabeled GABA and proline confirmed this conclusion and provided information on the kinetic parameters of the GabP carrier for both of these substrates.

Metabolite and transcriptome analysis of Campylobacter jejuni in vitro growth reveals a stationary-phase physiological switch

Campylobacter jejuni is a prevalent cause of food-borne diarrhoeal illness in humans. Understanding of the physiological and metabolic capabilities of the organism is limited. We report a detailed analysis of the C. jejuni growth cycle in batch culture. Combined transcriptomic, phenotypic and metabolic analysis demonstrates a highly dynamic ‘stationary phase’, characterized by a peak in motility, numerous gene expression changes and substrate switching, despite transcript changes that indicate a metabolic downshift upon the onset of stationary phase. Video tracking of bacterial motility identifies peak activity during stationary phase. Amino acid analysis of culture supernatants shows a preferential order of amino acid utilization. Proton NMR (1H-NMR) highlights an acetate switch mechanism whereby bacteria change from acetate excretion to acetate uptake, most probably in response to depletion of other substrates. Acetate production requires pta (Cj0688) and ackA (Cj0689), although the acs homologue (Cj1537c) is not required. Insertion mutants in Cj0688 and Cj0689 maintain viability less well during the stationary and decline phases of the growth cycle than wild-type C. jejuni, suggesting that these genes, and the acetate pathway, are important for survival.

Role of Pseudomonas putida tol-oprL gene products in uptake of solutes through the cytoplasmic membrane

Proteins of the Tol-Pal (Tol-OprL) system play a key role in the maintenance of outer membrane integrity and cell morphology in gram-negative bacteria. Here we describe an additional role for this system in the transport of various carbon sources across the cytoplasmic membrane. Growth of Pseudomonas putida tol-oprL mutant strains in minimal medium with glycerol, fructose, or arginine was impaired, and the growth rate with succinate, proline, or sucrose as the carbon source was lower than the growth rate of the parental strain. Assays with radiolabeled substrates revealed that the rates of uptake of these compounds by mutant cells were lower than the rates of uptake by the wild-type strain. The pattern and amount of outer membrane protein in the P. putida tol-oprL mutants were not changed, suggesting that the transport defect was not in the outer membrane. Consistently, the uptake of radiolabeled glucose and glycerol in spheroplasts was defective in the P. putida tol-oprL mutant strains, suggesting that there was a defect at the cytoplasmic membrane level. Generation of a proton motive force appeared to be unaffected in these mutants. To rule out the possibility that the uptake defect was due to a lack of specific transporter proteins, the PutP symporter was overproduced, but this overproduction did not enhance proline uptake in the tol-oprL mutants. These results suggest that the Tol-OprL system is necessary for appropriate functioning of certain uptake systems at the level of the cytoplasmic membrane.

Characterization of a functional bacterial homologue of sodium-dependent neurotransmitter transporters

The tnaT gene of Symbiobacterium thermophilum encodes a protein homologous to sodium-dependent neurotransmitter transporters. Expression of the tnaT gene product in Escherichia coli conferred the ability to accumulate tryptophan from the medium and the ability to grow on tryptophan as a sole source of carbon. Transport was Na(+)-dependent and highly selective. The K(m) for tryptophan was approximately 145 nm, and tryptophan transport was unchanged in the presence of 100 microM concentrations of other amino acids. Tryptamine and serotonin were weak inhibitors with K(I) values of 200 and 440 microM, respectively. By using a T7 promoter-based system, TnaT with an N-terminal His(6) tag was expressed at high levels in the membrane and was purified to near-homogeneity in high yield.

Sequence of the 165-kilobase catabolic plasmid pAO1 from Arthrobacter nicotinovorans and identification of a pAO1-dependent nicotine uptake system

The 165-kb catabolic plasmid pAO1 enables the gram-positive soil bacterium Arthrobacter nicotinovorans to grow on the tobacco alkaloid L-nicotine. The 165,137-nucleotide sequence, with an overall G+C content of 59.7%, revealed, besides genes and open reading frames (ORFs) for nicotine degradation, a complete set of ORFs for enzymes essential for the biosynthesis of the molybdenum dinucleotide cofactor, as well as ORFs related to uptake and utilization of carbohydrates, sarcosine, and amino acids. Of the 165 ORFs, approximately 50% were related to metabolic functions. pAO1 conferred to A. nicotinovorans the ability to take up L-[(14)C]nicotine from the medium, with an K(m) of 5.6 +/- 2.2 micro M. ORFs of putative nicotine transporters formed a cluster with the gene of the D-nicotine-specific 6-hydroxy-D-nicotine oxidase. ORFs related to replication, chromosome partitioning, and natural transformation functions (dprA) were identified on pAO1. Few ORFs showed similarity to known conjugation-promoting proteins, but pAO1 could be transferred by conjugation to a pAO1-negative strain at a rate of 10(-2) to 10(-3) per donor. ORFs with no known function represented approximately 35% of the pAO1 sequence. The positions of insertion sequence elements and composite transposons, corroborated by the G+C content of the pAO1 sequence, suggest a modular composition of the plasmid.

Functional analysis of salt-inducible proline transporter of barley roots

We cloned a cDNA encoding Hordeum vulgare Proline Transporter (HvProT) from salt-stressed barley roots by differential display. HvProT was 2,161 bp long and had an open reading frame encoding 450 amino acids. The deduced amino acid sequence of HvProT was similar to those of proline transporter proteins of rice (65.7%), Arabidopsis (57.7%) and tomato (42.0%). Northern blot analysis showed that the transcript level of HvProT was induced in roots at 30 min after 200 mM NaCl treatment and its peak was observed at 3 h. However, the transcript level was very low in leaves and did not increase by salt stress. The expression level of Delta(1)-pyrroline-5-carboxylate synthetase (P5CS), encoding a key enzyme of proline synthesis, was induced later than HvProT by salt stress. A transport assay using a yeast with mutation in proline uptake revealed that HvProT was a transporter with high affinity for L-proline (K(m) = 25 microM). HvProT was found to be a unique transporter with high affinity for L-proline. Since its transport activity was dependent on the pH gradient, HvProT was suggested to be a H(+)/amino acid symporter. In situ hybridization analysis showed that the HvProT mRNA was strongly expressed in root cap cells under salt stress. HvProT might play an important role in the transport of proline to root tip region urgently upon salt stress.

Towards the molecular mechanism of Na(+)/solute symport in prokaryotes

The Na(+)/solute symporter family (SSF, TC No. 2.A.21) contains more than 40 members of pro- and eukaryotic origin. Besides their sequence similarity, the transporters share the capability to utilize the free energy stored in electrochemical Na(+) gradients for the accumulation of solutes. As part of catabolic pathways most of the transporters are most probably involved in the acquisition of nutrients. Some transporters play a role in osmoadaptation. With a high resolution structure still missing, a combination of genetic, protein chemical and spectroscopic methods has been used to gain new insights into the structure and molecular mechanism of action of the transport proteins. The studies suggest a common 13-helix motif for all members of the SSF according to which the N-terminus is located in the periplasm and the C-terminus is directed into the cytoplasm (except for proteins containing a N- or C-terminal extension). Furthermore, an amino acid substitution analysis of the Na(+)/proline transporter (PutP) of Escherichia coli, a member of the SSF, has identified regions of particular functional importance. For example, amino acids of TM II of PutP proved to be critical for high affinity binding of Na(+) and proline. In addition, it was shown that ligand binding induces widespread conformational alterations in the transport protein. Taken together, the studies substantiate the common idea that Na(+)/solute symport is the result of a series of ligand-induced structural changes.

Sodium-substrate cotransport in bacteria

A variety of sodium-substrate cotransport systems are known in bacteria. Sodium enters the cell down an electrochemical concentration gradient. There is obligatory coupling between the entry of the ion and the entry of substrate with a stoichiometry (in the cases studied) of 1:1. Thus, the downhill movement of sodium ion into the cell leads to the accumulation of substrate within the cell. The melibiose carrier of Escherichia coli is perhaps the most carefully studied of the sodium cotransport systems in bacteria. This carrier is of special interest because it can also use protons or lithium ions for cotransport. Other sodium cotransport carriers that have been studied recently are for proline, glutamate, serine-threonine, citrate and branched chain amino acids.

Proline catabolism by Pseudomonas putida: cloning, characterization, and expression of the put genes in the presence of root exudates

Pseudomonas putida KT2442 is a root-colonizing strain which can use proline, one of the major components in root exudates, as its sole carbon and nitrogen source. A P. putida mutant unable to grow with proline as the sole carbon and nitrogen source was isolated after random mini-Tn5-Km mutagenesis. The mini-Tn5 insertion was located at the putA gene, which is adjacent to and divergent from the putP gene. The putA gene codes for a protein of 1,315 amino acid residues which is homologous to the PutA protein of Escherichia coli, Salmonella enterica serovar Typhimurium, Rhodobacter capsulatus, and several Rhizobium strains. The central part of P. putida PutA showed homology to the proline dehydrogenase of Saccharomyces cerevisiae and Drosophila melanogaster, whereas the C-terminal end was homologous to the pyrroline-5-carboxylate dehydrogenase of S. cerevisiae and a number of aldehyde dehydrogenases. This suggests that in P. putida, both enzymatic steps for proline conversion to glutamic acid are catalyzed by a single polypeptide. The putP gene was homologous to the putP genes of several prokaryotic microorganisms, and its gene product is an integral inner-membrane protein involved in the uptake of proline. The expression of both genes was induced by proline added in the culture medium and was regulated by PutA. In a P. putida putA-deficient background, expression of both putA and putP genes was maximal and proline independent. Corn root exudates collected during 7 days also strongly induced the P. putida put genes, as determined by using fusions of the put promoters to 'lacZ. The induction ratio for the putA promoter (about 20-fold) was 6-fold higher than the induction ratio for the putP promoter.

A conserved aspartate residue, Asp187, is important for Na+-dependent proline binding and transport by the Na+/proline transporter of Escherichia coli

Asp187 in the Na+/proline transporter of Escherichia coli (PutP) is conserved within the Na+/solute cotransporter family. Information on the role of this residue has been gained by amino acid substitution analysis. PutP with Glu, Asn, or Cys in place of Asp187 catalyzed Na+-coupled proline uptake at 75%, 25%, and 1.5%, respectively, of the Vmax of PutP-wild-type while the apparent Km for proline was only slightly altered. Importantly, acetylation or amidoacetylation of an engineered transporter containing a single Cys at position 187 stimulated proline uptake. Strikingly, PutP-D187C exhibited high-affinity proline binding even at very low Na+ concentrations (2 microM) while proline binding to PutP-wild-type, -D187E, and -D187N was strictly dependent on the Na+ concentration. The apparent independence of proline binding from the Na+ concentration can at least partially be attributed to an enhanced Na+ affinity of PutP-D187C. In addition, reaction of PutP containing a single Cys at position 187 with N-ethylmaleimide was inhibited by Na+ but not by Li+ or proline. The results indicate that electrostatic interactions of the amino acid side chain at position 187 in PutP with other parts of the transporter and/or the coupling ion are crucial for active proline transport. It is suggested that Asp187 is located close to the pathway of the coupling ion through the membrane and may be involved in the release of Na+ on the cytoplasmic side of the membrane.

Aspartate 55 in the Na+/proline permease of Escherichia coli is essential for Na+-coupled proline uptake

Four acidic residues in the N-terminal domain of Na+/proline permease of Escherichia coli (Asp33, Asp34, and Asp55 in putative loop 2, Glu75 in putative transmembrane domain II) were individually replaced with neutral or charged amino acid residues. Replacement of Glu75, the only residue in the permease presumed to be in the middle of a transmembrane domain, Asp33, or Asp34 had little or no influence on the kinetics of Na+-coupled proline transport. In contrast, removal of the carboxylate at position 55 (Asp55 --> Asn or Asp55 --> Cys permease) impaired proline uptake completely while lengthening of the side chain at this position by one methylene group (Asp55 --> Glu permease) allowed transport at a reduced initial rate. Importantly, all permease molecules were present in the membrane at concentrations comparable to the wild-type protein. Kinetic analysis of Na+-coupled proline transport catalyzed by Asp55 --> Glu permease revealed a 5-fold increase of the K(m) for proline and a 30-fold decrease of the V(max) compared to wild-type. Remarkably, replacement of Asp55 by Glu led to a 50-fold decrease of the apparent affinity of the permease for Na+. Furthermore, replacement of Asp55 with Cys or Asn blocked proline-induced Na+ uptake whereas significant Na+ transport was observed with Asp55 --> Glu permease. In addition, transport of proline down its concentration gradient was not detectable with deenergized cells containing Asp55 --> Glu permease at low Na+ concentrations. However, downhill transport activity was observed in the presence of high Na+ concentrations. Replacement of Asp55 with Asn or Cys impaired downhill transport under all conditions tested. The observations demonstrate that a carboxylate at position 55 of proline permease is essential for Na+-coupled proline transport. It is suggested that Asp55 may be involved in binding of the coupling ion.

Influence of Osmolarity on Phase Shift in Photorhabdus luminescens

The influence of osmolarity and other environmental factors like low oxygen levels, light, extreme pH values, and temperatures on phase variation of Photorhabdus luminescens, the symbiotic bacterium of entomopathogenic nematodes of the genus Heterorhabditis, was investigated. Only subculturing in low-osmolarity medium triggered a phase shift to secondary phase reliably.

Role of Na+ in transport of Hg2+ and induction of the Tn21 mer operon

The effects of sodium ions on the uptake of Hg2+ and induction of the Tn21 mer operon were studied by using Escherichia coli HMS174 harboring the reporter plasmids pRB28 and pOS14. Plasmid pRB28 carries merRT', and pOS14 carries merRTPC of the mer operon, both cloned upstream of a promoterless luciferase gene cassette in pUCD615. The bioluminescent response to 1 microM Hg2+ was significantly inhibited in E. coli HMS174(pRB28) in minimal medium supplemented with sodium ions at 10 to 140 mM. After initial acceleration, light emission declined at 50 nM Hg2+ in the presence of Na+. The mer-lux assay with resting cells carrying pRB28 and 203Hg2+ uptake experiments showed increased induction and enhanced mercury uptake, respectively, in media supplemented with sodium ions. The presence of Na+ facilitated maintenance of bioluminescence in resting HMS174(pRB28) cells induced with 50 nM Hg2+. External K+ stimulated bioluminescent response in HMS174(pRB28) and HMS174(pOS14) grown in sodium phosphate minimal medium devoid of potassium ions. Sodium ions appear to facilitate mercury transport. We propose that sodium-coupled transport of mercuric ions can be one of the mechanisms for mercury uptake by E. coli and that the Na+ gradient may energize the transport of Hg2+.

Transport of glycine-betaine by Listeria monocytogenes

Uptake of [14C]glycine-betaine by Listeria monocytogenes was stimulated by NaCl with optimal stimulation at 0.4-0.5 M. The glycine-betaine transport system had a Km of 22 microM and a Vmax of 11.7 nmol-1 min-1 mg-1 protein when grown in the absence of NaCl. When grown in the presence of 0.8 M NaCl the Vmax increased to 27.0 nmol-1 min-1 mg-1 protein in 0.8 M NaCl. At NaCl concentrations above 0.5 M the uptake rate of glycine-betaine was reduced. Measurement of intracellular K+ concentrations and fluorescent dye quenching indicated that higher NaCl concentrations also led to a decrease in the electrochemical potential difference across the cytoplasmic membrane. Uptake of glycine was also observed, but this was not stimulated by NaCl.

Energetics of alanine, lysine, and proline transport in cytoplasmic membranes of the polyphosphate-accumulating Acinetobacter johnsonii strain 210A

Amino acid transport in right-side-out membrane vesicles of Acinetobacter johnsonii 210A was studied. L-Alanine, L-lysine, and L-proline were actively transported when a proton motive force of -76 mV was generated by the oxidation of glucose via the membrane-bound glucose dehydrogenase. Kinetic analysis of amino acid uptake at concentrations of up to 80 microM revealed the presence of a single transport system for each of these amino acids with a Kt of less than 4 microM. The mode of energy coupling to solute uptake was analyzed by imposition of artificial ion diffusion gradients. The uptake of alanine and lysine was driven by a membrane potential and a transmembrane pH gradient. In contrast, the uptake of proline was driven by a membrane potential and a transmembrane chemical gradient of sodium ions. The mechanistic stoichiometry for the solute and the coupling ion was close to unity for all three amino acids. The Na+ dependence of the proline carrier was studied in greater detail. Membrane potential-driven uptake of proline was stimulated by Na+, with a half-maximal Na+ concentration of 26 microM. At Na+ concentrations above 250 microM, proline uptake was strongly inhibited. Generation of a sodium motive force and maintenance of a low internal Na+ concentration are most likely mediated by a sodium/proton antiporter, the presence of which was suggested by the Na(+)-dependent alkalinization of the intravesicular pH in inside-out membrane vesicles. The results show that both H+ and Na+ can function as coupling ions in amino acid transport in Acinetobacter spp.

Effect of novel compound, 1-methyl-1-piperidino methane sulfonate (MPMS), on the osmoprotectant activity of glycine betaine, choline and L-proline in Escherichia coli

A novel compound, 1-methyl-1-piperidino methane sulfonate (MPMS), was found to block the osmoprotectant activity of choline and L-proline, but not glycine betaine in Escherichia coli. MPMS was more active against salt-sensitive than salt-resistant strains, but had no effect on the salt tolerance of a mutant which was unable to transport choline, glycine betaine and proline. Growth of E. coli in NaCl was inhibited by MPMS and restored by glycine betaine, but not by choline or L-proline. Uptake of radiolabeled glycine betaine, choline or L-proline by cells grown at high osmolarity was not inhibited when MPMS and the radioactive substrates were added simultaneously. Preincubation for 5 min with MPMS reduced the uptake of choline and L-proline, but not glycine betaine. Similar incubation with MPMS had no effect on the uptake of radiolabeled glucose or succinate. The toxicity of MPMS was much lower than that of the L-proline analogues L-azetidine-2-carboxylic acid and 3,4-dehydro-DL-proline. The exact mechanism by which MPMS exerts its effect is not entirely clear. MPMS or a metabolite may interfere with the activity of several independent permeases involved in the uptake of osmoprotective compounds, or the conversion of choline to glycine betaine, or effect the expression of some of the osmoregulatory genes.

Tobramycin uptake in Escherichia coli is driven by either electrical potential or ATP

Aminoglycoside antibiotics such as streptomycin and tobramycin must traverse the bacterial cytoplasmic membrane prior to initiating lethal effects. Previous data on Escherichia coli, Staphylococcus aureus, and Bacillus subtilis have demonstrated that transport of aminoglycosides is regulated by delta psi, the electrical component of the proton motive force. However, several laboratories have observed that growth of bacterial cells can occur in the apparent absence of delta psi, and we wished to confirm these studies with E. coli and further investigate whether transport of aminoglycosides could occur in the absence of a membrane potential. Treatment of acrA strain CL2 with the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) dissipated delta psi, decreased intracellular ATP levels, and resulted in cessation of growth; after a variable period of time (3 to 7 h), growth resumed, ultimately achieving growth rates comparable to those of untreated cells. Absence of delta psi in these cells was confirmed by absence of [3H]tetraphenyl phosphonium+ uptake as measured by membrane filtration, lack of flagellar motion, and inability of these cells to transport proline (but not methionine). Regrowth was associated with restoration of normal intracellular ATP as measured by luciferin-luciferase bioluminescence assay. Unlike unacclimatized CL2 cells treated with CCCP, these cells transported [3H]tobramycin similarly to untreated cells; aminoglycoside-induced killing was seen in association with transport. These studies suggest that under certain circumstances aminoglycoside transport can be driven by ATP (or other high-energy activated phosphate donors) alone, in the absence of a measurable delta psi. delta uncBC mutants of CL2 incapable of interconverting delta psi and ATP were treated with CCCP, resulting in dissipation of delta psi but no alteration in ATP content. Despite maintenance of normal ATP, there was no transport of [3H] bramycin, confirming that under normal growth conditions ATP has no role in the transport of aminoglycosides.

Peptide transport and chemotaxis in Escherichia coli and Salmonella typhimurium: characterization of the dipeptide permease (Dpp) and the dipeptide-binding protein

The dipeptide permease (Dpp) is one of three genetically distinct peptide-transport systems in enteric bacteria. Dpp also plays a role in chemotaxis towards peptides. We have devised three selections for dpp mutations based on resistance to toxic peptides (bacilysin, valine-containing peptides, and bialaphos). All dpp mutations mapped to a single chromosomal locus between 77 and 78 min in Salmonella typhimurium and at 79.2 min in Escherichia coli. Expression of dpp was constitutive in both species but the absolute level of expression varied widely between strains. At least in part this difference in expression levels is determined by cis-acting sequences. The dpp locus of E. coli was cloned. The first gene in the operon, dppA, encodes a periplasmic dipeptide-binding protein (DBP) required for dipeptide transport and chemotaxis. Downstream of dppA are other genes required for transport but not for chemotaxis. The dipeptide-binding protein was found to share 26.5% sequence identity with the periplasmic oligopeptide-binding protein OppA.

Dissecting the molecular mechanism of ion-solute cotransport: substrate specificity mutations in the putP gene affect the kinetics of proline transport

Rare mutations that alter the substrate specificity of proline permease cluster in discrete regions of the putP gene, suggesting that they may replace amino acids at the active site of the enzyme. If putP substrate specificity mutations directly after the active site of proline permease, the mutants should show specific defects in the kinetics of proline transport. In order to test this prediction, we examined the kinetics of three putP substrate specificity mutants. One class of mutation increases the Km over 120 fold but only decreases the Vmax fourfold. Such Km mutants may be specifically defective in substrate recognition, thus identifying an amino acid critical for substrate binding. Another class of mutation decreases the Vmax 80-fold without changing the Km. Vmax mutants appear to alter the rate of substrate translocation without affecting the substrate binding site. The last class of mutation alters both the Km and Vmax of proline transport. These results indicate that substrate specificity mutations alter amino acids critical for Na+/proline symport.

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

Amino acid transport in membrane vesicles of Bacillus stearothermophilus was studied. A relatively high concentration of sodium ions is needed for uptake of L-alanine (Kt = 1.0 mM) and L-leucine (Kt = 0.4 mM). In contrast, the Na(+)-H(+)-L-glutamate transport system has a high affinity for sodium ions (Kt less than 5.5 microM). Lithium ions, but no other cations tested, can replace sodium ions in neutral amino acid transport. The stimulatory effect of monensin on the steady-state accumulation level of these amino acids and the absence of transport in the presence of nonactin indicate that these amino acids are translocated by a Na+ symport mechanism. This is confirmed by the observation that an artificial delta psi and delta mu Na+/F but not a delta pH can act as a driving force for uptake. The transport system for L-alanine is rather specific. L-Serine, but not L-glycine or other amino acids tested, was found to be a competitive inhibitor of L-alanine uptake. On the other hand, the transport carrier for L-leucine also translocates the amino acids L-isoleucine and L-valine. The initial rates of L-glutamate and L-alanine uptake are strongly dependent on the medium pH. The uptake rates of both amino acids are highest at low external pH (5.5 to 6.0) and decline with increasing pH. The pH allosterically affects the L-glutamate and L-alanine transport systems. The maximal rate of L-glutamate uptake (Vmax) is independent of the external pH between pH 5.5 and 8.5, whereas the affinity constant (Kt) increases with increasing pH. A specific transport system for the basic amino acids L-lysine and L-arginine in the membrane vesicles has also been observed. Transport of these amino acids occurs most likely by a uniport mechanism.

Activation of a new proline transport system in Salmonella typhimurium

Proline uptake can be mediated by three different transport systems in wild-type Salmonella typhimurium: a high-affinity proline transport system encoded by the putP gene and two glycine-betaine transport systems with a low affinity for proline encoded by the proP and proU genes. However, only the PutP permease transports proline well enough t allow growth on proline as a sole carbon or nitrogen source. By selecting for mutations that allow a putP mutant to grow on proline as a sole nitrogen source, we isolated mutants (designated proZ) that appeared to activate a cryptic proline transport system. These mutants enhanced the transport of proline and proline analogs but did not require the function of any of the known proline transport genes. The mutations mapped between 75 and 77.5 min on the S. typhimurium linkage map. Proline transport by the proZ mutants was competitively inhibited by isoleucine and leucine, which suggests that the ProZ phenotype may be due to unusual mutations that alter the substrate specificity of the branched-chain amino acid transport system encoded by the liv genes.

Mechanism of Na+/proline symport in Escherichia coli: reappraisal of the effect of cation binding to the Na+/proline symport carrier

The proton and sodium ion dependences of the proline binding and transport activities of the proline carrier in Escherichia coli were investigated in detail. The binding activity in cytoplasmic membrane vesicles from a carrier over-producing strain (PT21/pTMP5) was absolutely dependent on the presence of Na+, but did not necessarily require protonation of the carrier, in contrast to the model previously reported (Mogi, T., Anraku, Y. 1984. J. Biol. Chem. 259:7797-7801). Based on this and previous observations, we propose a modified model of the proline binding reaction of the proline carrier, in which a proton is supposed to be a regulatory factor for the binding activity. The apparent Michaelis constant of proline (Kt) of the transport activity of cytoplasmic membrane vesicles from the wild type E. coli strain driven by a respiratory substrate, ascorbate, showed dependence on a low concentration of sodium ion. The Michaelis constant of sodium ion for transport (KtNa) was estimated to be 25 microM. The proline transport activities in membrane vesicles and intact cells were modulated by H+ concentration, the inhibitory effect of protons (pKa approximately equal to 6) being similar to that observed previously (Mogi, T., Anraku, Y. 1984. J. Biol. Chem. 259:7802-7806). Based on these observations and the modified model of substrate binding to the proline carrier, a model of the proline/Na+ symport mechanism is proposed, in which a proton is postulated to be a regulatory factor of the transport activity.

Energy coupling to periplasmic binding protein-dependent transport systems: stoichiometry of ATP hydrolysis during transport in vivo

Periplasmic binding protein-dependent transport systems mediate the accumulation of many diverse substrates in prokaryotic cells. Similar transport systems, including the P-glycoprotein responsible for multidrug resistance in human tumors, are also found in eukaryotes. The mechanism by which energy is coupled to the accumulation of substrate by these transport systems has been controversial. In this paper we demonstrate that ATP hydrolysis occurs in vivo concomitantly with transport. These data strongly suggest that ATP hydrolysis directly energizes substrate accumulation by these transport systems. The apparent stoichiometry is one to two molecules of ATP hydrolyzed per molecule of substrate transported.

Binding-protein-dependent alanine transport in Rhodobacter sphaeroides is regulated by the internal pH

The properties of an L-alanine uptake system in Rhodobacter sphaeroides were studied and compared with those of H+/lactose symport in R. sphaeroides 4P1, a strain in which the lactose carrier of Escherichia coli has been cloned and functionally expressed (F. E. Nano, Ph.D. thesis, University of Illinois, Urbana, 1984). Previous studies indicated that both transport systems were active only when electron transfer took place in the respiratory or cyclic electron transfer chain, while uptake of L-alanine also required the presence of K+ (M. G. L. Elferink, Ph.D. thesis, University of Groningen, Groningen, The Netherlands, 1986). The results presented in this paper offer an explanation for these findings. Transport of the nonmetabolizable L-alanine analog 2-alpha-aminoisobutyric acid (AIB) is mediated by a shock-sensitive transport system. The apparently unidirectional uptake of AIB results in accumulation levels which exceed 7 x 10(3). The finding of L-alanine-binding activity in the concentrated crude shock fluid indicates that L-alanine is taken up by a binding-protein-dependent transport system. Transport of the nonmetabolizable lactose analog methyl-beta-D-thiogalactopyranoside (TMG) by the lactose carrier under anaerobic conditions in the dark was observed in cells and membrane vesicles. This indicates that the H+/lactose symport system is active without electron transfer. Uptake of AIB, but not that of TMG, is inhibited by vanadate with a 50% inhibitory concentration of 50 microM, which suggests a role of a phosphorylated intermediate in AIB transport. Uptake of TMG and AIB is regulated by the internal pH. The initial rates of uptake increased with the internal pH, and and pKa values of 7.2 for TMG and 7.8 for AIB. At an internal pH of 7, no AIB uptake occurred, and the rate of TMG uptake was only 30% of the rate at an internal pH of 8. In a previous study, we found that K+ plays an essential role in regulating the internal pH (T. Abee, K. J. Hellingwerf, and W. N. Konings, J. Bacteriol. 170:5647-5653, 1988). The dependence of solute transport in R. sphaeroides on both K+ and activity of an electron transfer chain can be explained by an effect of the internal pH, which subsequently influences the activities of the lactose-and binding-protein-dependent L-alanine transport system.

Influence of osmoregulation processes on starvation survival of Escherichia coli in seawater

The adaptation of enteric bacteria in seawater has previously been described in terms of nutrient starvation. In the present paper, we bring experimental arguments suggesting that survival of these microorganisms could also depend on their ability to overcome the effects of osmotic stress. We analyzed the influence of osmoregulatory mechanisms (potassium transport, transport and accumulation of organic osmolytes) on the survival of Escherichia coli in seawater microcosms by using mutants lacking components of the osmotic stress response. Long-term protection was afforded to cells by growth in a medium whose osmotic pressure was increased by either NaCl, LiCl, or saccharose. Achievement of the protection state depended at least partly on osmoregulatory mechanisms, but differed when these were activated or induced during prior growth or in resting cells suspended in phosphate buffer or in seawater. When achieved during growth, K+ transport, glycine-betaine (GBT) synthesis or transport, and trehalose synthesis helped increase the ability to survive in seawater. Protection by GBT was also obtained with resting cells in a phosphate buffer at high osmotic pressure. However, when added only to the seawater, GBT did not change the survival ability of cells no matter what their osmoregulation potential. These results showed that the survival of E. coli cells in seawater depends, at least partly, on whether they possess certain genes which enable them to regulate osmotic pressure and whether they can be stimulated to express those genes before or after their release into the environment. This expression requires nutrients as the substrates from which the corresponding gene products are made.

Active transport of maltose in membrane vesicles obtained from Escherichia coli cells producing tethered maltose-binding protein

Attempts to reconstitute periplasmic binding protein-dependent transport activity in membrane vesicles have often resulted in systems with poor and rather inconsistent activity, possibly because of the need to add a large excess of purified binding protein to the vesicles. We circumvented this difficulty by using a mutant which produces a precursor maltose-binding protein that is translocated across the cytoplasmic membrane but is not cleaved by the signal peptidase (J. D. Fikes and P. J. Bassford, Jr., J. Bacteriol. 169:2352-2359, 1987). The protein remains tethered to the cytoplasmic membrane, presumably through the hydrophobic signal sequence, and we show here that the spheroplasts and membrane vesicles prepared from this mutant catalyze active maltose transport without the addition of purified maltose-binding protein. In vesicles, the transport requires electron donors, such as ascorbate and phenazine methosulfate or D-lactate. However, inhibition by dicyclohexylcarbodiimide and stimulation of transport by the inculsion of ADP or ATP in the intravesicular space suggest that ATP (or compounds derived from it) is involved in the energization of the transport. The transport activity of intact cells can be recovered without much inactivation in the vesicles, and their high activity and ease of preparation will be useful in studies of the mechanism of the binding protein-dependent transport process.

Genetic studies of mutants in a high-affinity methionine transport system in Salmonella typhimurium

A total of 30 metP mutations defective in the high-affinity methionine transport system were linked in P1 transduction to the zaf-1351::Tn10 insertion mutation at min 5-6 on the Salmonella typhimurium chromosome map. The relationship of metP to several other markers in this region was studied. Methionine transport was strongly inhibited by arsenate, suggesting that the metP system belongs to the shock-sensitive category and possesses a periplasmic binding protein. However, other experiments provided less clear cut evidence. Transport activity was only slightly reduced by osmotic shock; a methionine binding activity was detected in shock fluids from the wild-type strain, and although this activity was reduced by 50% in 3 frameshift mutants, mutants without any activity were not found. No differences were detected in the shock fluids of the 30 mutants when examined by SDS-polyacrylamide gel electrophoresis.

Proline porters effect the utilization of proline as nutrient or osmoprotectant for bacteria

Proline is utilized by all organisms as a protein constituent. It may also serve as a source of carbon, energy and nitrogen for growth or as an osmoprotectant. The molecular characteristics of the proline transport systems which mediate the multiple functions of proline in the Gram negative enteric bacteria, Escherichia coli and Salmonella typhimurium, are now becoming apparent. Recent research on those organisms has provided both protocols for the genetic and biochemical characterization of the enzymes mediating proline transport and molecular probes with which the degree of homology among the proline transport systems of archaebacteria, eubacteria and eukaryotes can be assessed. This review has provided a detailed summary of recent research on proline transport in E. coli and S. typhimurium; the properties of other organisms are cited primarily to illustrate the generality of those observations and to show where homologous proline transport systems might be expected to occur. The characteristics of proline transport in eukaryotic microorganisms have recently been reviewed (Horak, 1986).

Mutations of putP that alter the lithium sensitivity of Salmonella typhimurium

The putP gene encodes the major proline permease in Salmonella typhimurium that couples transport of proline to the sodium electrochemical gradient. To identify residues involved in the cation binding site, we have isolated putP mutants that confer resistance to lithium during growth on proline. Wild-type S. typhimurium can grow well on proline as the sole carbon source in media supplemented with NaCl, but grows poorly when LiCl is substituted for NaCl. In contrast to the growth phenotype, proline permease is capable of transporting proline via Na+/proline or Li+/proline symport. Therefore, we selected mutants that grow well on media containing proline as the sole carbon source in the presence of lithium ions. All of the mutants assayed exhibit decreased rates of Li+/proline and Na+/proline cotransport relative to wild type. The location of each mutation was determined by deletion mapping: the mutations cluster in two small deletion intervals at the 5' and 3' termini of the putP gene. The map positions of these lithium resistance mutations are different from the locations of the previously isolated substrate specificity mutations. These results suggest that Lir mutations may define domains of the protein that fold to form the cation binding site of proline permease.

Regulation of proline utilization in Salmonella typhimurium: molecular characterization of the put operon, and DNA sequence of the put control region

The two genes required for proline utilization (put) in Salmonella typhimurium form a divergent operon. Extensive genetic evidence suggests that transcription of the put operon is autoregulated by the putA gene product, a membrane-associated dehydrogenase. In order to understand the mechanism of regulation, we characterized plasmid clones of the put operon. A 7.5 kb clone contains both of the put structural genes and regulatory sites. This clone only expressed two unique proteins corresponding to the putA and putP gene products. By comparing the physical and genetic maps of the put operon, the position of the put regulatory region was defined and the DNA sequence of this region was determined. Analysis of the DNA sequence indicated several potential regulatory sites for the put genes. Based on genetic and physical mapping studies, the most likely regulatory sites are two convergent promoters approximately 30 bp apart. A 27 bp palindrome located between the two promoters may be the operator for autoregulation by the PutA protein. The putA translational start site is 40 bp downstream of its putative mRNA start site. The putP promoter and its translational start site are separated by a 400 bp untranslated region.

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

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Molecular characterization of the oligopeptide permease of Salmonella typhimurium

The oligopeptide permease (Opp) of Salmonella typhimurium is a periplasmic binding protein-dependent transport system and handles any peptides containing from two to five amino acid residues. Opp plays an important nutritional role and is also required for the recycling of cell wall peptides. We have determined the nucleotide sequence of the opp operon. In addition to the four opp genes identified previously by genetic means (oppABCD) a fifth gene, oppF, is shown to be cotranscribed as part of the opp operon. Using reverse genetics, we show that oppF also encodes an essential component of the Opp transport system. The five proteins, OppABCDF, are shown to be the only proteins required for Opp function. Regulation of opp expression and of the differential expression of genes within the operon is investigated. We have devised a simple means of constructing lacZ gene fusions to any S. typhimurium chromosomal gene in vivo, using derivatives of bacteriophage Mu. Using this procedure, opp-lacZ gene fusions were selected. The resultant Opp-LacZ hybrid proteins were used to show that OppB, OppC and OppD are membrane-associated proteins. A detailed comparison of the Opp components with those of other binding protein-dependent transport systems provides insight into the mechanisms and evolution of these transport systems.

Mechanisms of halotolerance in microorganisms

Microorganisms have the ability to adapt to a wide range of NaCl concentrations. In general the NaCl tolerance shown by microbes far exceeds the salt tolerance of any other organism, procryote or eukaryote. There are at least three mechanisms available for adaptation to different salt concentrations. The first would be a passive one in which the cytoplasmic ion content would always equal that in the medium. A second mechanism which is used by many organisms involves concentrating compatible solutes to create an osmotic balance between the cytoplasm and the external environment. The third mechanism involves changing the cell physiology to control the movement of water allowing the cell to exist with an ionically dilute cytoplasm. This article will review the major developments and discuss the implications of increasing knowledge about salt tolerance in microorganisms.

Proline transport in Salmonella typhimurium: putP permease mutants with altered substrate specificity

The putP gene encodes a proline permease required for Salmonella typhimurium LT2 to grow on proline as the sole source of nitrogen. The wild-type strain is sensitive to two toxic proline analogs (azetidine-2-carboxylic acid and 3,4-dehydroproline) also transported by the putP permease. Most mutations in putP prevent transport of all three substrates. Such mutants are unable to grow on proline and are resistant to both of the analogs. To define domains of the putP gene that specify the substrate binding site, we used localized mutagenesis to isolate rare mutants with altered substrate specificity. The position of the mutations in the putP gene was determined by deletion mapping. Most of the mutations are located in three small (approximately 100-base-pair) deletion intervals of the putP gene. The sensitivity of the mutants to the proline analogs was quantitated by radial streaking to determine the affinity of the mutant permeases for the substrates. Some of the mutants showed apparent changes in the kinetics of the substrates transported. These results indicate that the substrate specificity mutations are probably due to amino acid substitutions at or near the active site of proline permease.

Osmotic regulation of transcription: induction of the proU betaine transport gene is dependent on accumulation of intracellular potassium

The proU locus, which encodes a high-affinity betaine transport system, and the kdp operon, which encodes a potassium transport system, are the principal osmoresponsive genes in Escherichia coli and Salmonella typhimurium. The kdp operon is known to be induced in response to changes in cell turgor. We have investigated the control of proU expression and shown that it differs from that of kdp in a number of fundamental ways. Rather than responding to changes in turgor, proU expression is principally determined by the intracellular accumulation of potassium ions. Potassium and betaine were shown to play distinct osmoprotective roles. Potassium serves as the principal osmoprotectant and is accumulated in response to low-level osmotic stress to restore turgor. As external osmolarity is increased to a level at which the corresponding increase in internal potassium concentrations is potentially deleterious to enzyme function, betaine (when available) is accumulated in preference to potassium. The different mechanisms of proU and kdp regulation reflect the different physiological roles of these two osmoprotectants.