Two Aspartate Transport Systems in Escherichia coli

Production of Succinic Acid from Amino Acids in Escherichia coli

Glutamate (Glu) and aspartate (Asp) are the most abundant amino acids in various sources of protein waste, recognized as a sustainable resource. In this study, Escherichia coli was engineered to produce succinic acid (SA) from Glu and Asp. Succinate dehydrogenase involved in the tricarboxylic acid was inactivated in the Glu-utilizing strain. To grow on Asp, this mutant strain was subjected to metabolic evolution. One resulting strain capable of metabolizing Asp was further evolved to improve the growth of Glu and Asp. After the deletion of arcA, the resulting strain was employed for the aerobic production of SA. The shake-flask culture was conducted with the minimal medium containing 10 g/L Glu and 10 g/L Asp. Finally, it resulted in the SA production, with a titer, the molar yield, and productivity reaching 72.8 mM (i.e., 8.6 g/L), 0.54 (ca. 75.4% of the theoretical yield), and 0.66 g/L/h, respectively. Overall, this study opens up a new avenue of the biorefinery platform based on renewable amino acids.

L-Aspartate as a high-quality nitrogen source in Escherichia coli: Regulation of L-aspartase by the nitrogen regulatory system and interaction of L-aspartase with GlnB

Escherichia coli uses the C4-dicarboxylate transporter DcuA for L-aspartate/fumarate antiport, which results in the exploitation of L-aspartate for fumarate respiration under anaerobic conditions and for nitrogen assimilation under aerobic and anaerobic conditions. L-Aspartate represents a high-quality nitrogen source for assimilation. Nitrogen assimilation from L-aspartate required DcuA, and aspartase AspA to release ammonia. Ammonia is able to provide by established pathways the complete set of intracellular precursors (ammonia, L-aspartate, L-glutamate, and L-glutamine) for synthesizing amino acids, nucleotides, and amino sugars. AspA was regulated by a central regulator of nitrogen metabolism, GlnB. GlnB interacted with AspA and stimulated its L-aspartate deaminase activity (NH₃ -forming), but not the reverse amination reaction. GlnB stimulation required 2-oxoglutarate and ATP, or uridylylated GlnB-UMP, consistent with the activation of nitrogen assimilation under nitrogen limitation. Binding to AspA was lost in the GlnB(Y51F) mutant of the uridylylation site. AspA, therefore, represents a new type of GlnB target that binds GlnB (with ATP and 2-oxoglutarate), or GlnB-UMP (with or without effectors), and both situations stimulate AspA deamination activity. Thus, AspA represents the central enzyme for nitrogen assimilation from L-aspartate, and AspA is integrated into the nitrogen assimilation network by the regulator GlnB.

Metabolomic analysis of riboswitch containing E. coli recombinant expression system

In this study we have employed metabolomics approaches to understand the metabolic effects of producing enhanced green fluorescent protein (eGFP) as a recombinant protein in Escherichia coli cells. This metabolic burden analysis was performed against a number of recombinant expression systems and control strains and included: (i) standard transcriptional recombinant expression control system BL21(DE3) with the expression plasmid pET-eGFP, (ii) the recently developed dual transcriptional-translational recombinant expression control strain BL21(IL3), with pET-eGFP, (iii) BL21(DE3) with an empty expression plasmid pET, (iv) BL21(IL3) with an empty expression plasmid, and (v) BL21(DE3) without an expression plasmid; all strains were cultured under various induction conditions. The growth profiles of all strains together with the results gathered by the analysis of the Fourier transform infrared (FT-IR) spectroscopy data, identified IPTG-dependent induction as the dominant factor hampering cellular growth and metabolism, which was in general agreement with the findings of GC-MS analysis of cell extracts and media samples. In addition, the exposure of host cells to the synthetic inducer ligand, pyrimido[4,5-d] pyrimidine-2,4-diamine (PPDA), of the orthogonal riboswitch containing expression system (BL21(IL3)) did not display any detrimental effects, and its detected levels in all the samples were at similar levels, emphasising the inability of the cells to metabolise PPDA. The overall results obtained in this study suggested that although the BL21(DE3)-EGFP and BL21(IL3)-EGFP strains produced comparable levels of recombinant eGFP, the presence of the orthogonal riboswitch seemed to be moderating the metabolic burden of eGFP production in the cells enabling higher biomass yield, whilst providing a greater level of control over protein expression.

C4-Dicarboxylate Utilization in Aerobic and Anaerobic Growth

C4-dicarboxylates and the C4-dicarboxylic amino acid l-aspartate support aerobic and anaerobic growth of Escherichia coli and related bacteria. In aerobic growth, succinate, fumarate, D- and L-malate, L-aspartate, and L-tartrate are metabolized by the citric acid cycle and associated reactions. Because of the interruption of the citric acid cycle under anaerobic conditions, anaerobic metabolism of C4-dicarboxylates depends on fumarate reduction to succinate (fumarate respiration). In some related bacteria (e.g., Klebsiella), utilization of C4-dicarboxylates, such as tartrate, is independent of fumarate respiration and uses a Na+-dependent membrane-bound oxaloacetate decarboxylase. Uptake of the C4-dicarboxylates into the bacteria (and anaerobic export of succinate) is achieved under aerobic and anaerobic conditions by different sets of secondary transporters. Expression of the genes for C4-dicarboxylate metabolism is induced in the presence of external C4-dicarboxylates by the membrane-bound DcuS-DcuR two-component system. Noncommon C4-dicarboxylates like l-tartrate or D-malate are perceived by cytoplasmic one-component sensors/transcriptional regulators. This article describes the pathways of aerobic and anaerobic C4-dicarboxylate metabolism and their regulation. The citric acid cycle, fumarate respiration, and fumarate reductase are covered in other articles and discussed here only in the context of C4-dicarboxylate metabolism. Recent aspects of C4-dicarboxylate metabolism like transport, sensing, and regulation will be treated in more detail. This article is an updated version of an article published in 2004 in EcoSal Plus. The update includes new literature, but, in particular, the sections on the metabolism of noncommon C4-dicarboxylates and their regulation, on the DcuS-DcuR regulatory system, and on succinate production by engineered E. coli are largely revised or new.

C4-Dicarboxylate Degradation in Aerobic and Anaerobic Growth

C4-dicarboxylates, like succinate, fumarate, L- and D-malate, tartrate, and the C4-dicarboxylic amino acid aspartate, support aerobic and anaerobic growth of Escherichia coli and related bacteria and can serve as carbon and energy sources. In aerobic growth, the C4-dicarboxylates are oxidized in the citric acid cycle. Due to the interruption of the citric acid cycle under anaerobic conditions, anaerobic metabolism of the C4-dicarboxylates depends on fumarate reduction to succinate. In some related bacteria (e.g., Klebsiella), degradation of C4-dicarboxylates, like tartrate, uses a different mechanism and pathway. It requires the functioning of an Na+-dependent and membrane-associated oxaloacetate decarboxylase. Due to the incomplete function of the citric acid cycle in anaerobic growth, succinate supports only aerobic growth of E. coli. This chapter describes the pathways of and differences in aerobic and anaerobic C4-dicarboxylate metabolism and the physiological consequences. The citric acid cycle, fumarate respiration, and fumarate reductase are discussed here only in the context of aerobic and anaerobic C4-dicarboxylate metabolism. Some recent aspects of C4-dicarboxylate metabolism, such as transport and sensing of C4-dicarboxylates, and their relationships are treated in more detail.

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.

Characterization of the l-alanine exporter AlaE of Escherichia coli and its potential role in protecting cells from a toxic-level accumulation of l-alanine and its derivatives

We previously reported that the alaE gene of Escherichia coli encodes the l-alanine exporter AlaE. The objective of this study was to elucidate the mechanism of the AlaE exporter. The minimum inhibitory concentration of l-alanine and l-alanyl-l-alanine in alaE-deficient l-alanine-nonmetabolizing cells MLA301ΔalaE was 4- and >4000-fold lower, respectively, than in the alaE-positive parent cells MLA301, suggesting that AlaE functions as an efflux pump to avoid a toxic-level accumulation of intracellular l-alanine and its derivatives. Furthermore, the growth of the alaE-deficient mutant derived from the l-alanine-metabolizing strain was strongly inhibited in the presence of a physiological level of l-alanyl-l-alanine. Intact MLA301ΔalaE and MLA301ΔalaE/pAlaE cells producing plasmid-borne AlaE, accumulated approximately 200% and 50%, respectively, of the [³H]l-alanine detected in MLA301 cells, suggesting that AlaE exports l-alanine. When 200 mmol/L l-alanine-loaded inverted membrane vesicles prepared from MLA301ΔalaE/pAlaE were placed in a solution containing 200 mmol/L or 0.34 μmol/L l-alanine, energy-dependent [³H]l-alanine accumulation occurred under either condition. This energy-dependent uphill accumulation of [³H]l-alanine was strongly inhibited in the presence of carbonyl cyanide m-chlorophenylhydrazone but not by dicyclohexylcarbodiimide, suggesting that the AlaE-mediated l-alanine extrusion was driven by proton motive force. Based on these results, physiological roles of the l-alanine exporter are discussed.

Selective (15)N-labeling of the side-chain amide groups of asparagine and glutamine for applications in paramagnetic NMR spectroscopy

The side-chain amide groups of asparagine and glutamine play important roles in stabilizing the structural fold of proteins, participating in hydrogen-bonding networks and protein interactions. Selective (15)N-labeling of side-chain amides, however, can be a challenge due to enzyme-catalyzed exchange of amide groups during protein synthesis. In the present study, we developed an efficient way of selectively labeling the side chains of asparagine, or asparagine and glutamine residues with (15)NH2. Using the biosynthesis pathway of tryptophan, a protocol was also established for simultaneous selective (15)N-labeling of the side-chain NH groups of asparagine, glutamine, and tryptophan. In combination with site-specific tagging of the target protein with a lanthanide ion, we show that selective detection of (15)N-labeled side-chains of asparagine and glutamine allows determination of magnetic susceptibility anisotropy tensors based exclusively on pseudocontact shifts of amide side-chain protons.

Prolonged stimuli alter the bacterial chemosensory clusters

The clustering of membrane-bound receptors plays an essential role in various biological systems. A notable model system for studying this phenomenon is the bacterial chemosensory cluster that allows motile bacteria to navigate along chemical gradients in their environment. While the basic structure of these chemosensory clusters is becoming clear, their dynamic nature and operation are not yet understood. By measuring the fluorescence polarization of tagged receptor clusters in live Escherichia coli cells, we provide evidence for stimulus-induced dynamics in these sensory clusters. We find that when a stimulus is applied, the packing of the receptors slowly decreases and that the process reverses when the stimulus is removed. Consistent with these physical changes we find that the effective cooperativity of the kinase response slowly evolves in the presence of a stimulus. Time-lapse fluorescence imaging indicates that, despite these changes, the receptor clusters do not generally dissociate upon ligand binding. These data reveal stimulus-dependent plasticity in chemoreceptor clusters.

Identification, source, and metabolism of N-ethylglutamate in Escherichia coli

N-Ethylglutamate (NEG) was detected in Escherichia coli BL21 cells grown on LB broth, and it was found to occur at a concentration of ∼4 mM in these cells under these conditions. The same cells grown on M9 glucose medium contained no detectable amount of NEG. Analysis of the LB broth showed the presence of NEG, a compound never before reported as a natural product. Isotope dilution analysis showed that it occurred at a concentration of 160 μM in LB broth. Analyses of yeast extract and tryptone, the organic components of LB broth, both showed the presence NEG. It was demonstrated that NEG can be generated during the autolysis of the yeast used in the preparation of the yeast extract. Growth of these E. coli cells in LB broth prepared in deuterated water showed no incorporation of deuterium into NEG, demonstrating that E. coli cells did not generate the NEG. Cell growth rates were not affected by the addition of 5 mM NEG to either LB or M9 glucose medium. l-[ethyl-(2)H(4)]NEG was found to be readily incorporated into the cells and metabolized by the cells. From these results, it was concluded that all of the NEG present in the cells was taken up from the medium. NEG could serve as the sole nitrogen source for E. coli when grown on M9 glucose medium in the presence of glucose but could not serve as the sole carbon source on M9 medium in the absence of glucose.

Enzymes involved in anaerobic respiration appear to play a role in Actinobacillus pleuropneumoniae virulence

Actinobacillus pleuropneumoniae, the etiological agent of porcine pleuropneumonia, is able to survive on respiratory epithelia, in tonsils, and in the anaerobic environment of encapsulated sequesters. It was previously demonstrated that a deletion of the anaerobic dimethyl sulfoxide reductase gene (dmsA) results in attenuation in acute disease (N. Baltes, S. Kyaw, I. Hennig-Pauka, and G. F. Gerlach, Infect. Immun. 71:6784-6792, 2003). In the present study, using two-dimensional polyacrylamide gel electrophoresis and quadrupole time-of-flight mass spectrometry, we identified an aspartate ammonia-lyase (AspA) which is upregulated upon induction with bronchoalveolar lavage fluid (BALF). This enzyme is involved in the production of fumarate, an alternative electron acceptor under anaerobic conditions. The coding gene (aspA) was cloned and shown to be present in all A. pleuropneumoniae serotype reference strains. The transcriptional start point was identified downstream of a putative FNR binding motif, and BALF-dependent activation of aspA was confirmed by construction of an isogenic A. pleuropneumoniae mutant carrying a chromosomal aspA::luxAB transcriptional fusion. Two aspA deletion mutants, A. pleuropneumoniae DeltaaspA and A. pleuropneumoniae DeltaaspADeltadmsA, were constructed, both showing reduced growth under anaerobic conditions in vitro. Pigs challenged with either of the two mutants in an aerosol infection model showed a lower lung lesion score than that of the A. pleuropneumoniae wild-type (wt) controls. Pigs challenged with A. pleuropneumoniae DeltaaspADeltadmsA had a significantly lower clinical score, and this mutant was rarely reisolated from unaltered lung tissue; in contrast, A. pleuropneumoniae DeltaaspA and the A. pleuropneumoniae wt were consistently reisolated in high numbers. These results suggest that enzymes involved in anaerobic respiration are necessary for the pathogen's ability to persist on respiratory tract epithelium and play an important role in A. pleuropneumoniae pathogenesis.

Biosynthesis of Glutamate, Aspartate, Asparagine, L-Alanine, and D-Alanine

Glutamate, aspartate, asparagine, L-alanine, and D-alanine are derived from intermediates of central metabolism, mostly the citric acid cycle, in one or two steps. While the pathways are short, the importance and complexity of the functions of these amino acids befit their proximity to central metabolism. Inorganic nitrogen (ammonia) is assimilated into glutamate, which is the major intracellular nitrogen donor. Glutamate is a precursor for arginine, glutamine, proline, and the polyamines. Glutamate degradation is also important for survival in acidic environments, and changes in glutamate concentration accompany changes in osmolarity. Aspartate is a precursor for asparagine, isoleucine, methionine, lysine, threonine, pyrimidines, NAD, and pantothenate; a nitrogen donor for arginine and purine synthesis; and an important metabolic effector controlling the interconversion of C3 and C4 intermediates and the activity of the DcuS-DcuR two-component system. Finally, L- and D-alanine are components of the peptide of peptidoglycan, and L-alanine is an effector of the leucine responsive regulatory protein and an inhibitor of glutamine synthetase (GS). This review summarizes the genes and enzymes of glutamate, aspartate, asparagine, L-alanine, and D-alanine synthesis and the regulators and environmental factors that control the expression of these genes. Glutamate dehydrogenase (GDH) deficient strains of E. coli, K. aerogenes, and S. enterica serovar Typhimurium grow normally in glucose containing (energy-rich) minimal medium but are at a competitive disadvantage in energy limited medium. Glutamate, aspartate, asparagine, L-alanine, and D-alanine have multiple transport systems.

Elucidation of a PTS-carbohydrate chemotactic signal pathway in Escherichia coli using a time-resolved behavioral assay

Chemotaxis of Escherichia coli toward phosphotransferase systems (PTSs)-carbohydrates requires phosphoenolpyruvate-dependent PTSs as well as the chemotaxis response regulator CheY and its kinase, CheA. Responses initiated by flash photorelease of a PTS substrates D-glucose and its nonmetabolizable analog methyl alpha-D-glucopyranoside were measured with 33-ms time resolution using computer-assisted motion analysis. This, together with chemotactic mutants, has allowed us to map out and characterize the PTS chemotactic signal pathway. The responses were absent in mutants lacking the general PTS enzymes EI or HPr, elevated in PTS transport mutants, retarded in mutants lacking CheZ, a catalyst of CheY autodephosphorylation, and severely reduced in mutants with impaired methyl-accepting chemotaxis protein (MCP) signaling activity. Response kinetics were comparable to those triggered by MCP attractant ligands over most of the response range, the most rapid being 11.7 +/- 3.1 s-1. The response threshold was <10 nM for glucose. Responses to methyl alpha-D-glucopyranoside had a higher threshold, commensurate with a lower PTS affinity, but were otherwise kinetically indistinguishable. These facts provide evidence for a single pathway in which the PTS chemotactic signal is relayed rapidly to MCP-CheW-CheA signaling complexes that effect subsequent amplification and slower CheY dephosphorylation. The high sensitivity indicates that this signal is generated by transport-induced dephosphorylation of the PTS rather than phosphoenolpyruvate consumption.

Chemotactic responses of Escherichia coli to small jumps of photoreleased L-aspartate

Computer-assisted motion analysis coupled to flash photolysis of caged chemoeffectors provides a means for time-resolved analysis of bacterial chemotaxis. Escherichia coli taxis toward the amino acid attractant L-aspartate is mediated by the Tar receptor. The physiology of this response, as well as Tar structure and biochemistry, has been studied extensively. The beta-2, 6-dinitrobenzyl ester of L-aspartic acid and the 1-(2-nitrophenyl)ethyl ether of 8-hydroxypyrene-1,3,6-tris-sulfonic acid were synthesized. These compounds liberated L-aspartate and the fluorophore 8-hydroxypyrene 1,3,6-tris-sulfonic acid (pyranine) upon irradiation with near-UV light. Photorelease of the fluorophore was used to define the amplitude and temporal stability of the aspartate jumps employed in chemotaxis experiments. The dependence of chemotactic adaptation times on aspartate concentration, determined in mixing experiments, was best fit by two Tar aspartate-binding sites. Signal processing (excitation) times, amplitudes, and adaptive recovery of responses elicited by aspartate jumps producing less than 20% change in receptor occupancy were characterized in photorelease assays. Aspartate concentration jumps in the nanomolar range elicited measurable responses. The response threshold and sensitivity of swimming bacteria matched those of bacteria tethered to glass by a single flagellum. Stimuli of similar magnitude, delivered either by rapid mixing or photorelease, evoked responses of similar strength, as assessed by recovery time measurements. These times remained proportional to change in receptor occupancy close to threshold, irrespective of prior occupancy. Motor excitation responses decayed exponentially with time. Rates of excitation responses near threshold ranged from 2 to 7 s-1. These values are consistent with control of excitation signaling by decay of phosphorylated pools of the response regulator protein, CheY. Excitation response rates increased slightly with stimulus size up to values limited by the instrumentation; the most rapid was measured to be 16 +/- 3 (SE) s-1. This increase may reflect simultaneous activation of CheY dephosphorylation, together with inhibition of its phosphorylation.

Chemotactic-like response of Escherichia coli cells lacking the known chemotaxis machinery but containing overexpressed CheY

We describe a chemotactic-like response of Escherichia coli strains lacking most of the known chemotaxis machinery but containing high levels of the response regulator CheY. The bacteria accumulated in aspartate-containing capillaries, they formed rings on tryptone-containing semisolid agar, and the probability of counterclockwise flagellar rotation transiently increased in response to stimulation with aspartate (10(-10)-10(-5) M; the response was inverted at > 10(-4) M). The temporal response was partial and delayed, as was the response of a control wild-type strain having a high CheY level. alpha-Methyl-DL-aspartate, a non-metabolizable analogue of aspartate as well as other known attractants of E. Coli, glucose and, to a lesser extent, galactose, maltose and serine caused a similar response. So did low concentrations of acetate and benzoate (which, at higher concentrations, act as repellents for wild-type E. coli). Other tested repellents such as indole, Ni2+ and CO2+ increased the clockwise bias. These observations raise the possibility that, at least when the conventional signal transduction components are missing, a non-conventional chemotactic signal transduction pathway might be functional in E. coli. Potential molecular mechanisms are discussed.

Anaerobic fumarate transport in Escherichia coli by an fnr-dependent dicarboxylate uptake system which is different from the aerobic dicarboxylate uptake system

Escherichia coli grown anaerobically with fumarate as electron acceptor is able to take up C4-dicarboxylates by a specific transport system. The system differs in all tested parameters from the known aerobic C4-dicarboxylate transporter. The anaerobic transport system shows higher transport rates (95 mumol/g [dry weight] per min versus 30 mumol/g/min) and higher Kms (400 versus 30 microM) for fumarate than for the aerobic system. Mutants lacking the aerobic dicarboxylate uptake system are able to grow anaerobically at the expense of fumarate respiration and transport dicarboxylates with wild-type rates after anaerobic but not after aerobic growth. Transport by the anaerobic system is stimulated by preloading the bacteria with dicarboxylates. The anaerobic transport system catalyzes homologous and heterologous antiport of dicarboxylates, whereas the aerobic system operates only in the unidirectional mode. The anaerobic antiport is measurable only in anaerobically grown bacteria with fnr+ backgrounds. Additionally, the system is inhibited by incubation of resting bacteria with physiological electron acceptors such as O2, nitrate, dimethyl sulfoxide, and fumarate. The inhibition is reversed by the presence of reducing agents. It is suggested that the physiological role of the system is a fumarate/succinate antiport under conditions of fumarate respiration.

Characterization of the specific pyruvate transport system in Escherichia coli K-12

A mutant of Escherichia coli K-12 lacking pyruvate dehydrogenase and phosphoenolpyruvate synthase was used to study the transport of pyruvate by whole cells. Uptake of pyruvate was maximal in mid-log phase cells, with a Michaelis constant for transport of 20 microM. Pretreatment of the cells with respiratory chain poisons or uncouplers, except for arsenate, inhibited transport up to 95%. Lactate and alanine were competitive inhibitors, but at nonphysiological concentrations. The synthetic analogs 3-bromopyruvate and pyruvic acid methyl ester inhibited competitively. The uptake of pyruvate was also characterized in membrane vesicles from wild-type E. coli K-12. Transport required an artificial electron donor system, phenazine methosulfate and sodium ascorbate. Pyruvate was concentrated in vesicles 7- to 10-fold over the external concentration, with a Michaelis constant of 15 microM. Energy poisons, except arsenate, inhibited the transport of pyruvate. Synthetic analogs such as 3-bromopyruvate were competitive inhibitors of transport. Lactate initially appeared to be a competitive inhibitor of pyruvate transport in vesicles, but this was a result of oxidation of lactate to pyruvate. The results indicate that uptake of pyruvate in E. coli is via a specific active transport system.

Evaluation of active versus passive uptake of metabolites by Rhizobium japonicum bacteroids

Rhizobium japonicum bacteroids were isolated anaerobically from soybean [Glycine max (L.) Merr] nodules. The bacteroids, which were capable of acetylene reduction and respiration, were used to study the uptake of metabolites by a method which permits correction for nonspecific adsorption of metabolites and estimation of total cell volume. These determinations permit active uptake to be assessed from metabolite accumulation against a concentration gradient. Succinate, malate, alpha-ketoglutarate, and glutamate were absorbed via an active mechanism. Plots of 1/V versus 1/[S] for succinate and malate indicated the presence of two uptake components: a saturable and presumably active or carrier-mediated component and a nonsaturable and presumably passive component. The uptake of glucose, malonate, D-pinitol, myo-inositol, and glucose 6-phosphate was slow and not active.

The acidic amino acid transport system of the baby hamster kidney cell line BHK21-C13

The uptake of L-glutamate into BHK21-C13 cells in culture has been studied. This amino acid appears to be transported via a relatively high affinity, low capacity, Na+-dependent transport system capable of the rapid accumulation of substrate amino acids. Kinetic studies of the inhibition of L-glutamate uptake has provided information as to the substrate and the molecular configuration required for transport via the glutamate transport system. This system exhibited marked substrate specificity and was only capable of transporting L-glutamate and aspartate and certain closely related acidic amino acid analogues.

1. Membrane vesicles of Escherichia coli K-12 CS7, a strain gentically derepressed for glutamate permease, maintain low aspartate transport activity, like that of prep

1. Membrane vesicles of Escherichia coli K-12 CS7, a strain gentically derepressed for glutamate permease, maintain low aspartate transport activity, like that of preparations of the wild-type parent. Growth of the parent CS101 on aspartate as the source of carnon or nitrogen results in derepression of both asparatate and glytamate transport. Growth of strain CS7 on aspartate derepresses aspartate transport to the same extent as in strains CS101, but only slightly increases the derepressed level of glutamate transport activity. 2. The affinity of the membrane transport system for glutamate is enhanced by sodium, while that for asparate is not. 3. Although the affinities for glutamate (23 muM) and aspartate (12 muM) are similar, aspartate does not inhibit glutamate transport, while glutamate competitively inhibits aspartate transport. 4. Aspartate transport, but not glutamate transport, is competitively inhibited by C4 dicarboxylic acids, whereas 2-oxoglutarate competitively inhibits glutamate transport, but not aspartate transport. 5. Competitive inhibition of L-aspartate transport by L-glutamate and by the 5-methyl ester of L-glutamate is abolished in the presence of 2-oxoglutarate. However, 2-oxoglutarate does not affect the competitive inhibition of L-aspartate transport by D-aspartate and by DL-threo-3-hydroxyaspartate. The relationship between the two dicarboxylic amino acid transport systems and the spatial characteristics of the aspartate carrier are discussed in the light of these findings.

Glutamate transport in membrane vesicles of the wild-type strain and glutamate-utilizing mutants of Escherichia coli

A highly specific energy-dependent glutamate transport system was demonstrated in membrane vesicles of glutamate-utilizing Escherichia coli K-12 mutants. The glutamate transport activity of membranes from the parent strain, unable to grow on glutamate, was very low. With ascorbate-phenazine methosulfate as the electron donor, mutant preparations displayed 17 to 20 times higher activity than did the wild type. However, the affinity of the mutant carrier for L-glutamate remained the same as in the parent strain. Comparative inhibition analysis of glutamate transport in whole cells and membrane vesicles and of in vitro binding of glutamate to a specific periplasmic-binding protein suggests that under certain conditions the latter may be a component of the E. coli K-12 glutamate transport system.

Soluble and membrane-bound aspartate-binding activities in Salmonella typhimurium

The specificities of the soluble and membrane aspartate-binding activities were compared with each other and with the specificity of aspartate chemotaxis and were found to be distinct. The soluble aspartate-binding protein was purified to homogeneity and had a molecular weight of 30,000. The dissociation constant was 10(-6) M for aspartate, and the protein bound glutamate, cysteic acid, and 2-amino-3-phosphonopropionate. Aspartate transport was inhibited by cysteic acid.

Transport of biosynthetic intermediates: regulation of homoserine and threonine uptake in Escherichia coli

Homoserine is transported by a single system that it shares with alanine, isoleucine, leucine, phenylalanine, threonine, valine and perhaps cysteine, methionine, serine, and tyrosine. We investigated the regulation of this transport system and found that alanine, isoleucine, leucine, methionine, and valine each repress the homoserine-transporting system. From the concentration resulting in 50% repression of this transport system and the maximal amount of repression, we ranked the amino acids according to their effectiveness in repressing homoserine transport (in decreasing order): leucine>methionine>alanine>valine>isoleucine. The exponential rate of decrease in transport capacity after leucine addition equals the exponential growth rate of the culture, and protein synthesis is necessary for the derepression seen when leucine is removed. Threonine, in addition to using the above system, is transported by a second system shared with serine. We present further evidence for this serine-threonine transport system and show that it is not regulated like the homoserine-transporting system.

Properties of alpha-aminoisobutyric acid transport in a thermophilic microorganism

Uptake of alpha-aminoisobutyric acid (AIB) by a leucine-tyrosine auxotroph of a thermophilic microorganism starved for leucine was studied. AIB was taken up by the cells against a substantial concentration gradient (300:1) and was present there in a free and unchanged form. Various energy inhibitors and sulfhydryl reagents strongly inhibited the accumulation of AIB. AIB uptake obeyed saturation kinetics, and the Lineweaver-Burk plot is characterized by a biphasic curve. AIB most probably shares a common transport system(s) with alanine, serine, and glycine. A mutant defective in l-alanine uptake was isolated by using the suicide effect due to accumulation of the tritiated substrate. The mutant also exhibited impaired transport activity towards AIB, glycine, and l-serine, but not to phenylalanine or valine. The transport of AIB, glycine, l-alanine, and l-serine was induced by d-alanine (5 x 10(-3) M) during growth in a succinate- and ammonia-containing medium. De novo protein synthesis was required for the induction of AIB transport; the induction was inhibited when growth occurred in glucose-containing media. The apparent differential rate of synthesis of the AIB transport system was decreased considerably in glucose-grown cells as compared to succinate-grown cells. A common genetic basis of either the regulatory or structural nature for the transport of AIB, alanine, glycine, and serine in a thermophilic microorganism is suggested.

Asparagine utilization in Escherichia coli

Asparagine-requiring auxotrophs of Escherichia coli K-12 that have an active cytoplasmic asparaginase do not conserve asparagine supplements for use in protein synthesis. Asparagine molecules entering the cell in excess of the pool required for use of this amino acid in protein synthesis are rapidly degraded rather than accumulated. Supplements are conserved when asparagine degradation is inhibited by the asparagine analogue 5-diazo-4-oxo-l-norvaline (DONV) or mutation to cytoplasmic asparaginase deficiency. A strain deficient in cytoplasmic asparaginase required approximately 260 mumol of asparagine for the synthesis of 1 g of cellular protein. The cytoplasmic asparaginase (asparaginase I) is required for growth of cells when asparagine is the nitrogen source. This enzyme has an apparent K(m) for l-asparagine of 3.5 mM, and asparaginase activity is competitively inhibited by DONV with an apparent K(i) of 2 mM. The analogue provides a time-dependent, irreversible inhibition of cytoplasmic asparaginase activity in the absence of asparagine.

Multiplicity of isoleucine, leucine, and valine transport systems in Escherichia coli K-12

The kinetics of isoleucine, leucine, and valine transport in Escherichia coli K-12 has been analyzed as a function of substrate concentration. Such analysis permits an operational definition of several transport systems having different affinities for their substrates. The identification of these transport systems was made possible by experiments on specific mutants whose isolation and characterization is described elsewhere. The transport process with highest affinity was called the "very-high-affinity"process. Isoleucine, leucine, and valine are substrates of this transport process and their apparent K(m) values are either 10(-8), 2 x 10(-8), or 10(-7) M, respectively. Methionine, threonine, and alanine inhibit this transport process, probably because they are also substrates. The very-high-affinity transport process is absent when bacteria are grown in the presence of methionine, and this is due to a specific repression. Methionine and alanine were also found to affect the pool size of isoleucine and valine. Another transport process is the "high-affinity" process. Isoleucine, leucine, and valine are substrates of this transport process, and their apparent K(m) value is 2 x 10(-6) M for all three. Methionine and alanine cause very little or no inhibition, whereas threonine appears to be a weak inhibitor. Several structural analogues of the branched-chain amino acids inhibit the very-high-affinity or the high-affinity transport process in a specific way, and this confirms their existence as two separate entities. Three different "low-affinity" transport processes, each specific for either isoleucine or leucine or valine, show apparent K(m) values of 0.5 x 10(-4) M. These transport processes show a very high substrate specificity since no inhibitor was found among other amino acids or among many branched-chain amino acid precursors or analogues tried. The evolutionary significance of the observed redundancy of transport systems is discussed.

Specificity of the tyrosine-phenylalanine transport system in Bacillus subtilis

l-Tyrosine and l-phenylalanine enter cells of Bacillus subtilis via a system of active transport that exhibits complex kinetic behavior. The specificity of the transport system was characterized both at low concentrations of transport substrate (where affinity for l-tyrosine or l-phenylalanine is high but capacity is low) and at high concentrations (where affinity is low but capacity is high). Specificity was not found to differ significantly as a function of either l-tyrosine or l-phenylalanine concentration. Kinetic analysis showed that the relationship between the uptake of l-phenylalanine and l-tyrosine is strictly competitive. Neither l-tyrosine nor l-phenylalanine uptake was competitively inhibited by other naturally occurring l-amino acids, indicating the importance of the phenyl side chain to uptake specificity. Hence, it is concluded that l-tyrosine and l-phenylalanine are transported by a common system that is specific for these two amino acids. The abilities of analogue derivatives of l-tyrosine and l-phenylalanine to inhibit the uptake of l-[(14)C]tyrosine and l-[(14)C]phenylalanine competitively were determined throughout a wide range of substrate and inhibitor concentrations. In this manner, the contributions of the side chain, the alpha-amino group and the carboxyl group to uptake specificity were established. It is concluded that the positively charged alpha-amino group contributes more significantly to uptake specificity than does the negatively charged carboxyl group. The recognition of a phenyl ring is an essential feature of specificity; other amino acids with aromatic side chains, such as the indole and imidazole rings of l-tryptophan and l-histidine, do not compete with l-tyrosine and l-phenylalanine for uptake. The presence of the p-hydroxy substitutent in the side chain (as in l-tyrosine) enhances the uptake of the aryl amino acid analogues investigated.