Interactions of a glutamate-aspartate binding protein with the glutamate transport system of Escherichia coli

Production of 5-aminolevulinic acid from glutamate by overexpressing HemA1 and pgr7 from Arabidopsis thaliana in Escherichia coli

The important metabolic intermediate 5-aminolevulinic acid (ALA) is useful for cancer treatment or plant growth regulation and has consequently received much attention. In this study, we introduced the HemA1 and pgr7 genes from the higher plant Arabidopsis thaliana into recombinant Escherichia coli to overproduce extracellular 5-aminolevulinic acid via the C5 pathway. In the E. coli BL21 (DE3) strain background, the ALA concentration of the strain expressing both HemA1 and pgr7 was the highest and reached 3080.62 mg/L. Among the 7 tested hosts, ALA production was the highest in E. coli Transetta (DE3). In E. coli Transetta GTR/GBP, the expression levels of zwf, gnd, pgl and RhtA were upregulated. Glutamate induced the expression of the GltJ, GltK, GltL and GltS genes that are in involved in glutamate uptake. The recombinant E. coli Transetta GTR/GBP was able to produce 7642 mg/L ALA in modified minimal medium supplemented with 10 g/L glutamate and 15 g/L glucose after 48 h of fermentation at 22 °C. The results provide persuading evidence for the efficient production of ALA from glucose and glutamate in E. coli expressing A. thaliana HemA1 and pgr7. Further optimization of the fermentation process should be done to improve the ALA production to an industrially relevant level.

Identification of a meningococcal L-glutamate ABC transporter operon essential for growth in low-sodium environments

GdhR is a meningococcal transcriptional regulator that was previously shown to positively control the expression of gdhA, encoding the NADP-specific L-glutamate dehydrogenase (NADP-GDH), in response to the growth phase and/or to the carbon source. In this study we used reverse transcriptase-PCR-differential display (to identify additional GdhR-regulated genes. The results indicated that GdhR, in addition to NADP-GDH, controls the expression of a number of genes involved in glucose catabolism by the Entner-Doudoroff pathway and in l-glutamate import by an unknown ABC transport system. The genes encoding the putative periplasmic substrate-binding protein (NMB1963) and the permease (NMB1965) of the ABC transporter were genetically inactivated. Uptake experiments demonstrated an impairment of L-glutamate import in the NMB1965-defective mutant in the absence or in the presence of a low sodium ion concentration. In contrast, at a sodium ion concentration above 60 mM, the uptake defect disappeared, possibly because the activity of a sodium-driven secondary transporter became predominant. Indeed, the NMB1965-defective mutant was unable to grow at a low sodium ion concentration (<20 mM) in a chemically defined medium containing L-glutamate and four other amino acids that supported meningococcal growth, but it grew when the sodium ion concentration was raised to higher values (>60 mM). The same growth phenotype was observed in the NMB1963-defective mutant. Cell invasion and intracellular persistence assays and expression data during cell invasion provided evidence that the l-glutamate ABC transporter, tentatively named GltT, was critical for meningococcal adaptation in the low-sodium intracellular environment.

The Campylobacter jejuni PEB1a adhesin is an aspartate/glutamate-binding protein of an ABC transporter essential for microaerobic growth on dicarboxylic amino acids

The PEB1a protein of the gastrointestinal pathogen Campylobacter jejuni mediates interactions with epithelial cells and is an important factor in host colonization. Cell fractionation and immunoblotting showed that PEB1a is most abundant in the periplasm of C. jejuni, and is detectable in the culture supernatant but not in the inner or outer membrane. The protein is homologous with periplasmic-binding proteins associated with ABC transporters and we show by fluorescence spectroscopy that purified recombinant PEB1a binds L-aspartate and L-glutamate with sub microM K(d) values. Binding of L-14C-aspartate or L-14C-glutamate was strongly out-competed by excess unlabelled aspartate or glutamate but only poorly by asparagine and glutamine. A mutant in the Cj0921c gene, encoding PEB1a, was completely unable to transport 5 microM L-14C-glutamate and showed a large reduction (approximately 20-fold) in the rate of L-14C-aspartate transport compared with the wild type. Although microaerobic growth of this mutant was little affected in complex media, growth on aspartate or glutamate in defined media was completely prevented, whereas growth with serine was similar to wild type. 1H-NMR analysis of the culture supernatants of the Cj0921c mutant showed some utilization of aspartate but not glutamate, consistent with the transport data. It is concluded that in addition to the established role of PEB1a as an adhesin, the PEB1 transport system plays a key role in the utilization of aspartate and glutamate, which may be important in vivo carbon sources for this pathogen.

Carbon-source-dependent nitrogen regulation in Escherichia coli is mediated through glutamine-dependent GlnB signalling

The P(II) signal transduction proteins GlnB and GlnK are uridylylated/deuridylylated in response to the intracellular glutamine level, the primary signal of the cellular nitrogen status. Furthermore, GlnB was shown to be allosterically regulated by 2-oxoglutarate, and thus GlnB was suggested to integrate signals of the cellular carbon and nitrogen status. Receptors of GlnB signal transduction in Escherichia coli are the NtrB/NtrC two-component system and GlnE, an enzyme which adenylylates/deadenylylates glutamine synthetase. In this study, the authors investigated the effect of different carbon sources on the expression of the NtrC-dependent genes glnA and glnK and on the uridylylation status of GlnB and GlnK. With glutamine as nitrogen source, high levels of glnA and glnK expression were obtained when glucose was used as carbon source, but expression was strongly decreased when the cells were grown with poor carbon sources or when cAMP was present. This response correlated with the uridylylation status of GlnB, suggesting that the carbon/cAMP effect was mediated through GlnB uridylylation, a conclusion that was confirmed by mutants of the P(II) signalling pathway. When glutamine was replaced by low concentrations of ammonium as nitrogen source, neither glnAglnK expression nor GlnB uridylylation responded to the carbon source or to cAMP. Furthermore, glutamine synthetase could be rapidly adenylylated in vivo by the external addition of glutamine; however, this occurred only when cells were grown in the presence of cAMP, not in its absence. Together, these results suggest that poor carbon sources, through cAMP signalling, favour glutamine uptake. The cellular glutamine signal is then transduced by uridylyltransferase and GlnB to modulate NtrC-dependent gene expression.

Bacterial evolution through the selective loss of beneficial Genes. Trade-offs in expression involving two loci

The loss of preexisting genes or gene activities during evolution is a major mechanism of ecological specialization. Evolutionary processes that can account for gene loss or inactivation have so far been restricted to one of two mechanisms: direct selection for the loss of gene activities that are disadvantageous under the conditions of selection (i.e., antagonistic pleiotropy) and selection-independent genetic drift of neutral (or nearly neutral) mutations (i.e., mutation accumulation). In this study we demonstrate with an evolved strain of Escherichia coli that a third, distinct mechanism exists by which gene activities can be lost. This selection-dependent mechanism involves the expropriation of one gene's upstream regulatory element by a second gene via a homologous recombination event. Resulting from this genetic exchange is the activation of the second gene and a concomitant inactivation of the first gene. This gene-for-gene expression tradeoff provides a net fitness gain, even if the forfeited activity of the first gene can play a positive role in fitness under the conditions of selection.

Cloning and sequencing of a gene encoding a glutamate and aspartate carrier of Escherichia coli K-12

A gene encoding a carrier protein for glutamate and aspartate was cloned into Escherichia coli K-12 strain BK9MDG by using the high-copy-number plasmid pBR322. The gene (designated gltP) is probably identical to a gene recently cloned from E. coli B (Y. Deguchi, I. Yamato, and Y. Anraku, J. Bacteriol. 171:1314-1319). A 1.6-kilobase DNA fragment containing gltP was subcloned into the expression plasmids pT7-5 and pT7-6, and its product was identified by a phage T7 RNA polymerase-T7 promoter coupled system (S. Tabor and C. C. Richardson, Proc. Natl. Acad. Sci. USA 82:1074-1078) as a polypeptide with an apparent mass of 38 kilodaltons. A portion of the gltP polypeptide was associated with the cytoplasmic membrane. The nucleotide sequence of the 1.6-kilobase fragment was determined. It contained an open reading frame capable of encoding a highly hydrophobic polypeptide of 395 amino acids, containing four possible transmembrane segments. Uptake of glutamate and aspartate was increased 5.5- and 4.5-fold, respectively, in strains containing gltP plasmids. Glutamate uptake was insensitive to the concentration of Na+ and was inhibited by L-cysteate and beta-hydroxyaspartate. These results suggest that gltP is a structural gene for a carrier protein of the Na(+)-independent, binding-protein-independent glutamate-aspartate transport system.

Genetic dissection of catalytic activities of the Salmonella typhimurium mannitol enzyme II

Approximately 60 mutants of Salmonella typhimurium were isolated which exhibited altered levels of the activities of the mannitol enzyme II. The mutants were grouped into six distinct categories based on their mannitol fermentation, transport, chemotaxis, and phosphorylation activities.

Genetic control of lysine permeases in Saccharomycopsis lipolytica

In order to obtain strains of Saccharomycopsis lipolytica impaired in the active transport of L-lysine, mutants resistant to a mixture of L-canavanine, L-4-5-transdehydrolysine and L-S-amino ethylcysteine, taken either all three or two by two, were isolated. These compounds were shown previously to be competitive inhibitors of L-lysine uptake. The resistance patterns and excretion capacity of the mutants were established. All mutants behaved as monogenic. Recombination tests indicated that four genes at least were involved. All mutants were impaired in both high and low affinity L-lysine transport systems. Several hypotheses on the functions of these genes are put forward and discussed.

Role of Na+ and Li+ in thiomethylgalactoside transport by the melibiose transport system of Escherichia coli

Thiomethyl-beta-galactoside (TMG) accumulation via the melibiose transport system was studied in lactose transport-negative strains of Escherichia coli. TMG uptake by either intact cells or membrane vesicles was markedly stimulated by Na+ or Li+ between pH 5.5 and 8. The Km for uptake of TMG was approximately 0.2 mM at an external Na+ concentration of 5 mM (pH 7). The alpha-galactosides, melibiose, methyl-alpha-galactoside, and o-nitrophenyl-alpha-galactoside had a high affinity for this system whereas lactose, maltose and glucose had none. Evidence is presented for Li+-TMG or Na+-TMG cotransport.

Sodium-dependent methyl 1-thio-beta-D-galactopyranoside transport in membrane vesicles isolated from Salmonella typhimurium

Membrane vesicles isolated from Salmonella typhimurium G-30 grown in the presence of melibiose catalyze methyl 1-thio-beta-D-galactopyranoside (TMG) transport in the presence of sodium or lithium, as shown initially with intact cells by Stock and Roseman (Stock, J., and Roseman, S. (1971), Biochem. Biophys. Res. Commun. 44, 132). TMG-dependent sodium uptake is also observed, but only when a potassium diffusion potential (interior negative) is induced across the vesicle membrane. Cation-dependent TMG accumulation varies with the electrochemical gradient of protons generated as a result of D-lactate oxidation, and the vesicles catalyze D-lactate-dependent sodium efflux in a manner which is consistent with the operation of a proton-sodium exchange mechanism. Although the stoichiometry between sodium and TMG appears to be 1:1 when transport is induced by a potassium diffusion potential, evidence is presented which indicates that the relationship may exceed unity under certain conditions. The results are explained in terms of a model in which TMG-sodium (lithium) symport is driven by an electrochemical gradient of protons which functions to maintain a low intravesicular sodium or lithium concentration through proton--sodium (lithium) antiport.

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.

Effect of growth conditions on glutamate transport in the wild-type strain and glutamate-utilizing mutants of Escherichia coli

The effects of growth conditions on the glutamate transport activity of intact cells and membrane vesicles and on the levels of glutamate-binding protein in wild-type Escherichia coli K-12 CS101 and in two glutamate-utilizing mutants, CS7 and CS2TC, were studied. Growth of CS101 on aspartate as the sole source of carbon or nitrogen resulted in a severalfold increase in glutamate transport activity of intact cells and membrane preparations to levels characteristic of the operator-constitutive mutant CS7. The high glutamate transport activity of mutant CS7 was not depressed further by growth on aspartate. Synthesis of glutamate-binding protein was not enhanced by aspartate in either strain. Mutant CS2TC produces a heat-labile repressor of glutamate permease synthesis and is therefore able to grow on glutamate at 42 C but not at 30 C. CS2TC cells grown in a glycerol-minimal medium at the restrictive temperature (30 C) exhibit low glutamate transport activity. Growth on aspartate at 30 C results in derepressed synthesis of glutamate permease. Cells grown on glycerol at 42 C have high glutamate transport activity. No further derepression is obtained upon growth on aspartate. Growth of CS101 and CS7 in "rich broth" greatly reduces the levels of glutamate-binding protein but does not appreciably affect glutamate transport by whole cells or membrane preparations. The identity of the carrier and the role of the binding protein in glutamate transport are discussed in the light of these findings.

On describing microbial growth kinetics from continuous culture data: Some general considerations, observations, and concepts

Analysis of continuous culture methodology suggests that this potentially powerful tool for kinetic analysis can be improved by minimizing several inherent shortcomings. Medium background substrates - organic carbon, phosphate, and manganese - were shown to dominate kinetic observations at concentrations below chemical detection methods. Reactor wall growth, culture size distribution changes, sample removal-induced steady state perturbations, and limiting substrate leakage from organisms are treated in terms of kinetic measurement errors. Large variations in maximal growth rates and substrate uptake rates found are attributed to experimental protocol-induced transient states. Relationships are presented for correcting limiting substrate concentrations for lability during sampling, contamination with unreacted medium, and background substrate effects. Analytical procedures are discussed for improved measurement of limiting substrate kinetics involving enzymes, isotopes, and material balance manipulation. Relaxation methods as applied to continuous culture are introduced as a means for isolating separate rate constants describing net substrate transport and for evaluating cellular metabolite leakage. Low velocity growth, multiple substrate metabolism, and endogenous metabolism are discussed along with measurements showing that 1-month generation times for aquatic microorganisms can be quite normal and that the kinetics are compatible withμg/liter limiting substrate concentrations. The concept of regarding growth kinetics as the sum of several net accumulation processes is suggested.