The purification and properties of the glutamine synthetase from the cytosol of Soya-bean root nodules

A phytoplasma effector destabilizes chloroplastic glutamine synthetase inducing chlorotic leaves that attract leafhopper vectors

Leaf yellowing is a well-known phenotype that attracts phloem-feeding insects. However, it remains unclear how insect-vectored plant pathogens induce host leaf yellowing to facilitate their own transmission by insect vectors. Here, we report that an effector protein secreted by rice orange leaf phytoplasma (ROLP) inhibits chlorophyll biosynthesis and induces leaf yellowing to attract leafhopper vectors, thereby presumably promoting pathogen transmission. This effector, designated secreted ROLP protein 1 (SRP1), first secreted into rice phloem by ROLP, was subsequently translocated to chloroplasts by interacting with the chloroplastic glutamine synthetase (GS2). The direct interaction between SRP1 and GS2 disrupts the decamer formation of the GS2 holoenzyme, attenuating its enzymatic activity, thereby suppressing the synthesis of chlorophyll precursors glutamate and glutamine. Transgenic expression of SRP1 in rice plants decreased GS2 activity and chlorophyll precursor accumulation, finally inducing leaf yellowing. This process is correlated with the previous evidence that the knockout of GS2 expression in rice plants causes a similar yellow chlorosis phenotype. Consistently, these yellowing leaves attracted higher numbers of leafhopper vectors, caused the vectors to probe more frequently, and presumably facilitate more efficient phytoplasma transmission. Together, these results uncover the mechanism used by phytoplasmas to manipulate the leaf color of infected plants for the purpose of enhancing attractiveness to insect vectors.

Nitrogen Assimilation and Transport by Ex Planta Nitrogen-Fixing Bradyrhizobium diazoefficiens Bacteroids Is Modulated by Oxygen, Bacteroid Density and l-Malate

Symbiotic nitrogen fixation requires the transfer of fixed organic nitrogen compounds from the symbiotic bacteria to a host plant, yet the chemical nature of the compounds is in question. Bradyrhizobium diazoefficiens bacteroids were isolated anaerobically from soybean nodules and assayed at varying densities, varying partial pressures of oxygen, and varying levels of l-malate. Ammonium was released at low bacteroid densities and high partial pressures of oxygen, but was apparently taken up at high bacteroid densities and low partial pressures of oxygen in the presence of l-malate; these later conditions were optimal for amino acid excretion. The ratio of partial pressure of oxygen/bacteroid density of apparent ammonium uptake and of alanine excretion displayed an inverse relationship. Ammonium uptake, alanine and branch chain amino acid release were all dependent on the concentration of l-malate displaying similar K0.5 values of 0.5 mM demonstrating concerted regulation. The hyperbolic kinetics of ammonium uptake and amino acid excretion suggests transport via a membrane carrier and also suggested that transport was rate limiting. Glutamate uptake displayed exponential kinetics implying transport via a channel. The chemical nature of the compounds released were dependent upon bacteroid density, partial pressure of oxygen and concentration of l-malate demonstrating an integrated metabolism.

New isoforms and assembly of glutamine synthetase in the leaf of wheat (Triticum aestivum L.)

Glutamine synthetase (GS; EC 6.3.1.2) plays a crucial role in the assimilation and re-assimilation of ammonia derived from a wide variety of metabolic processes during plant growth and development. Here, three developmentally regulated isoforms of GS holoenzyme in the leaf of wheat (Triticum aestivum L.) seedlings are described using native-PAGE with a transferase activity assay. The isoforms showed different mobilities in gels, with GSII>GSIII>GSI. The cytosolic GSI was composed of three subunits, GS1, GSr1, and GSr2, with the same molecular weight (39.2kDa), but different pI values. GSI appeared at leaf emergence and was active throughout the leaf lifespan. GSII and GSIII, both located in the chloroplast, were each composed of a single 42.1kDa subunit with different pI values. GSII was active mainly in green leaves, while GSIII showed brief but higher activity in green leaves grown under field conditions. LC-MS/MS experiments revealed that GSII and GSIII have the same amino acid sequence, but GSII has more modification sites. With a modified blue native electrophoresis (BNE) technique and in-gel catalytic activity analysis, only two GS isoforms were observed: one cytosolic and one chloroplastic. Mass calibrations on BNE gels showed that the cytosolic GS1 holoenzyme was ~490kDa and likely a dodecamer, and the chloroplastic GS2 holoenzyme was ~240kDa and likely a hexamer. Our experimental data suggest that the activity of GS isoforms in wheat is regulated by subcellular localization, assembly, and modification to achieve their roles during plant development.

Knockout mutants as a tool to identify the subunit composition of Arabidopsis glutamine synthetase isoforms

Glutamine synthetase (GS) is a key enzyme in nitrogen assimilation, which catalyzes the formation of glutamine from ammonia and glutamate. Plant GS isoforms are multimeric enzymes, recently shown to be decamers. The Arabidopsis genome encodes five cytosolic (GS1) proteins labeled as GLN1;1 through GLN1;5 and one chloroplastic (GS2) isoform, GLN2;0. However, as many as 11 GS activity bands were resolved from different Arabidopsis tissues by Native PAGE and activity staining. Western analysis showed that all 11 isoforms are composed exclusively of 40 kDa GS1 subunits. Of five GS1 genes, only GLN1;1, GLN1;2 and GLN1;3 transcripts accumulated to significant levels in vegetative tissues, indicating that only subunits encoded by these three genes produce the 11-band zymogram. Even though the GS2 gene also had significant expression, the corresponding activity was not detected, probably due to inactivation. To resolve the subunit composition of 11 active GS1 isoforms, homozygous knockout mutants deficient in the expression of different GS1 genes were selected from the progeny of T-DNA insertional SALK and SAIL lines. Comparison of GS isoenzyme patterns of the selected GS1 knockout mutants indicated that all of the detected isoforms consist of varying proportions of GLN1;1, GLN1;2 and GLN1;3 subunits, and that GLN1;1 and GLN1;3, as well as GLN1;2 and GLN1;3 and possibly GLN1;1 and GLN1;2 proteins combine in all proportions to form active homo- and heterodecamers.

Glutamine synthetase in legumes: recent advances in enzyme structure and functional genomics

Glutamine synthetase (GS) is the key enzyme involved in the assimilation of ammonia derived either from nitrate reduction, N(2) fixation, photorespiration or asparagine breakdown. A small gene family is encoding for different cytosolic (GS1) or plastidic (GS2) isoforms in legumes. We summarize here the recent advances carried out concerning the quaternary structure of GS, as well as the functional relationship existing between GS2 and processes such as nodulation, photorespiration and water stress, in this latter case by means of proline production. Functional genomic analysis using GS2-minus mutant reveals the key role of GS2 in the metabolic control of the plants and, more particularly, in carbon metabolism.

Interaction of cytosolic glutamine synthetase of soybean root nodules with the C-terminal domain of the symbiosome membrane nodulin 26 aquaglyceroporin

Nodulin 26 (nod26) is a major intrinsic protein that constitutes the major protein component on the symbiosome membrane (SM) of N(2)-fixing soybean nodules. Functionally, nod26 forms a low energy transport pathway for water, osmolytes, and NH(3) across the SM. Besides their transport functions, emerging evidence suggests that high concentrations of major intrinsic proteins on membranes provide interaction and docking targets for various cytosolic proteins. Here it is shown that the C-terminal domain peptide of nod26 interacts with a 40-kDa protein from soybean nodule extracts, which was identified as soybean cytosolic glutamine synthetase GS(1)beta1 by mass spectrometry. Fluorescence spectroscopy assays show that recombinant soybean GS(1)beta1 binds the nod26 C-terminal domain with a 1:1 stoichiometry (K(d) = 266 nm). GS(1)beta1 also binds to isolated SMs, and this binding can be blocked by preincubation with the C-terminal peptide of nod26. In vivo experiments using either a split ubiquitin yeast two-hybrid system or bimolecular fluorescence complementation show that the four cytosolic GS isoforms expressed in soybean nodules interact with full-length nod26. The binding of GS, the principal ammonia assimilatory enzyme, to the conserved C-terminal domain of nod26, a transporter of NH(3), is proposed to promote efficient assimilation of fixed nitrogen, as well as prevent potential ammonia toxicity, by localizing the enzyme to the cytosolic side of the symbiosome membrane.

Crystallization and preliminary crystallographic characterization of glutamine synthetase from Medicago truncatula

The condensation of ammonium and glutamate into glutamine catalyzed by glutamine synthetase (GS) is a fundamental step in nitrogen metabolism in all kingdoms of life. In plants, this is preceded by the reduction of inorganic nitrogen to an ammonium ion and therefore effectively articulates nitrogen fixation and metabolism. Although the three-dimensional structure of the dodecameric bacterial GS was determined quite some time ago, the quaternary architecture of the plant enzyme has long been assumed to be octameric, mostly on the basis of low-resolution electron-microscopy studies. Recently, the crystallographic structure of a monocotyledonous plant GS was reported that revealed a homodecameric organization. In order to unambiguously establish the quaternary architecture of GS from dicotyledonous plants, GS1a from the model legume Medicago truncatula was overexpressed, purified and crystallized. The collection of synchrotron diffraction data to 2.35 A resolution allowed the determination of the three-dimensional structure of this enzyme by molecular replacement.

The three-dimensional structure of an eukaryotic glutamine synthetase: Functional implications of its oligomeric structure

The structure of the prokaryotic glutamine synthetases type I (GS-I), key enzymes in nitrogen metabolism, was determined several years ago by X-ray diffraction, and consists of a double hexameric ring. The structure of the eukaryotic GS from the plant Phaseolus vulgaris (Glutamine synthetase type II; GS-II) has now been determined at low-resolution using electron microscopy and image processing, and consists of an octamer composed of two tetramers placed back-to-back and rotated 90 degrees with respect to each other. The oligomeric structure possesses a twofold symmetry, very suggestive of each tetramer being composed of two dimers. This is reinforced by the fact that dimers are isolated as a stable albeit non-functional species during the purification procedure. Given the fact that the active site of all types of GS is formed by highly conserved residues located in the interface of two interacting monomers, the geometry of the reconstructed tetramer suggests that it only contains two functional active sites, i.e., an active site per dimer. This is supported by biochemical data, which reveal that while the octamer binds eight ATP molecules, it only binds four molecules of the transition state analogue and GS inhibitor methionine-(S)-sulfoximine-P (MetSox-P). All this suggests for the GS-II enzyme an oligomeric structure containing four active sites and four possible regulatory sites, which might point to a complex regulatory behavior.

Comprehensive survey of proteins targeted by chloroplast thioredoxin

Possible target proteins of chloroplast thioredoxin (Trx) have been investigated in the stroma lysate of spinach chloroplasts. For that purpose, we immobilized a mutant of m-type Trx in which an internal cysteine at the active site was substituted with serine, on cyanogen bromide-activated resin. By using this resin, the target proteins in chloroplast were efficiently acquired when they formed the mixed-disulfide intermediates with the immobilized Trxs. We could acquire Rubisco activase (45 kDa) and 2-Cys-type peroxiredoxin (Prx), which were recently identified as targets of chloroplast Trxs. Glyceraldehyde-3-phosphate dehydrogenase and sedoheputulose 1,7-bisphosphatase, well-known thiol enzymes in the Calvin cycle, also were recognized among the collected proteins, suggesting the method is applicable for our purpose. Furthermore, four proteins were identified from a homology search of the NH(2)-terminal sequence of the acquired proteins: glutamine synthetase, a protein homologous to chloroplast cyclophilin, a homolog of Prx-Q, and the Rubisco small subunit. The Trx susceptibilities of the recombinant cyclophilin and Prx-Q of Arabidopsis thaliana were then examined. The method developed in the present study is thus applicable to investigate the various redox networks via Trxs and the related enzymes in the cell.

Some processes related to nitrogen fixation in nodulated legumes

We have summarized information in four areas of the broad topic of legume- Rhizobium symbiosis. These include: carbon substrates provided to nodule bacteroids by the host, assimilation of fixed nitrogen by the host, O 2 metabolism in legume nodules and involvement of H 2 in nodule metabolism. Although nodules contain a variety of carbon substrates, both biochemical and genetic evidence indicate that C4 dicarboxylates are the major carbon substrates that support N 2 fixation in nodules. The biochemical pathways for utilization of products of N 2 fixation are fairly well understood but relatively little is known about the regulation of the assimilation of fixed nitrogenous compounds at the gene level. Ureides are primary nitrogenous compounds exported from nodules of the tropical legumes. Because the catabolism of these products may involve the hydrolysis of urea by nickel-dependent urease, the possible importance of nickel as a trace element in the nutrition of legumes is raised. The O 2 supply to nodule bacteroids is regulated by a barrier to free-O 2 diffusion and by leghaemoglobin. Progress has been made in understanding of the molecular genetics and biochemistry of leghaemoglobin but little is known about the mechanisms that control the physical barrier to O 2 diffusion. Legume nodules contain mechanisms for the disposition of peroxide and free radicals of oxygen. The importance of these systems as protective mechanisms for the O 2 -labile nitrogenase is discussed. Some strains of Rhizobium form nodules which recycle the H 2 produced as a byproduct of N 2 fixation. The genes necessary for H 2 oxidation have been cloned and transferred within and among species of Rhizobium . The advantages and disadvantages of H 2 recycling in legume nodules are discussed.

Energy status and functioning of phosphorus-deficient soybean nodules

Characterization of the effects of long-term P deficiency and of onset and recovery from P deficiency on bacteroid mass and number per unit nodule mass and energy status of soybean (Glycine max L. Merr.) nodules was used to investigate the mechanisms by which P deficiency decreases symbiotic N(2) fixation. The continuous P deficiency treatment (0.05 millimolar P) significantly decreased the whole plant dry mass, P, and N by 62, 90, and 78%, respectively, relative to the P-sufficient control (1.0 millimolar) at 44 days after transplanting. Specific nitrogenase activity was decreased an average of 28% over a 16-day experimental period by P deficiency. Whole nodules of P-deficient controls contained 70 to 75% lower ATP concentrations than nodules of P-sufficient controls. Energy charge and ATP concentrations in the bacteroid fraction of nodules were not significantly affected by P treatment. However, ATP and total adenylate concentrations and energy charge in the plant cell fraction of nodules were significantly decreased 91, 62, and 50%, respectively, by the P deficiency treatment. Specific nitrogenase activity, energy charge, and ATP concentration in the plant cell fraction increased to the levels of nonstressed controls within 2, 2, and 4 days, respectively, after alleviation of external P limitation, whereas bacteroid mass per unit nodule mass and bacteroid N concentration did not increase to the level of nonstressed controls until 7 days after alleviation of external P limitation. All of these parameters except bacteroid mass per unit nodule mass decreased to the levels of the P-deficient controls by 11 days after onset of external P limitation. Concentration of ATP in the bacteroid fraction was not significantly affected by alteration in the external P supply. Energy charge in the bacteroid fraction from plants recovering from P deficiency was decreased to a small (10%) but significant extent (P < 0.05) at two sampling dates relative to P-sufficient controls. These ATP concentration and energy charge measurements indicate that P deficiency impaired oxidative phosphorylation in the plant cell fraction of nodules to a much greater extent than in the bacteroids. The concurrence of significant changes in specific nitrogenase activity (2 days) and in the energy charge (2 days) and ATP concentration (4 days) in the plant cell fraction during recovery from external P limitation is consistent with the conclusion that P deficiency decreases the specific nitrogenase activity by inhibiting an energy-dependent reaction(s) in the plant cell fraction of the nodules.

Purification and Properties of Glutamine Synthetase in Leaves and Roots of Pinus banksiana Lamb

A method is described for the purification of glutamine synthetase (GS; EC. 6.3.1.2) from the leaves and roots of Pinus banksiana Lamb., a conifer which utilizes ammonium as its primary nitrogen source. The enzyme was purified to apparent homogeneity by a procedure involving salt fractionation as well as ion-exchange, size exclusion, and affinity chromatography. Since the final preparation produced two bands on SDS polyacryamide gels but only one band on a nondenaturating gel, it is concluded that the two subunits (44 and 40 kilodaltons, respectively) are part of a single enzymatic protein which shows GS activity. The pH optimum for leaf GS ranged between 6.2 and 6.5, one pH unit lower than the values reported for higher plants which utilize primarily nitrate nitrogen. Magnesium requirements for GS in P. banksiana were different for leaves and roots, showing V(max)/2 values of 2.5 and 8 millimolar, respectively at 5 millimolar ATP. Furthermore, K(m) values for ammonium were higher for the enzyme in leaves (33.1 micromolar) than in roots (19.2 micromolar). K(m) values for ATP and for glutamate, on the other hand, were similar for the two tissues. A polyclonal antibody was produced against the purified leaf GS. Western blots of leaf homogenates produced two bands, the lighter one being more abundant. The same pattern was found when immunodetection was performed using an anti GS IgG produced against purified GS from Phaseolus nodules thus indicating common antigenic determinants. At least 30% of total GS was recovered in a plastid-fraction of dark-grown calli produced from the basal part of P. banksiana hypocotyls.

Negative staining of proteins

Negative staining, some closely related alternative preparation techniques and radiation stability are considered. An attempt is made to clarify the mechanism of action and ultimate resolution limit of negative staining. The results of electron diffraction investigation of thermitase microcrystals embedded in glucose and glucose + stains are presented. It is shown that at doses not exceeding 10 electrons/nm2 electron diffraction from thermitase crystals demonstrate diffraction fields up to 0.2 nm. When adding heavy-atom salts to glucose or using negative staining, the relative intensities of reflections change and electron diffraction patterns for every type of heavy-atom additive (or negative stain) have their specific features. Such characteristic changes of reflection intensities indicate specific interaction of these additives (or stains) with the object. In the case of electron diffraction from the crystals stained using the routine negative staining technique the ordering was preserved down to 0.4-0.5 nm. Increasing the dose up to the normal value results in fading of distant reflections. Thus, negative staining with radiation doses less than the critical one could yield resolution down to 0.4 nm. Yet, the structure may change due to interaction with the stain. Nevertheless, the possibility that such resolution could be obtained for a limited number of objects should not be excluded. Some examples of the application of negative staining for investigation of quaternary and domain structure of proteins (nitrogenase, glutamine synthetase, mitochondrial ATP-synthase, membrane monooxygenase enzymes), tubular and two-dimensional protein crystals (catalase, phosphorylase, HWV protein, hydrogenase), as well as ribosomes and bacteriophages are given in the review.

Subunit Composition of Glutamine Synthetase Isozymes from Root Nodules of Bean (Phaseolus vulgaris L.)

Glutamine synthetase from bean nodules can be separated into two isoforms, GS(n1) and GS(n2). A purification protocol has been developed. It included protamine sulfate precipitation, ammonium sulfate fractionation, anthranilate-affinity chromatography, Dye-Matrex (Orange A) chromatography, and diethylaminoethyl-cellulose ion-exchange chromatography. GS(n1) and GS(n2) have been purified to homogeneity. Subunit structure analysis using two-dimensional polyacrylamide gel electrophoresis revealed that GS(n1) was composed of two different types of subunit polypeptides. They differed in isoelectric points (6.0 and 6.3) but had the same molecular weights (46,000 Daltons). GS(n2) was composed of only one type of subunit polypeptide. It had an isoelectric point of 6.0 and a molecular weight of 46,000 Daltons. It was apparently identical to one of the polypeptides found in GS(n1). Glutamine synthetase holoenzyme consisted of eight subunits. In the nodule there are two different types of glutamine synthetase subunit polypeptides. Random combinations of the polypeptides should generate nine different isozymes. Our electrophoretic analysis revealed that GS(n2) was but one of the isozymes, and GS(n1) was a composite of the other eight. Hence, nodule glutamine synthetase isozymes were homo-octameric as well as hetero-octameric.

Estimation of ammonium concentration in the cytosol of soybean nodules

Analysis of ammonium concentration in the cytosol of soybean (Glycine max [L.] Merr.) root nodules gave high levels of error variation. When the separation of cytosol and bacteroids was deliberately delayed following nodule maceration, a large increase in ammonium concentration was found in the cytosol. When a series of samples was subjected to delay intervals of 0 to 60 minutes, extrapolation of the regression line to time zero indicated that the ammonium concentration in cytosol at the time of nodule maceration was essentially nil. The source of ammonium buildup following maceration was not found, but hydrolysis of free amino acids or ureides was ruled out. An extremely low concentration of ammonium in the cytosol is consistent with a model for movement of ammonia (or ammonium) from bacteroids to host cytoplasm by diffusion.

Effects of Tabtoxinine-beta-Lactam on Nitrogen Metabolism in Avena sativa L. Roots

The effects of tabtoxinine-beta-lactam (T-beta-L) on nitrate uptake and glutamine synthetase (GS) and nitrate reductase (NR) activities in roots of Avena sativa seedlings were determined. Seven-day-old oat seedlings placed in a 10 mm KNO(3) and 0.5 mm T-beta-L solution for 24 hours took up T-beta-L and lost approximately 90% of their root GS activity. [(3)H]-T-beta-L taken up by roots of seven-day-old oat seedlings was associated with GS immunoprecipitated from the extract of these roots. Total nitrate uptake and in vivo NR activity were decreased approximately 50% in the T-beta-L treated roots. However, T-beta-L uptake did not affect the induction phases of nitrate uptake or reduction, nor did it inhibit in vitro NR activity. Thus, the decrease in nitrate uptake and reduction is a secondary effect of T-beta-L action. Roots of seven-day-old oat seedlings were inoculated with Pseudomonas syringae pv tabaci (Tox+) and the pathogen population in the rhizosphere was estimated by dilution plate count; 6 x 10(13) bacteria were recovered after 3 days, as compared to the original inoculation with 7 x 10(9) bacteria, indicating a significant growth of the pathogen in the rhizosphere. The bacteria recovered from the rhizosphere caused chlorosis in tobacco leaves and produced T-beta-L in culture; 1 x 10(14) bacteria were recovered from roots of seedlings inoculated with P. syringae pv tabaci (Tox-) using the same inoculation and assay procedure as for the pv tabaci (Tox+). Extracts of surface-sterilized roots previously inoculated with P. syringae pv tabaci (Tox+) did not produce viable bacterial cultures when plated out on a complete medium. Oat seedlings growing in sand culture and inoculated with P. syringae pv tabaci (Tox+) had developed chlorosis, and root GS activity had declined to less than 10% of controls after 3 days. Conversely, seedlings inoculated with P. syringae pv tabaci (Tox-) never developed chlorosis and maintained normal levels of GS activity. All oat plants inoculated with P. syringae pv tabaci (Tox+) died within 7 days after inoculation as compared to the plants inoculated with P. syringae pv tabaci (Tox-) which grew to maturity.

Isozymes of Glutamine Synthetase in Phaseolus vulgaris L. and Phaseolus lunatus L. Root Nodules

The glutamine synthetase (GS) isozymes in the plant fraction of nodule extracts from 62 cultivars of Phaseolus vulgaris L. and one cultivar of Phaseolus lunatus L. were analyzed by polyacrylamide gel electrophoresis. All P. vulgaris nodule extracts displayed two GS activity bands: a nodule-specific band (GS(n1)) and a band (GS(n2)) similar to the single band (GS(r)) present in root extracts. In nodule extracts of P. lunatus, the GS(n1) band was detected, but the GS(n2) band was barely detectable. In contrast to P. vulgaris, the GS(n2) band and the GS(r) band of P. lunatus appeared to be different. The electrophoretic mobility of the GS(n1) band in P. vulgaris was governed by both the plant cultivar and the development stage of the nodule. In nodule extracts of P. vulgaris and P. lunatus, the zone of GS(n1) activity coincided with six to nine distinct protein bands as revealed after treatment of gels, which had previously been stained for GS activity, with Coomassie blue. All these protein bands were shown to consist of polypeptides of identical molecular weight (approximately 47,000 daltons) by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Our results indicate that P. vulgaris continuously generates isozymes of GS(n1) of increasing electrophoretic mobility during the course of nodule development.

Purification and properties of glutamine synthetase from spinach leaves

The chloroplastic glutamine synthetase of spinach leaves has been purified to homogeneity using affinity chromatography. This involves a tandem ;reactive blue A-agarose' and ;reactive red-A-agarose' as the final step in the procedure. This procedure results in a yield of 18 milligrams of pure glutamine synthetase per kilogram of starting material. The purity of our enzyme has been demonstrated on both one- and two-dimensional polyacrylamide gels.Purified glutamine synthetase has a molecular weight of 360,000 daltons and consists of eight 44,000 dalton subunits. The K(m) is 6.7 millimolar for glutamate, 1.8 millimolar for ATP (synthetase assay), and 37.6 millimolar for glutamine (transferase assay). The isoelectric point is 6.5 and the pH optima are 7.3 in the synthetase assay and 6.4 in the transferase assay. The irreversible, competitive inhibitors methionine sulfoxamine and phosphinothricin have K(i) values of 0.1 millimolar and 6.1 micromolar, respectively. Amino acid analysis has been carried out and the results compared with published analyses for other isoforms of glutamine synthetase.

Heterogeneity of Glutamine Synthetase Polypeptides in Phaseolus vulgaris L

Glutamine synthetases from roots, nodules, and leaves of Phaseolus vulgaris L. have been purified to homogeneity and their polypeptide composition determined.The leaf enzyme is composed of six polypeptides. The cytosolic fraction contains two 43,000 dalton polypeptides and the chloroplastic enzyme is formed by four 45,000 dalton polypeptides. Root glutamine synthetase consists only of the same two polypeptides of 43,000 dalton that are present in the leaf enzyme. The nodule enzyme is formed by two polypeptides of 43,000 dalton, one is common to the leaf and root enzyme but the other is specific for N(2)-fixing nodule tissue. The two glutamine synthetase forms of the nodule contain a different proportion of the 43,000 dalton polypeptides.

Cytosolic glutamine synthetase in higher plants. A comparative immunological study

Cytosolic glutamine synthetase (GS1) was purified to homogeneity from etiolated barley leaves by DEAE-Sephacel and hydroxyapatite chromatography, gel filtration and polyacrylamide gel electrophoresis. Specific antibodies against the purified protein were raised by the immunization of rabbits. Immunoprecipitation experiments demonstrated that cytosolic glutamine synthetases isolated from the leaves of different plant species were very similar proteins. Good recognition of other cytosolic glutamine synthetases from roots, root nodular tissue and seeds by barley GS1 antibodies was obtained, suggesting that they too are all quite similar proteins. In contrast, chloroplast glutamine synthetase (GS2) was considered to be a different protein in view of its low level of recognition by barley GS1 antibodies.

Glutamine synthetase, glutamate synthase and glutamate dehydrogenase in Rhizobium japonicum strains grown in cultures and in bacteroids from root nodules of Glycine max

The growth yields of three strains of Rhizobium japonicum (CB 1809, CC 723, CC 705) in culture solutions containing L-glutamate were about twice those grown with ammonium. The activities of glutamine synthetase (GS; EC 6.3.1.2) and glutamate dehydrogenase (GDH; EC 1.4.1.4) were dependent on the nitrogen source in the medium and also varied with growth. Both NADPH-and NADH-dependent glutamate synthase (GOGAT; EC 1.4.1.13) and NADPH-dependent GDH were found in strains grown with either glutamate or ammonium but NADH-linked GDH was only detected in glutamate-grown cells. Glutamine synthetase was adenylylated in cells grown with NH 4 (+) (90%) and to lesser extent in those grown with L-glutamate (50%). In root nodules produced by the three strains in Glycine max (L.) Merr., the bulk of GS was located in the nodule cytosol (60-85%). The enzyme was adenylylated in bacteroids (43-75%) and in the nodule tissues (52-68%). The enzyme in cell-free extracts of Rh. japonicum (CC 705) grown in culture solutions containing glutamate and in bacteroids (CC 705) was deadenylylated by snake-venom phosphodiesterase. L-methionine-DL-sulfoximine restricted the incoporation of (15)N-labelled (NH4)2SO4 into cells of strains CB 1809 and CC 705, as well as in bacteroids of strain CC 705. It is noteworthy that appreciable activities for GDH were found in the free-living rhizobia grown on glutamate. Thus the presence of an enzyme does not necessarily imply that a particular pathway is operative in assimilating ammonium into cell nitrogen. Based on (15)N studies, the GS-GOGAT pathway of rhizobia (strains CB 1809 and CC 705) is important when grown in culture solutions as well as in bacteroids from root nodules of G. max.

Appearance of a novel form of plant glutamine synthetase during nodule development in Phaseolus vulgaris L

The activities of glutamine synthetase (GS), nitrogenase and leghaemoglobin were measured during nodule development in Phaseolus vulgaris infected with wild-type or two non-fixing (Fix(-)) mutants of Rhizobium phaseoli. The large increase in GS activity which was observed during nodulation with the wild-type rhizobial strain occurred concomitantly with the detection and increase in activity of nitrogenase and the amount of leghaemoglobin. Moreover, this increase in GS was found to be due entirely to the appearance of a novel form of the enzyme (GSn1) in the nodule. The activity of the form (GSn2) similar to the root enzyme (GSr) remained constant throughout the experiment. In nodules produced by infection with the two mutant strains of Rhizobium phaseoli (JL15 and JL19) only trace amounts of GSn1 and leghaemoglobin were detected.

Purification and properties of two forms of glutamine synthetase from the plant fraction of Phaseolus root nodules

Two forms of glutamine synthetase (GS) have been purified to apparent homogeneity from the plant fraction of Phaseolus vulgaris root nodules. One of these forms appears identical to the form of the enzyme found in roots but the other is probably specifically associated with the nodule. Free-living Rhizobium phaseoli also contain two forms of GS both of which have different molecular weights from the plant enzymes. Bacteroids contain solely the higher-molecular-weight form of rhizobial GS. There are only minor differences between the plant enzymes in Km or S0.5 values for the synthetase-reaction substrates and both forms have identical molecular weights of the holoenzyme (380,000 daltons) and its sub-units (41,000 daltons). They can be separated by ion-exchange chromatography on diethylaminoethyl-Sephacel and by native polyacrylamide-gel electrophoresis. The only other distinguishing feature observed is that the ratio of transferase: synthetase activity of the root form is threefold greater than that of the nodule-specific GS.

Purification and comparison of glutamine synthetase from chicken liver, brain and neural retina

Glutamine synthetase (L-glutamate: ammonia ligase, EC 6.3.1.2) was isolated from chicken liver, brain and neural retina. The specific activities of the purified enzyme preparations from the three different sources were similar. They were composed of subunits of the same molecular weight (43 K) and were immunologically indistinguishable. Slight differences were detectable among them in relation to the amino acid compositions and regulation of their activities by the several effectors tested.

Rhythms in glutamine synthetase activity, energy charge, and glutamine in sunflower roots

Roots of sunflower plants (Helianthus annuus L. var. Mammoth Russian) subjected to L12:D12, L18:D6, and L12:D12 followed by continuous light all display rhythms of about 12 hours for glutamine synthetase (GS) activity (transferase reaction) with one peak in the ;light phase' and one in the ;dark phase.' Root energy charge (EC = ATP+(1/2)ADP/ATP+ADP+AMP) is directly correlated with GS, but the GS rhythm is better explained as the result of a rhythmic adenine nucleotide ratio (ATP/ADP+AMP) that regulates enzyme activity through allosteric modification. When L12:D12 plants are subjected to free-running conditions in continuous darkness, only diurnal rhythms for GS and EC, with peaks in the dark phase, remain. The 12-hour root rhythms for GS and EC appear to be composed of two alternating rhythms, one a diurnal, light-dependent, incompletely circadian light phase rhythm and the other a light-independent, circadian dark phase rhythm.Only glutamine, of the root amino acids, displays cyclical changes in concentration, maintaining under all conditions a 12-hour rhythm that is consistently synchronized with, but nearly always inversely correlated with, GS and EC rhythms.

Isolation and Immunochemical Characterization of Plant Glutamine Synthetase in Alfalfa (Medicago sativa L.) Nodules

Host plant glutamine synthetase (GS) has been purified 100-fold from N(2)-fixing alfalfa (Medicago sativa L.) nodules by a new procedure involving preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) as a final step. An SDS-polypeptide fraction corresponding to plant GS was identified and consisted of two major polypeptides of 40,000 to 45,000 molecular weight. Antibodies to the SDS-polypeptide fraction were raised in mice by intraperitoneal injection, and antisera were collected as ascitic fluid. Crude extracts of soluble protein from the plant fraction of nodules were resolved by SDS-PAGE and then subjected to electrophoresis in the second dimension into antibody-containing agarose gel. A single immunochemically active protein species was observed using this crossed immunoelectrophoresis method, even though both major GS SDS-polypeptides were apparently resolved in the first (SDS-PAGE) dimension. Plant GS protein in crude nodule extracts was quantitated immunochemically by comparison with immunoprecipitin arcs of similarly treated amounts of pure antigen. Using this technique, it was determined that plant GS was present at 150 micrograms per gram fresh weight or 1.2% of total plant soluble protein in N(2)-fixing alfalfa nodules.Results suggest that alfalfa nodule plant GS consists of two major subunit polypeptides, but only a single immunochemically active native protein was observed. The crossed immunoelectrophoresis procedure described here should be generally applicable for immunochemical detection of lower abundance components of crude plant extracts.

Root Nodule Enzymes of Ammonia Assimilation in Alfalfa (Medicago sativa L.) : DEVELOPMENTAL PATTERNS AND RESPONSE TO APPLIED NITROGEN

Nitrogenase-dependent acetylene reduction activity of glasshouse-grown alfalfa (Medicago sativa L.) decreased rapidly in response both to harvesting (80% shoot removal) and applied NO(3) (-) at 40 and 80 kilograms N per hectare. Acetylene reduction activity of harvested plants grown on 0 kilogram N per hectare began to recover by day 15 as shoot regrowth became significant. In contrast, acetylene reduction activity of all plants treated with 80 kilograms NO(3) (-)-N per hectare and harvested plants treated with 40 kilograms NO(3) (-)-N per hectare remained low for the duration of the experiment. Acetylene reduction of unharvested alfalfa treated with 40 kilograms N per hectare declined to an intermediate level and appeared to recover slightly by day 15. Changes in N(2)-fixing capacity were accompanied by similar changes in levels of nodule soluble protein.After an initial lag of 24 hours, specific activities of alfalfa nodule glutamine synthetase, NADH-glutamate synthase, and NAD-glutamate dehydrogenase (oxidative amination) decreased similar to but less rapidly than acetylene reduction activity. Increased specific activities of these nodule enzymes occurred as acetylene reduction activity increased and shoot growth resumed. The observed rates of glutamine synthetase and glutamate synthase were sufficient to assimilate ammonia produced via symbiotic N(2) fixation. Nodule NADH-dependent glutamate dehydrogenase (reductive amination) specific activity was not associated with changes in acetylene reduction activity.The data indicate that host plant glutamine synthetase and NADH-glutamate synthase function to assimilate symbiotically fixed N and that NADH-dependent glutamate dehydrogenase may function in ammonia assimilation during senescence in alfalfa nodules.

Electron microscopy of glutamine synthetase from pea leaf chloroplasts

The structure of pea leaf chloroplast glutamine synthetase was studied by electron microscopy. The enzyme is shown to consist of eight elongated subunits which are arranged with point 42 symmetry at the vertices of two squares. These squares are twisted about the 4-fold axis at 40 degrees relative to each other.

Glutamine Synthetase in Rice: A COMPARATIVE STUDY OF THE ENZYMES FROM ROOTS AND LEAVES

Chromatographic, kinetic, and regulatory properties of glutamine synthetase in rice were investigated. By DEAE-Sephacel column chromatography, two forms (glutamine synthetase 1 and glutamine synthetase 2) were identified in leaves and one form (glutamine synthetase R) was identified in roots. Purification on hydroxyapatite and gel electrophoresis showed that glutamine synthetase R was distinct from the leaf enzymes. The three isoforms were purified to similar specific activities and their properties were studied. Heat lability, pH optimum about 8, K(m) for l-glutamate of 20 millimolar, and inhibition by glucosamine 6-phosphate were the main characteristics of glutamine synthetase 2. Heat stability, pH optimum about 7.5, K(m) for l-glutamate of 2 millimolar, and no effect of glucosamine 6-phosphate differentiated glutamine synthetase 1 from glutamine synthetase 2. Glutamine synthetase R was also a labile protein but its kinetic and regulatory properties were quite similar to those of glutamine synthetase 1. These results clearly demonstrate the existence of three isoforms of glutamine synthetase in rice, two of which are located in the leaves and the third in the roots.