Identification of two forms of glutamine synthetase in barley (Hordeum vulgare)

Glutamine Metabolism, Sensing and Signaling in Plants

Glutamine (Gln) is the first amino acid synthesized in nitrogen (N) assimilation in plants. Gln synthetase (GS), converting glutamate (Glu) and NH4+ into Gln at the expense of ATP, is one of the oldest enzymes in all life domains. Plants have multiple GS isoenzymes that work individually or cooperatively to ensure that the Gln supply is sufficient for plant growth and development under various conditions. Gln is a building block for protein synthesis and an N-donor for the biosynthesis of amino acids, nucleic acids, amino sugars and vitamin B coenzymes. Most reactions using Gln as an N-donor are catalyzed by Gln amidotransferase (GAT) that hydrolyzes Gln to Glu and transfers the amido group of Gln to an acceptor substrate. Several GAT domain-containing proteins of unknown function in the reference plant Arabidopsis thaliana suggest that some metabolic fates of Gln have yet to be identified in plants. In addition to metabolism, Gln signaling has emerged in recent years. The N regulatory protein PII senses Gln to regulate arginine biosynthesis in plants. Gln promotes somatic embryogenesis and shoot organogenesis with unknown mechanisms. Exogenous Gln has been implicated in activating stress and defense responses in plants. Likely, Gln signaling is responsible for some of the new Gln functions in plants.

An Overview of Important Enzymes Involved in Nitrogen Assimilation of Plants

Nitrogen (N) is a macro-nutrient that is essential for growth development and resistance against biotic and abiotic stresses of plants. Nitrogen is a constituent of amino acids, proteins, nucleic acids, chlorophyll, and various primary and secondary metabolites. The atmosphere contains huge amounts of nitrogen but it cannot be taken up directly by plants. Plants can take up nitrogen in the form of nitrate, ammonium, urea, nitrite, or a combination of all these forms. In addition, in various leguminous rhizobia, bacteria can convert atmospheric nitrogen to ammonia and supply it to the plants. The form of nitrogen nutrition is also important in plant growth and resistance against pathogens. Nitrogen content has an important function in crop yield. Nitrogen deficiency can cause reduced root growth, change in root architecture, reduced plant biomass, and reduced photosynthesis. Hence, understanding the function and regulation of N metabolism is important. Several enzymes and intermediates are involved in nitrogen assimilation. Here we provide an overview of the important enzymes such as nitrate reductase, nitrite reductase, glutamine synthase, GOGAT, glutamate dehydrogenase, and alanine aminotransferase that are involved in nitrogen metabolism.

Rice Ferredoxin-Dependent Glutamate Synthase Regulates Nitrogen-Carbon Metabolomes and Is Genetically Differentiated between japonica and indica Subspecies

Plants assimilate inorganic nitrogen absorbed from soil into organic forms as Gln and Glu through the glutamine synthetase/glutamine:2-oxoglutarate amidotransferase (GS/GOGAT) cycle. Whereas GS catalyzes the formation of Gln from Glu and ammonia, GOGAT catalyzes the transfer of an amide group from Gln to 2-oxoglutarate to produce two molecules of Glu. However, the regulatory role of the GS/GOGAT cycle in the carbon-nitrogen balance is not well understood. Here, we report the functional characterization of rice ABNORMAL CYTOKININ RESPONSE 1 (ABC1) gene that encodes a ferredoxin-dependent (Fd)-GOGAT. The weak mutant allele abc1-1 mutant shows a typical nitrogen-deficient syndrome, whereas the T-DNA insertional mutant abc1-2 is seedling lethal. Metabolomics analysis revealed the accumulation of an excessive amount of amino acids with high N/C ratio (Gln and Asn) and several intermediates in the tricarboxylic acid cycle in abc1-1, suggesting that ABC1 plays a critical role in nitrogen assimilation and carbon-nitrogen balance. Five non-synonymous single-nucleotide polymorphisms were identified in the ABC1 coding region and characterized as three distinct haplotypes, which have been highly and specifically differentiated between japonica and indica subspecies. Collectively, these results suggest that ABC1/OsFd-GOGAT is essential for plant growth and development by modulating nitrogen assimilation and the carbon-nitrogen balance.

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.

The Stable Level of Glutamine synthetase 2 Plays an Important Role in Rice Growth and in Carbon-Nitrogen Metabolic Balance

Glutamine synthetase 2 (GS2) is a key enzyme involved in the ammonium metabolism in plant leaves. In our previous study, we obtained GS2-cosuppressed plants, which displayed a normal growth phenotype at the seedling stage, while at the tillering stage they showed a chlorosis phenotype. In this study, to investigate the chlorosis mechanism, we systematically analyzed the plant growth, carbon-nitrogen metabolism and gene expressions between the GS2-cosuppressed rice and wild-type plants. The results revealed that the GS2-cosuppressed plants exhibited a poor plant growth phenotype and a poor nitrogen transport ability, which led to nitrogen accumulation and a decline in the carbon/nitrogen ratio in the stems. Interestingly, there was a higher concentration of soluble proteins and a lower concentration of carbohydrates in the GS2-cosuppressed plants at the seedling stage, while a contrasting result was displayed at the tillering stage. The analysis of the metabolic profile showed a significant increase of sugars and organic acids. Additionally, gene expression patterns were different in root and leaf of GS2-cosuppressed plants between the seedling and tillering stage. These results indicated the important role of a stable level of GS2 transcription during normal rice development and the importance of the carbon-nitrogen metabolic balance in rice growth.

Accumulated expression level of cytosolic glutamine synthetase 1 gene (OsGS1;1 or OsGS1;2) alter plant development and the carbon-nitrogen metabolic status in rice

Maintaining an appropriate balance of carbon to nitrogen metabolism is essential for rice growth and yield. Glutamine synthetase is a key enzyme for ammonium assimilation. In this study, we systematically analyzed the growth phenotype, carbon-nitrogen metabolic status and gene expression profiles in GS1;1-, GS1;2-overexpressing rice and wildtype plants. Our results revealed that the GS1;1-, GS1;2-overexpressing plants exhibited a poor plant growth phenotype and yield and decreased carbon/nitrogen ratio in the stem caused by the accumulation of nitrogen in the stem. In addition, the leaf SPAD value and photosynthetic parameters, soluble proteins and carbohydrates varied greatly in the GS1;1-, GS1;2-overexpressing plants. Furthermore, metabolite profile and gene expression analysis demonstrated significant changes in individual sugars, organic acids and free amino acids, and gene expression patterns in GS1;1-, GS1;2-overexpressing plants, which also indicated the distinct roles that these two GS1 genes played in rice nitrogen metabolism, particularly when sufficient nitrogen was applied in the environment. Thus, the unbalanced carbon-nitrogen metabolic status and poor ability of nitrogen transportation from stem to leaf in GS1;1-, GS1;2-overexpressing plants may explain the poor growth and yield.

Functional analysis and comparative genomics of expressed sequence tags from the lycophyte Selaginella moellendorffii

BACKGROUND

The lycophyte Selaginella moellendorffii is a member of one of the oldest lineages of vascular plants on Earth. Fossil records show that the lycophyte clade arose 400 million years ago, 150-200 million years earlier than angiosperms, a group of plants that includes the well-studied flowering plant Arabidopsis thaliana. S. moellendorffii has a genome size of approximately 100 Mbp, as small or smaller than that of A. thaliana. S. moellendorffii has the potential to provide significant comparative information to better understand the evolution of vascular plants.

RESULTS

We sequenced 2181 Expressed Sequence Tags (ESTs) from a S. moellendorffii cDNA library. One thousand three hundred and one non-redundant sequences were assembled, containing 291 contigs and 1010 singletons. Approximately 75% of the ESTs matched proteins in the non-redundant protein database. Among 1301 clusters, 343 were categorized according to Gene Ontology (GO) hierarchy and were compared to the GO mapping of A. thaliana tentative consensus sequences. We compared S. moellendorffii ESTs to the A. thaliana and Physcomitrella patens EST databases, using the tBLASTX algorithm. Approximately 60% of the ESTs exhibited similarity with both A. thaliana and P. patens ESTs; whereas, 13% and 1% of the ESTs had exclusive similarity with A. thaliana and P. patens ESTs, respectively. A substantial proportion of the ESTs (26%) had no match with A. thaliana or P. patens ESTs.

CONCLUSION

We discovered 1301 putative unigenes in S. moellendorffii. These results give an initial insight into its transcriptome that will aid in the study of the S. moellendorffii genome in the near future.

Distribution of the glutamine synthetase isozyme GSp1 in maize (Zea mays)

In maize (Zea mays L.), GSp1, the predominant GS isozyme of the developing kernel, is abundant in the pedicel and pericarp, but absent from the endosperm and embryo. Determinations of GSp1 tissue distribution in vegetative tissues have been limited thus far to root and leaves, where the isozyme is absent. However, the promoter from the gene encoding GSp1 has been shown to drive reporter gene expression not only in the maternal seed-associated tissues in transgenic maize plants, but also in the anthers, husks and pollen (Muhitch et al. 2002, Plant Sci 163: 865-872). Here we report chromatographic evidence that GSp1 resides in immature tassels, dehiscing anthers, kernel glumes, ear husks, cobs and stalks of maize plants, but not in mature, shedding pollen grains. RNA blot analysis confirmed these biochemical data. In stalks, GSp1 increased in the later stages of ear development, suggesting that it plays a role in nitrogen remobilization during grain fill.

Mitochondrial oxidative phosphorylation is required for ammonium assimilation in light in a marine diatom

Ammonium assimilation in plants occurs via the glutamine synthetase (GS, EC 6.3.1.2)/glutamine 2-oxoglutarate aminotransferase (GOGAT, EC 1.4.1.13 + 1.4.1.14 + 1.4.7.1) pathway. Rates of in vivo ammonium assimilation were measured in the marine diatom Phaeodactylum tricornutum by a recently developed technique that uses the protonophore carbonyl cyanide m-chlorophenylhydrazone to release unassimilated ammonium from the cells. In nitrogen-replete cells of P. tricornutum, there was a poor relationship between uptake and in vivo assimilation of ammonium, with the rate of uptake decreasing and the rate of assimilation increasing with time in the presence of ammonium. Ammonium uptake and assimilation were markedly light dependent, with assimilation inhibited by 77% in darkness. Oligomycin (5 micro g ml-1), an inhibitor of the mitochondrial ATPase, had no effect on the rate of photosynthesis, the maximum endogenous ammonium pool or GS activity in Phaeodactylum, but inhibited respiration by 24-27%. In the light, oligomycin inhibited ammonium assimilation by 55-70% and growth rate by 52%. One possible explanation for these results, namely that mitochondrial ATP is required to sustain activity of the cytosolic isoform of GS, is discussed.

Isolation of photorespiratory mutants from Lotus japonicus deficient in glutamine synthetase

A mutagenesis programme using ethyl methanesulphonate (EMS) was carried out on Lotus japonicus (Regel) Larsen cv. Gifu in order to isolate photorespiratory mutants in this model legume. These mutants were able to grow in a CO2-enriched atmosphere [0.7% (v/v) CO2] but showed stress symptoms when transferred to air. Among them, three mutants displayed low levels of glutamine synthetase (GS; EC 6.3.1.2) activity in leaves. The mutants accumulated ammonium in leaves upon transfer from 0.7% (v/v) CO2 to air. F1 plants of back crosses to wild type were viable in air and F2 populations segregated 3 : 1 (viable in air : air-sensitive) indicative of a single Mendelian recessive trait. Complementation tests showed that the three mutants obtained were allelic. Chromatography on DEAE-Sephacel used to separate the cytosolic and plastidic GS isoenzymes together with immunological data showed that: (1) mutants were specifically affected in the plastidic GS isoform, and (2) in L. japonicus the plastidic GS isoform eluted at lower ionic strength than the cytosolic isoform, contrary to what happens in most plants. The plastidic GS isoform present in roots of wild type L. japonicus was also absent in roots of the mutants, indicating that this plastidic isoform from roots was encoded by the same gene than the GS isoform expressed in leaf tissue. Viability of mutant plants in high-CO2 conditions indicates that plastidic GS is not essentially required for primary ammonium assimilation. Nevertheless, mutant plants did not grow as well as wild type plants in high-CO2 conditions.

MACRONUTRIENT UTILIZATION BY PHOTOSYNTHETIC EUKARYOTES AND THE FABRIC OF INTERACTIONS

Organisms acclimate to a continually fluctuating nutrient environment. Acclimation involves responses specific for the limiting nutrient as well as responses that are more general and occur when an organism experiences different stress conditions. Specific responses enable organisms to efficiently scavenge the limiting nutrient and may involve the induction of high-affinity transport systems and the synthesis of hydrolytic enzymes that facilitate the release of the nutrient from extracellular organic molecules or from internal reserves. General responses include changes in cell division rates and global alterations in metabolic activities. In photosynthetic organisms there must be precise regulation of photosynthetic activity since when severe nutrient limitation prevents continued cell growth, excitation of photosynthetic pigments could result in the formation of reactive oxygen species, which can severely damage structural and functional features of the cell. This review focuses on ways that photosynthetic eukaryotes assimilate the macronutrients nitrogen, sulfur, and phosphorus, and the mechanisms that govern assimilatory activities. Also discussed are molecular responses to macronutrient limitation and the elicitation of those responses through integration of environmental and cellular cues.

Carbon and amino acids reciprocally modulate the expression of glutamine synthetase in Arabidopsis

In bacteria and yeast, glutamine synthetase (GS) expression is tightly regulated by the metabolic status of the cell, both at the transcriptional and posttranscriptional levels. We discuss the relative contributions of light and metabolic cues on the regulation of members of the GS gene family (chloroplastic GS2 and cytosolic GS1) in Arabidopsis. These studies reveal that the dramatic induction of mRNA for chloroplastic GS2 by light is mediated in part by phytochrome and in part by light-induced changes in sucrose (Suc) levels. In contrast, the modest induction of mRNA for cytosolic GS1 by light is primarily mediated by changes in the levels of carbon metabolites. Suc induction of mRNA for GS2 and GS1 occurs in a time- and dose-dependent manner. Suc-induced changes in GS mRNA levels were also observed at the level of GS enzyme activity. In contrast, amino acids were shown to antagonize the Suc induction of GS, both at the level of mRNA accumulation and that of enzyme activity. For GS2, the gene whose expression was the most dramatically regulated by metabolites, we used a GS2 promoter-beta-glucuronidase fusion to demonstrate that transcriptional control is involved in this metabolic regulation. Our results suggest that the metabolic regulation of GS expression in plants is controlled by the relative abundance of carbon skeletons versus amino acids. This would allow nitrogen assimilation into glutamine to proceed (or not) according to the metabolic status and biosynthetic needs of the plant. This type of GS gene regulation is reminiscent of the nitrogen regulatory system in bacteria, and suggests an evolutionary link between metabolic sensing and signaling in bacteria and plants.

Molecular identification and characterization of cytosolic isoforms of glutamine synthetase in maize roots

In maize, a small multigene family encodes the cytosolic isoforms of glutamine synthetase (GS), and five cDNAs, designated pGS1a, pGS1b, pGS1c, pGS1d, and pGS1e, have been cloned (Sakakibara, H., Kawabata, S., Takahashi, H., Hase, T., and Sugiyama, T. (1992) Plant Cell Physiol. 33, 49-58; Li, M., Villemur, R., Hussey, P. J., Silflow, C. D., Gantt, J. S., and Snustad, D. P. (1993) Plant Mol. Biol. 23, 401-407). This report describes the identification and enzymatic characterization of the cytosolic isoforms of GS in maize roots, namely GS1 and GSr. The purified isoforms, as well as recombinant enzymes that had been overexpressed in Escherichia coli, were analyzed by capillary liquid chromatography/electrospray ionization-mass spectrometry, and GS1 and GSr were identified as the products of the GS1a/GS1b and GS1c/GS1d genes, respectively. Upon the addition of ammonia to the culture medium, significant amounts of GSr accumulated and a preferential increase in GS synthetase activity, as compared to GS transferase activity, was found in the root extract. Assays with the purified recombinant enzymes confirmed that the specific biosynthetic and synthetase activities of GSr were 1.6-fold higher than those of GS1. Marked differences in stability were also found between the two isoforms: GSr was more sensitive to heat than GS1 and octameric aggregates of the subunits of GSr were easily dissociated to monomers than those of GS1 at low concentrations of Mn2+ and Mg2+ ions. These characteristics of the ammonia-induced isoform of GS seem to be physiologically important for the primary assimilation of external ammonia by roots.

Glutamine synthetase isozymes in germinating barley seeds

Glutamine synthetase (GS; EC 6.3.1.2) is a key enzyme of ammonia assimilation in higher plants. In the present study the subunit composition and localization of GS in germinating barley (Hordeum vulgare) seed have been clarified. Analysis of the GS polypeptide composition by immunoblotting revealed two different polypeptides. A and B, with a molecular mass of 42 and 40 kDa, respectively. In the scutellum subunit A was already present in the ungerminated seed and remained unchanged, whereas subunit B appeared on day 2 and increased about 5-fold during germination. Polypeptide B also appeared later during germination in the aleurone layer, roots and weakly in the etiolated shoots. By immunogold microscopy, GS was detected in the scutellum and the aleurone layer of barley seeds during germination. Subcellular localization of GS on ultrathin cryosections showed that a cytosolic isozyme was present in the scutellum. Our study confirms that only a cytosolic GS is expressed in barley seed, and its subunit composition changes during germination.

Glutamine synthetases of green and etiolated leaves ofSinapis alba : Evidence of the identity of the respective enzyme proteins

Studies on the glutamine synthetases (GS, EC 6.3.1.2) of green (GS2) and etiolated leaves (GSet) ofSinapis alba L. (cv. Steinacher) revealed striking similarities between the respective enzyme proteins. The enzymes showed corresponding chromatographic properties, both on dimethylaminoethyl-Sephacel and on hydroxylapatite columns. The purified GS proteins were also identical with regard to the molecular weight of their subunits. Isoelectrofocusing of pure GSet yielded two distinct polypeptide bands in the pH 5.6 region of the gels. This pattern corresponded to the two strong bands of GS2. Two charge variants of GS polypeptides could be detected by Western-blot analysis of the soluble protein of green leaves using antibodies against mustard GS2. In immunoprecipitation experiments, the holoenzymes of GS2 and GSet were recognized with identical affinities by this antiserum. We conclude that strong similarities exist between the proteins of the GS enzymes in green and etiolated leaves of mustard. Most probably only one GS form, namely the plastidic enzyme, can be found in the epigeal organs ofSinapis. The polypeptides of the GS2 subunits showed no differences in the hydrophobicity of the polypeptide chains. Neither glucosyl nor mannosyl residues could be detected.

Glutamine synthetase isoenzymes of Phaseolus vulgaris L.: subunit composition in developing root nodules and plumules

In the legume Phaseolus vulgaris L., glutamine synthetase (GS) (EC.6.3.1.2.) occurs as three cytosolic polypeptides, α, β and γ, and a plastidic polypeptide, δ. This paper describes the subunit composition of active octameric GS isoenzymes from root nodules and plumules using ionexchange high-performance liquid chromatography followed by two-dimensional denaturing gel electrophoresis and Western immunodetection. Root nodules contained four separable GS activities, three of which were composed mainly of cytosolic γ, γ/β and β GS polypeptides, whereas the fourth activity, consisted of plastidic δ GS polypeptides. The increase in GS activity during nodulation was due largely to the appearance of γ-containing isoenzymes, and to a lesser extent on the δ isoenzyme, whereas the β-isoenzyme activity remained approximately constant throughout. Plumule GS from imbibed seeds was found to be composed of separate α and β isoenzymes, but 2 d after germination, plumule GS consisted of a mixture of α, α/β and β isoenzymes. The results from both nodules and plumules indicate that different cytosolic GS polypeptides in P. vulgaris are able to assemble into both homo-octameric and heterooctameric isoenzymes. Moreover, the changes in the patterns of isoenzymes observed during nodule development and plumule growth are interpreted to be caused both by temporal changes in the denovo synthesis of the polypeptides and also by their spatial separation in different cell types.

Glutamine synthetase isoforms in the green hydra symbiosis

The occurrence of glutamine synthetase isoforms in the green hydra symbiosis has been investigated using ion-exchange chromatography. There were three isoforms which were designated algal cytosolic, chloroplastic and host isoenzymes. Comparison of the proportions of algal isoforms from the intact association with those from nitrogen-replete and nitrogen-starved cultures of a Chlorella strain originally isolated from green hydra, suggests that symbionts in the intact association may be nitrogen-deficient.

Regulation of the appearance of glutamine synthetase in mustard (Sinapis alba L.) cotyledons by light, nitrate and ammonium

During transformation of mustard seedlings cotyledons from storage organs to photosynthetically competent leaves, a process which occurs during the first 4 d after sowing, total glutamine-synthetase (GS, EC 6.3.1.2) activity increases from zero to the high level usually observed in green leaves. In the present study we have used ion-exchange chromatography to separate possible isoforms of GS during the development of the cotyledons. The approach failed since we could only detect a single form of GS, presumably plastidic GS, under all circumstances tested. The technique of selective photooxidative destruction of plastids in situ was applied to solve the problem of GS localization. It was inferred from the data that the GS as detected by ion-exchange chromatography is plastidic GS.The regulatory role, if any, of light, nitrate and ammonium in the process of the appearance of GS in the developing cotyledons was investigated. The results show that nitrate and ammonium play only minor roles. Light, operating via phytochrome, is the major regulatory factor.

Characterization of a cDNA clone for barley leaf glutamine synthetase

A barley cDNA clone (1182 bp) encoding chloroplastic glutamine synthetase was isolated with a heterologous cDNA probe of the gene specifying the enzyme from alfalfa. The clone, named pGS8, was found in a lambda gtII cDNA library prepared from dark grown barley leaves even though the chloroplastic glutamine synthetase is absent from such leaves. In agreement therewith the clone hybridized in Northern blot analyses with a 1.7 kb mRNA species present the in poly A+ mRNA fraction of both dark grown and greened primary leaves of barley. The nucleotide sequence of the barley clone reveals 75% identity to the Phaseolus vulgaris and Pisum sativum clones encoding chloroplastic glutamine synthetase, while only 69% identity is observed in comparisons with the clones specifying the cytosolic isozymes. At the amino acid level 85% identity is found between the deduced barley glutamine synthetase sequence and that of the corresponding chloroplastic isoenzymes from bean and pea. The chloroplastic glutamine synthetases contain cysteins in the putative ATP and and substrate binding sites. In the cytosolic forms these positions are occupied by alanine residues.

Genetic determinants of glutamine synthetase in Drosophila melanogaster: a gene for glutamine synthetase I resides in the 21B3-6 region

Recombinational and deletion mapping of electrophoretic variants of the glutamine synthetase I isozyme (GSI) in Drosophila melanogaster locates the gene in the 21B region on the second chromosome. We have conducted a genetic analysis of the region extending cytologically from 21A to 21B4-6. Recessive lethal mutations were generated by ethyl methanesulfonate (EMS) and ethyl nitrosourea (ENU) mutagenesis and by hybrid dysgenesis (HD). These lethals fall into seven functional groups, which were partially ordered by complementation with cytologically defined deficiencies of this region generated by hybrid dysgenesis. Two of the EMS- and two of the ENU-induced lethals fulfill biochemical criteria expected for null alleles of the GSI gene.

Cysteine residues at the active site of glutamine synthetase from spinach leaves

Titration of cysteine residues of spinach glutamine synthetase with 5-5' dithiobis (2-nitrobenzoic acid) indicates that there are five such residues per monomer of enzyme and that two of these five are on the surface of the molecule. The presence of substrates, or either of the competitive inhibitors methionine sulfoximine or phosphinothricin, completely protects both of the surface sulfhydryls from titration. This suggests that both are located at the active site. In the absence of Mg2+ and ATP, both surface sulfhydryls must be modified before loss of activity. We conclude that while both of the cysteine residues are located at the active site, only one of them may be involved in catalysis. Because the cysteine residue which is implicated in catalysis can be protected by Mg2+ and ATP, we believe that it may be located at or near the binding site of these ligands.

Changes in the activities of chloroplast and cytosolic isoenzymes of glutamine synthetase during normal leaf growth and plastid development in wheat

Soluble protein extracts and chloroplasts from a serial sequence of transverse sections of a 7-d-old wheat leaf (Triticum aestivum cv. Maris Huntsman) were used to study changes in the activity of glutamine synthetase (GS; EC 6.3.1.2) during cell and chloroplast development. Glutamine synthetase activity increased more than 50-fold per cell from the base to the tip of the wheat leaf. Two isoenzymes of GS were separated using fast protein liquid chromatography (FPLC). Glutamine synthetase localized in the cytoplasm (GS1) eluted at about 0.21 M NaCl, and the isoenzyme localized in the chloroplast (GS2) eluted at about 0.33 M NaCl. The increase in GS activity during leaf development was found to be caused primarily by an increase in the activity of the chloroplast GS2. The activity of the cytoplasmic GS1 remained constant as the cells were displaced from the base to the tip of the leaf, whereas GS2 activity increased within the chloroplast throughout development. At the base of the leaf, 26% of total GS activity was cytoplasmic; the remaining 74% was in the chloroplast. At 10 cm from the base, only 4% of the activity was cytoplasmic, and 96% was in the chloroplast. The results indicate that the chloroplast GS2 is probably responsible for most of the ammonia assimilation in the mature wheat leaf, whereas cytoplasmic GS1 may serve a role in immature developing leaf cells.

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.

Four isoforms of glutamine synthetase in light-grown soybeans

Glutamine synthetase (GS) activity recovered from linear sucrose gradients was associated with the cytosol of cells isolated from etiolated soybean hypocotyls whereas light-grown tissue contained increased GS activity localized in both the cytosol and chloroplasts. DEAE-cellulose chromatography indicated two GS isoforms in etiolated hypocotyls whereas light-grown hypocotyls and primary leaves contained four isoforms. Only one GS isoform was recovered from both etiolated and light-grown cotyledons.

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.

Evidence for a cytosolic-dependent light induction of chloroplastic glutamine synthetase during greening of etiolated rice leaves

During the greening of etiolated rice leaves, total glutamine synthetase activity increases about twofold, and after 48 h the level of activity usually observed in green leaves is obtained. A density-labeling experiment with deuterium demonstrates that the increase in enzyme activity is due to a synthesis of the enzyme. The enhanced activity obtained upon greening is the result of two different phenomena: there is a fivefold increase of chloroplastic glutamine synthetase content accompanied by a concommitant decrease (twofold) of the cytosolic glutamine synthetase. The increase of chloroplastic glutamine synthetase (GS2) is only inhibited by cycloheximide and not by lincomycin. This result indicates a cytosolic synthesis of GS2. The synthesis of GS2 was confirmed by a quantification of the protein by an immunochemical method. It was demonstrated that GS2 protein content in green leaves is fivefold higher than in etiolated leaves.

Intracellular distribution of enzymes associated with nitrogen assimilation in roots

The cellular distribution of enzymes involved in nitrogen assimilation: nitrate reductase (EC 1.6.6.2), nitrite reductase (EC 1.6.6.4), glutamine synthetase (EC 6.3.1.2), glutamate synthase (EC 2.6.1.53), and glutamate dehydrogenase (EC 1.4.1.3) has been studied in the roots of five plants: maize (Zea mays L. hybrid W 64A x W 182E), rice (Oryza sativa L. cv. Delta), bean (Phaseolus vulgaris L. cv. Contender), pea (Pisum sativum L. cv. Demi-nain), and barley (Hordeum vulgare L.). Initially, cell organelles were separated from soluble proteins by differential centrifugation. Cell organelles were also subjected to sucrose density gradients. The results obtained by these two methods indicate that nitrite reductase and glutamate synthase are localized in plastids, nitrate reductase and glutamine synthetase are present in the cytosol, and glutamate dehydrogenase is a mitochondrial enzyme.