cDNA sequence and differential expression of the gene encoding the glutamine synthetase γ polypeptide ofPhaseolus vulgaris L

The plant nitrogen mobilization promoted by Colletotrichum lindemuthianum in Phaseolus leaves depends on fungus pathogenicity

Nitrogen plays an essential role in the nutrient relationship between plants and pathogens. Some studies report that the nitrogen-mobilizing plant metabolism that occurs during abiotic and biotic stress could be a 'slash-and-burn' defence strategy. In order to study nitrogen recycling and mobilization in host plants during pathogen attack and invasion, the Colletotrichum lindemuthianum/Phaseolus vulgaris interaction was used as a model. C. lindemuthianum is a hemibiotroph that causes anthracnose disease on P. vulgaris. Non-pathogenic mutants and the pathogenic wild-type strain were used to compare their effects on plant metabolism. The deleterious effects of infection were monitored by measuring changes in chlorophyll, protein, and amino acid concentrations. It was shown that amino acid composition changed depending on the plant-fungus interaction and that glutamine accumulated mainly in the leaves infected by the pathogenic strain. Glutamine accumulation correlated with the accumulation of cytosolic glutamine synthetase (GS1 alpha) mRNA. The most striking result was that the GS1 alpha gene was induced in all the fungus-infected leaves, independent of the strain used for inoculation, and that GS1 alpha expression paralleled the PAL3 and CHS defence gene expression. It is concluded that a role of GS1 alpha in plant defence has to be considered.

The 3' untranslated region of a soybean cytosolic glutamine synthetase (GS1) affects transcript stability and protein accumulation in transgenic alfalfa

Higher plants assimilate nitrogen in the form of ammonia through the concerted activity of glutamine synthetase (GS) and glutamate synthase (GOGAT). The GS enzyme is either located in the cytoplasm (GS1) or in the chloroplast (GS2). Glutamine synthetase 1 is regulated in different plants at the transcriptional level and there are some reports of regulation at the level of protein stability. Here we present data that clearly establish that GS1 in plants is also regulated at the level of transcript turnover and at the translational level. Using a Glycine max (soybean) GS1 transgene, with and without its 3' untranslated region (UTR), driven by the constitutive CaMV 35S promoter in Medicago sativa (alfalfa) and Nicotiana tabacum (tobacco), we show that the 3' UTR plays a major role in both transcript turnover and translation repression in both the leaves and the nodules. Our data suggest that the 3' UTR mediated turnover of the transcript is regulated by a nitrogen metabolite or carbon/nitrogen ratios. We also show that the 3' UTR of the gene for the soybean GS1 confers post-transcriptional regulation on a reporter gene. Our dissection of post-transcriptional and translational levels of regulation of GS in plants shows that the situation in plants strongly resembles that in other organisms where GS is regulated at almost all levels. Multistep regulation of GS shows the high priority given by organisms to regulating and ensuring optimal control of nitrogen substrates and preventing overproduction of glutamine and drainage of the glutamate pool.

Novel expression pattern of cytosolic Gln synthetase in nitrogen-fixing root nodules of the actinorhizal host, Datisca glomerata

Gln synthetase (GS) is the key enzyme of primary ammonia assimilation in nitrogen-fixing root nodules of legumes and actinorhizal (Frankia-nodulated) plants. In root nodules of Datisca glomerata (Datiscaceae), transcripts hybridizing to a conserved coding region of the abundant nodule isoform, DgGS1-1, are abundant in uninfected nodule cortical tissue, but expression was not detectable in the infected zone or in the nodule meristem. Similarly, the GS holoprotein is immunolocalized exclusively to the uninfected nodule tissue. Phylogenetic analysis of the full-length cDNA of DgGS1-1 indicates affinities with cytosolic GS genes from legumes, the actinorhizal species Alnus glutinosa, and nonnodulating species, Vitis vinifera and Hevea brasilensis. The D. glomerata nodule GS expression pattern is a new variant among reported root nodule symbioses and may reflect an unusual nitrogen transfer pathway from the Frankia nodule microsymbiont to the plant infected tissue, coupled to a distinctive nitrogen cycle in the uninfected cortical tissue. Arg, Gln, and Glu are the major amino acids present in D. glomerata nodules, but Arg was not detected at high levels in leaves or roots. Arg as a major nodule nitrogen storage form is not found in other root nodule types except in the phylogenetically related Coriaria. Catabolism of Arg through the urea cycle could generate free ammonium in the uninfected tissue where GS is expressed.

Short-term metabolic responses of soybean root nodules to nitrate

Soybean (Glycine max L. Merr.) plants exposed to 10 mM KNO(3) for a 4 d period were used to test the correlation between nitrogenase activity, gene expression and sucrose metabolism. Nitrate caused the down-regulation of sucrose synthase (SS) transcripts within 1 d, although a decline in nodule SS activity and an increase in nodule sucrose content only occurred after 3-4 d. In a second experiment, plants were exposed to (15)N-labelled nitrate for 48 h to determine the time period during which nitrate was taken up, and to relate this to the decline in apparent nitrogenase activity (H(2) production in air) and the reduction in SS gene transcript levels. The peak of nitrate uptake appeared to be between 8 h and 14 h whilst apparent nitrogenase activity began to decline at about 17.5 h. The SS mRNA signal declined markedly between 14 h and 24 h. The correlative association of these factors is clear. However, SS activity per se does not appear to be related to the initial decline in apparent nitrogenase activity as a result of nitrate uptake. These findings, therefore, do not support the hypothesis that the regulation of nodule function is mediated by the regulation of SS activity.

Cytosolic glutamine synthetase in soybean is encoded by a multigene family, and the members are regulated in an organ-specific and developmental manner

Gln synthetase (GS) is the key enzyme in N metabolism and it catalyzes the synthesis of Gln from glutamic acid, ATP, and NH4+. There are two major isoforms of GS in plants, a cytosolic form (GS1) and a chloroplastic form (GS2). In leaves, GS2 functions to assimilate ammonia produced by nitrate reduction and photorespiration, and GS1 is the major isoform assimilating NH3 produced by all other metabolic processes, including symbiotic N2 fixation in the nodules. GS1 is encoded by a small multigene family in soybean (Glycine max), and cDNA clones for the different members have been isolated. Based on sequence divergence in the 3'-untranslated region, three distinct classes of GS1 genes have been identified (alpha, beta, and gamma). Genomic Southern analysis and analysis of hybrid-select translation products suggest that each class has two distinct members. The alpha forms are the major isoforms in the cotyledons and young roots. The beta forms, although constitutive in their expression pattern, are ammonia inducible and show high expression in N2-fixing nodules. The gamma1 gene appears to be more nodule specific, whereas the gamma2 gene member, although nodule enhanced, is also expressed in the cotyledons and flowers. The two members of the alpha and beta class of GS1 genes show subtle differences in the expression pattern. Analysis of the promoter regions of the gamma1 and gamma2 genes show sequence conservation around the TATA box but complete divergence in the rest of the promoter region. We postulate that each member of the three GS1 gene classes may be derived from the two ancestral genomes from which the allotetraploid soybean was derived.

Constitutive overexpression of cytosolic glutamine synthetase (GS1) gene in transgenic alfalfa demonstrates that GS1 may be regulated at the level of RNA stability and protein turnover

Glutamine synthetase (GS) catalyzes the ATP-dependent condensation of NH4+ with glutanate to yield glutamine. Gene constructs consisting of the cauliflower mosaic virus (CaMV) 35S promoter driving a cytosolic isoform of GS (GS1) gene have been introduced into alfalfa (Medicago sativa). Although transcripts for the transgene were shown to accumulate to high levels in the leaves, they were undetectable in the nodules. However, significant amounts of beta-glucuronidase activity could be detected in nodules of plants containing the CaMV 35S promoter-beta-glucuronidase gene construct, suggesting that the transcript for the GS1 transgene is not stable in the root nodules. Leaves of alfalfa plants with the CaMV 35S promoter-GS1 gene showed high levels of accumulation of the transcript for the transgene when grown under low-nitrogen conditions and showed a significant drop in the level of GS1 transcripts when fed with high levels of NO3-. However, no increase in GS activity or polypeptide level was detected in the leaves of transgenic plants. The results suggest that GS1 is regulated at the level of RNA stability and protein turnover.

Oxidative turnover of soybean root glutamine synthetase. In vitro and in vivo studies

Glutamine synthetase (GS) is the key enzyme in ammonia assimilation and catalyzes the ATP-dependent condensation of NH3 with glutamate to produce glutamine. GS in plants is an octameric enzyme. Recent work from our laboratory suggests that GS activity in plants may be regulated at the level of protein turnover (S.J. Temple, T.J. Knight, P.J. Unkefer, C. Sengupta-Gopalan [1993] Mol Gen Genet 236: 315-325; S.J. Temple, S. Kunjibettu, D. Roche, C. Sengupta-Gopalan [1996] Plant Physiol 112: 1723-1733; S.J. Temple, C. Sengupta-Gopalan [1997] In C.H. Foyer, W.P. Quick, eds, A Molecular Approach to Primary Metabolism in Higher Plants. Taylor & Francis, London, pp 155-177). Oxidative modification of GS has been implicated as the first step in the turnover of GS in bacteria. By incubating soybean (Glycine max) root extract enriched in GS in a metal-catalyzed oxidation system to produce the.OH radical, we have shown that GS is oxidized and that oxidized GS is inactive and more susceptible to degradation than nonoxidized GS. Histidine and cysteine protect GS from metal-catalyzed inactivation, indicating that oxidation modifies the GS active site and that cysteine and histidine residues are the site of modification. Similarly, ATP and particularly ATP/glutamate give the enzyme the greatest protection against oxidative inactivation. The roots of plants fed ammonium nitrate showed a 3-fold increase in the level of GS polypeptides and activity compared with plants not fed ammonium nitrate but without a corresponding increase in the GS transcript level. This would suggest either translational or posttranslational control of GS levels.

Nitrogen metabolism in actinorhizal nodules of Alnus glutinosa: expression of glutamine synthetase and acetylornithine transaminase

Two nodule cDNA clones representing genes involved in Alnus glutinosa nitrogen metabolism were analysed. ag11 encoded glutamine synthetase (GS), the enzyme responsible for ammonium assimilation, while ag118 encoded acetylornithine transaminase (AOTA), an enzyme involved in the biosynthesis of citrulline, the nitrogen transport form in Alnus. GS mRNA was found at highest levels in root nodules, where it was present in the infected cells as well as in the cells of the pericycle of the vascular system. AOTA transcripts were found at high levels in nodules, confined to the infected cells, suggesting that in nodules of A. glutinosa, citrulline biosynthesis takes place mainly in the infected cells.

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.

Influence of indoleacetic-acid-producing Bacillus isolates on the nodulation of Phaseolus vulgaris by Rhizobium etli under gnotobiotic conditions

Twenty-two Bacillus spp. isolates from the rhizosphere of Phaseolus vulgaris 'Contender' were identified using Biolog™, gas chromatographic fatty acid methyl ester, and 23S rDNA analyses. Some of the Bacillus isolates produced significant amounts of the phytohormone indoleacetic acid (IAA) when grown in a liquid culture medium supplemented with 100 μg L-tryptophan/L; less IAA was produced in culture medium not supplemented with L-tryptophan. Thin-layer chromatography, high-performance liquid chromatography, gas chromatography – mass spectrometry, and the avena coleoptile bioassay were used to identify and quantify IAA produced by Bacillus isolates. Significant differences were observed in the amounts of IAA produced by different strains of Bacillus, with amounts varying from 0.40 to 4.88 μg/mL. α-Methyltryptophan-resistant mutants of Bacillus exhibited altered IAA production and excreted tryptophan into the growing medium. The IAA-producing Bacillus isolates promoted root growth and (or) nodulation when coinoculated with Rhizobium etli (TAL 182) on Phaseolus vulgaris 'Contender' under gnotobiotic conditions in growth chambers. Coinoculation resulted in increased nodule number, nodule fresh weight, nitrogenase activity, leghemoglobin content, and total soluble protein content in the root nodules of Phaseolus vulgaris. In contrast, coinoculation with α-methyltryptophan mutants resulted in decreased nodulation, indicating that Bacillus isolates have a direct effect on either the Rhizobium or the plant and the effect may not be singularly attributed to their ability to produce IAA in vitro.Key words: Bacillus, indoleacetic acid production, nodulation enhancement.

Characterization of Vitis vinifera L. glutamine synthetase and molecular cloning of cDNAs for the cytosolic enzyme

Grapevine (Vitis vinifera L.) glutamine synthetase (GS) was analysed into two distinct classes of isoforms; one of them was present in both leaf and root tissues while the other one showed leaf specificity. Western blot analysis revealed that grapevine GS consists of three types of polypeptides of distinct size and differential tissue specificity. Two structurally distinct cDNA clones, pGS1;1 and pGS1;2, encoding grapevine GS were isolated from a cell suspension library and characterized. Both clones contained open reading frames encoding for polypeptides of 356 amino acids with a predicted molecular mass of about 39 kDa. Although the coding sequences of pGS1;1 and pGS1;2 were 84% similar, their 5'- and 3'-untranslated sequences showed only 40% similarity. The coding sequences of the two clones and the derived amino acid sequences showed higher homology to cytosolic than to chloroplastic GSs of other higher plants indicating that the cDNAs isolated encode for cytosolic isoforms of grapevine GS. Southern blot analysis suggested the existence of more than two GS genes in the grapevine genome. In northern blots both clones were hybridized to mRNAs of about 1.4 kb that are differentially expressed in the various tissues. Supply of nitrate or ammonium in the cell suspension culture medium, as a sole nitrogen source, resulted in differential response of the pGS1;1- and pGS1;2-related genes.

Localization of tobacco cytosolic glutamine synthetase enzymes and the corresponding transcripts shows organ- and cell-specific patterns of protein synthesis and gene expression

The subcellular localization of glutamine synthetase in tobacco and the differential expression of two genes encoding cytosolic enzyme was investigated using both immunocytochemistry and in situ hybridization. Two full length cDNA clones each encoding cytosolic GS (Gln 1-3 and Gln 1-5) were isolated from a tobacco seeding cDNA library. A strong homology was found in the coding region of the two clones whereas the 3'- and 5'-untranslated sequences were dissimilar. In order to determine the levels of transcription, specific sequences from Gln1-3 and Gln1-5 were used in an RNAse protection assay. This experiment clearly showed that the gene encoding Gln1-3 is expressed in roots and flowers whereas the gene encoding Gln1-5 is transcribed at a high level in stems and at a lower level in roots and flowers. Immunogold labelling was used to examine the subcellular and cellular distribution of glutamine synthetase in vegetative and reproductive organs of tobacco plants. In mature leaf tissue or petals and sepals, plastidic GS was visualised only in the stroma matrix of chloroplasts and plastids. Cytosolic GS was detected in a number of vegetative or reproductive organs including leaves and flowers. In leaves cytosolic GS was preferentially located in the vascular tissue. In situ hybridization was performed using sections of tobacco organs and specific antisense RNA probes to the genes encoding Gln1-3 and Gln1-5. Gln1-5 transcripts were localised in the vascular tissues of stems and roots whereas Gln1-3 transcripts were detected in all root cells and floral organs including petals, sepals and anthers.

Molecular evolution of duplicate copies of genes encoding cytosolic glutamine synthetase in Pisum sativum

Here, we describe two nearly identical expressed genes for cytosolic glutamine synthetase (GS3A and GS3B) in Pisum sativum L. RFLP mapping data indicates that the GS3A and GS3B genes are separate loci located on different chromosomes. DNA sequencing of the GS3A and GS3B genes revealed that the coding regions are 99% identical with only simple nucleotide substitutions resulting in three amino acid differences. Surprisingly, the non-coding regions (5' non-coding leader, the 11 introns, and 3' non-coding tail) all showed a high degree of identity (96%). In these non-coding regions, 25% of the observed differences between the GS3A and GS3B genes were deletions or duplications. The single difference in the 3' non-coding regions of the GS3A and GS3B genes was a 25 bp duplication of an AU-rich element in the GS3B gene. As the GS3B mRNA accumulates to lower levels than the GS3A gene, we tested whether this sequence which resembles an mRNA instability determinant functioned as such in the context of the GS mRNA. Using the GS3B 3' tail as part of a chimeric gene in transgenic plants, we showed that this AU-rich sequence has little effect on transgene mRNA levels. To determine whether the GS3A/GS3B genes represent a recent duplication, we examined GS3-like genes in genomic DNA of ancient relatives of P. sativum. We observed that several members of the Viceae each contain two genomic DNA fragments homologous to the GS3B gene, suggesting that this is an ancient duplication event. Gene conversion has been invoked as a possible mechanism for maintaining the high level of nucleotide similarity found between GS3A and GS3B genes. Possible evolutionary reasons for the maintenance of these 'twin' GS genes in pea, and the general duplication of genes for cytosolic GS in all plant species are discussed.

Identification of several soybean cytosolic glutamine synthetase transcripts highly or specifically expressed in nodules: expression studies using one of the corresponding genes in transgenic Lotus corniculatus

A DNA fragment containing sequences hybridizing to the 5' region of GS15, a gene encoding soybean cytosolic glutamine synthetase, was isolated from a soybean genomic library. Mapping and partial sequence analysis of the genomic clone revealed that it encodes a cytosolic GS gene, GS21, which is different from GS15. In parallel, a number of cDNA clones encoding cytosolic GS were isolated using the coding region of pGS20 as a probe (pGS20 is a cDNA clone which corresponds to a transcript of the GS15 gene). Two new full-length cDNAs designated pGS34 and pGS38 were isolated and sequenced. In the 5' non-coding region a strong homology was found between the two clones and the GS21 gene. However, none of these sequences were identical, which suggests that there are at least three members in this group of genes. In order to determine their relative levels of transcription, specific sequences from pGS34, pGS38 and GS21 were used in an RNAse protection assay. This experiment clearly showed that GS21 and the gene encoding pGS38 are specifically expressed in young or mature nodules, whereas the gene encoding pGS34 is highly transcribed in nodules and constitutively expressed at a lower level in other soybean organs. In order to further analyse the molecular mechanisms controlling GS21 transcription, different fragments of the promoter region were fused to the Escherichia coli reporter gene encoding beta-glucuronidase (GUS) and the constructs were introduced into Lotus corniculatus via Agrobacterium rhizogenes-mediated transformation. Analysis of GUS activity showed that the GS21 promoter-GUS constructs were expressed in the vasculature of all vegetative organs. This result is discussed in relation to species-specific metabolic and developmental characteristics of soybean and Lotus.

Cloning and characterization of a nodule-enhanced glutamine synthetase-encoding gene from Lupinus luteus

Glutamine synthetase (GS)-encoding genes in Lupinus luteus constitute a small family of genes showing different expression patterns [Boroń et al., Acta Biochim. Polon. 36 (1989) 295-301]. One member of this family, the LlNGS1 gene, is strongly induced in root nodules close to the onset of nitrogen fixation and is referred to as a nodule-enhanced GS gene. We present here the structure of the nodule-enhanced LlNGS1 gene, the first gene of this class which has been sequenced. LlNGS1 is composed of twelve exons and shows structural similarity to the GS gene from Medicago sativa, indicating structure conservation of GS genes in legumes. Comparison of protein coding regions, as well as 5'-untranslated regions derived from LlNGS1 and a Lupinus angustifolius pGS5 GS cDNA clone [Grant et al., Plant Mol. Biol. 13 (1989) 481-490], revealed a high degree of shared identity between both genes, indicating that they are orthologous. The sequence of the LlNGS1 5'-flanking region (2.3 kb) contains several elements implicated in regulation of nodulin genes, as well as other characteristic DNA motifs. RNA blot hybridization analysis carried out using a probe corresponding to the LlNGS1 3'-untranslated region revealed that this gene is also transcribed in leaves, but at a barely detectable level.

Two classes of differentially regulated glutamine synthetase genes are expressed in the soybean nodule: a nodule-specific class and a constitutively expressed class

We have characterized two sets of cDNA clones representing the glutamine synthetase (GS) mRNA in soybean nodules. Using the 3'-untranslated regions of a representative member of each set, as gene member(s) specific probes, we have shown that one set of the GS genes are expressed in a nodule-specific manner, while the other set is expressed in other tissues, besides the nodules. The nodule-specific GS genes are expressed in a developmentally regulated manner in the nodules, independent of the onset of nitrogen fixation. The other class of GS genes is expressed constitutively in all tissues tested, but its expression level is dramatically enhanced in nodules following onset of N2 fixation. The latter set of genes is also expressed in cotyledons of germinating seedlings in a developmentally regulated manner. Analysis of hybrid select translation products and genomic Southern blots suggests that multiple gene members in each class are expressed in the nodules.

A survey of transcripts expressed specifically in root nodules of broadbean (Vicia faba L.)

More than 600 potentially nodule-specific clones have been detected by differential hybridization of a broadbean cDNA library constructed from root nodule poly(A)+ RNA. These isolated cDNAs belong to at least 28 different clone groups containing cross-hybridizing sequences. The number of clones within a clone group varies from about 200 to only one single clone. Northern hybridization experiments revealed nodule-specific transcripts for 14 clone groups and markedly nodule-enhanced transcripts for another 7 clone groups. Sequence homologies indicate that three transcript sequences code for different leghemoglobins. Two other transcripts encode a nodule-specific sucrose synthase and a nodule-enhanced asparagine synthetase, respectively. Four deduced gene products are proline-rich, two of them being the homologues of PsENOD2 and PsENOD12. The third proline-rich protein (PRP) is composed of similar amino acid repeats as the nodule-specific PsENOD12 but is expressed in nodules and roots in comparable amounts. The fourth PRP is a nodule-enhanced extensin-type protein built up by Ser-Pro4 repeats. Two further nodule-specific transcripts encode gene products showing some similarity to structural glycine-rich proteins. Additionally, transcripts could be identified for broadbean homologues of the nodulins MsNOD25, PsENOD3 and PsENOD5 and transcripts specifying a nodule-enhanced lipoxygenase and a translation elongation factor EF-1 alpha, which is expressed in all broadbean tissues tested.

Molecular characterization of a cDNA clone encoding glutamine synthetase from a gymnosperm, Pinus sylvestris

A full-length cDNA clone (pGSP114) encoding glutamine synthetase was isolated from a lambda gt11 library of the gymnosperm Pinus sylvestris. Nucleotide sequence analysis showed that pGSP114 contains an open reading frame encoding a protein of 357 amino acid residues with a calculated molecular mass of 39.5 kDa. The derived amino acid sequence was more homologous to cytosolic (GS1) (78-82%) than to chloroplastic (GS2) (71-75%) glutamine synthetase in angiosperms. The lack of N-terminal presequence and C-terminal extension which define the primary structure of GS2, also supports that the isolated cDNA encodes cytosolic GS. Southern blot analysis of genomic DNA from P. sylvestris and P. pinaster suggests that GS may be encoded by a small gene family in pine. GS mRNA was more abundant in cotyledons and stems than in roots of both Scots and maritime pines. Western blot analysis in P. sylvestris seedlings showed that only one GS polypeptide, similar in size to GS1 in P. pinaster, could be detected in several different tissues. Our results suggest that cytosolic GS is mainly responsible for glutamine biosynthesis in pine seedlings.

Forcing expression of a soybean root glutamine synthetase gene in tobacco leaves induces a native gene encoding cytosolic enzyme

Glutamine synthetase (GS; EC 6.3.1.2) is present in different subcellular compartments in plants. It is located in the cytoplasm in root and root nodules while generally present in the chloroplasts in leaves. The expression of GS gene(s) is enhanced in root nodules and in soybean roots treated with ammonia. We have isolated four genes encoding subunits of cytosolic GS from soybean (Glycine max L. cv. Prize). Promoter analysis of one of these genes (GS15) showed that it is expressed in a root-specific manner in transgenic tobacco and Lotus corniculatus, but is induced by ammonia only in the legume background. Making the GS15 gene expression constitutive by fusion with the CaMV-35S promoter led to the expression of GS in the leaves of transgenic tobacco plants. The soybean GS was functional and was located in the cytoplasm in tobacco leaves where this enzyme is not normally present. Forcing this change in the location of GS caused concomitant induction of the mRNA for a native cytosolic GS in the leaves of transgenic tobacco. Shifting the subcellular location of GS in transgenic plants apparently altered the nitrogen metabolism and forced the induction in leaves of a native GS gene encoding a cytosolic enzyme. The latter is normally expressed only in the root tissue of tobacco. This phenomenon may suggest a hitherto uncharacterized metabolic control on the expression of certain genes in plants.

Characterization of the gene encoding the plastid-located glutamine synthetase of Phaseolus vulgaris: regulation of beta-glucuronidase gene fusions in transgenic tobacco

The gln-delta gene, which encodes the plastid-located glutamine synthetase of Phaseolus vulgaris, was cloned and its promoter region was sequenced. Primer extension analysis was used to map the two major transcription initiation sites which are about 90 nucleotides apart. A fusion of 2.3 kb of the upstream region of the gln-delta gene to the reporter gene uidA encoding beta-glucuronidase was shown to be expressed in the chlorophyllous cell types of leaves and stems and in the root meristem region of transgenic tobacco. Analysis of a series of three 5' promoter deletion fusions revealed the presence of a region essential for promoter activity between -786 and -327 and regions involved in tissue-specific regulation and light regulation between -786 and +43.

The glutamine synthetase gene family of Arabidopsis thaliana: light-regulation and differential expression in leaves, roots and seeds

Glutamine synthetase (GS) plays an important role in the assimilation of nitrogen by higher plants. We present here a molecular analysis of the GS polypeptides, mRNAs, and genes of Arabidopsis thaliana. Western blot analysis of leaf and root protein extracts revealed at least two distinct GS polypeptides; 43 kDa and 39 kDa GS polypeptides were present in leaves, while only a 39 kDa GS was detected in roots. The 43 kDa GS polypeptide is light-inducible. In etiolated seedlings only the 39 kDa GS was detected. However, upon greening the 43 kDa GS increased to levels comparable to those observed in light-grown plants. Four distinct GS cDNA clones, lambda Atgsl1, lambda Atgsr1, lambda Atgsr2 and lambda Atkb6 were isolated and characterized. Their complete nucleotide and deduced amino acid sequences are presented. The coding sequences of the four clones are 70-88% similar while their 5' and 3' untranslated regions exhibit less than 50% similarity. Northern blots of leaf, root and germinated seed RNA revealed that the four cDNAs hybridize to mRNAs which are differentially expressed in the organs of Arabidopsis thaliana. lambda Atgsl1 is leaf-specific and hybridizes to a 1.6 kb mRNA. Both lambda Atgsr1 and lambda Atgskb6 hybridize to 1.4 kb mRNAs which are expressed in both roots and germinated seeds. lambda Atgsr2 hybridizes to a 1.4 kb mRNA, which is primarily expressed in roots with low levels of expression in seeds and leaves. lambda Atgsl1, which represents the leaf-specific mRNA, is induced by light. lambda Atgsl1 mRNA levels increase during the greening of etiolated seedlings while lambda Atgsr1 levels remain constant. Southern blot analysis indicated that the Arabidopsis genome contains at least four and possibly five distinct GS genes.

A promoter sequence involved in cell-specific expression of the pea glutamine synthetase GS3A gene in organs of transgenic tobacco and alfalfa

The DNA sequence of the pea cytosolic glutamine synthetase GS3A gene promoter has been determined and the start of transcription mapped using S1 nuclease. The full-length promoter and a series of 5' deletions were fused to beta-glucuronidase (GUS) and introduced into transgenic tobacco and alfalfa. In transgenic tobacco the GS3A promoter directed GUS expression in the phloem cells of the vasculature in leaves, stems and roots. GUS expression was also detected in the vasculature of cotyledons and the root tips of germinating T1 seedlings. The promoter conferred a similar expression pattern in transgenic alfalfa, and expression was also observed in root nodules. Nodule expression was located in nodule primordia, as well as the meristem, symbiotic zone, and vasculature of mature nodules. The promoter was found to be active even when deleted to -132 relative to the start of transcription. DNA mobility-shift analysis identified a protein present in nuclear and whole-cell plant extracts which bound to a 17 bp DNA element contained within the minimal -132 promoter required for expression.

Analyses of Phaseolus vulgaris L. and P. coccineus Lam. hybrids by RFLP: preferential transmission of P. vulgaris alleles

Restriction fragment length polymorphism (RFLP) was determined among P. vulgaris genotypes and Phaseolus species using 19 probes. The incidence of polymorphism was high (70-86%) between species, but relatively low (22-26%) between genotypes of P. vulgaris. Suitable probes were identified for the analysis of P. vulgaris and P. coccineus hybrids. The segregation pattern in F2 populations was Mendelian for two probes (LHB and VEE20) and non-Mendelian for GS-g, CHS, and CHI. Statistical analyses indicated gametic selection with preferential transmission of the P. vulgaris alleles, which may account for the selective recovery of P. vulgaris progeny types observed earlier. The available hybrids of P. vulgaris and P. coccineus and the high degree of interspecific RFLP will facilitate the construction of a linkage map for Phaseolus.

Targeting of glutamine synthetase to the mitochondria of transgenic tobacco

Two transgenic tobacco lines were genetically engineered to contain chimaeric genes encoding the glutamine synthetase (GS) gamma polypeptide of Phaseolus vulgaris (French bean), expressed from the cauliflower mosaic virus 35S promoter. One (MIT-1) contained two copies of a construct including the first 60 amino acids of the Nicotiana plumbaginifolia beta-F1 ATPase to target the GS polypeptide to the mitochondrion. The other (CYT-4) contained a single copy of a cytosolic GS construct. Leaves of in vitro plantlets expressed the constructs and contained a novel GS polypeptide, which assembled into active GS isoenzymes constituting about 25% of the total GS activity. In in vitro plantlets of MIT-1, but not CYT-4, the novel polypeptide was found to be associated with the mitochondria. Moreover in MIT-1, the size of the novel polypeptide was not that predicted of the precursor (44.9 kDa) but was about 39 kDa, the same size as the authentic GS gamma polypeptide in CYT-4. These results are consistent with the precursor being imported into the mitochondria and cleaved near the fusion junction between the two sequences. These experiments have therefore shown that the presequence of the beta-F1 ATPase has successfully targeted the GS gamma polypeptide to the mitochondria of transgenic tobacco where it has assembled into an active isoenzyme. However, in fully regenerated plants growing photoautotrophically in growth-room conditions, although the constructs were still expressed, the gamma polypeptide did not accumulate to the same levels as in in vitro plantlets and new isoenzyme activities were now barely detectable. Moreover in leaves of the mature MIT-1 plants, the gamma polypeptide was found to be associated with the insoluble fraction of the mitochondria. The results of these experiments are discussed.

Expression of three plant glutamine synthetase cDNA in Escherichia coli. Formation of catalytically active isoenzymes, and complementation of a glnA mutant

Three cDNA clones encoding the closely related glutamine synthetase (GS) alpha, beta and gamma polypeptides of Phaseolus vulgaris (French bean) were recombinantly expressed in Escherichia coli. The GS expression plasmids correctly synthesised the recombinant alpha, beta and gamma polypeptides which then assembled into catalytically active homo-octameric isoenzymes. These isoenzymes behaved similarly to their native homologues on ion-exchange and gel-filtration chromatography. Furthermore, the alpha and gamma isoenzymes complemented a GS(glnA)-deficient mutant, thus demonstrating their physiological activity in E. coli. Differences were observed between the three recombinant GS plasmids in their quantitative expression of the GS polypeptides and their ability to complement the E. coli mutant. These differences were correlated to the degree of solubility of the polypeptide, which was observed to be dependent on the temperature of expression. The production of active GS isoenzymes in E. coli facilitates the isolation and characterisation of the individual P. vulgaris homo-octameric GS isoenzymes.

The ENOD12 gene product is involved in the infection process during the pea-Rhizobium interaction

The pea cDNA clone pPsENOD12 represents a gene involved in the infection process during Pisum sativum L.-Rhizobium leguminosarum bv. viciae symbiosis. The ENOD12 protein is composed of pentapeptides containing two hydroxyprolines. The expression of the ENOD12 gene is induced in cells through which the infection thread is migrating, but also in cells that do not yet contain an infection thread. Soluble compounds from Rhizobium are involved in eliciting ENOD12 gene expression. Rhizobium common and host-specific nodulation genes are essential for the production of these compounds. Two ENOD12 genes are expressed in nodules and in stem tissue of uninoculated plants. The gene represented by the cloned ENOD12 mRNA is also expressed in flowers, but a different transcription start may be used.

Chimeric genes and transgenic plants are used to study the regulation of genes involved in symbiotic plant-microbe interactions (nodulin genes)

Nodulin genes are plant genes specifically activated during the formation of nitrogen-fixing nodules on leguminous plants. These genes are interesting to study since they are not only induced in a specific developmental fashion by signals coming directly or indirectly from the rhizobial symbiont, but are also expressed in a tissue-specific manner. By examining the expression of chimeric nodulin-reporter genes in transgenic legume plants it has been shown that nodule specific expression is mediated by DNA sequences present in the 5 upstream region of several nodulin genes. Here we summarize the available data on these cis-acting elements and the trans-acting factors interacting with them. We also review experiments designed to identify rhizobial "signals" which may play a role in nodule specific gene expression.

Location of two isoenzymes of NADH-dependent glutamate synthase in root nodules of Phaseolus vulgaris L

The two isoenzymes of NADH-dependent glutamate synthase (NADH-GOGAT; EC 1.4.1.14), previously identified in root nodules of Phaseolus vulgaris L., have both been shown to be located in root-nodule plastids. The nodule specific NADH-GOGAT II accounts for the majority of the activity in root nodules, and is present almost exclusively in the central tissue of the nodule. However about 20% of NADH-GOGAT I activity is present in the nodule cortex, at about the same specific activity as this isoenzyme is found in the central tissue. Glutamine synthetase (GS; EC 6.3.1.2) occurs predominantly as the γ polypeptide in the central tissue, whereas in the cortex, the enzyme is represented mainly by the β polypeptide. Over 90% of both GS and NADH-GOGAT activities are located in the central tissue of the nodule and GS activity exceeds NADH-GOGAT activity by about twofold in this region. Using the above information, a model for the subcellular location and stoichiometry of nitrogen metabolism in the central tissue of P. vulgaris root nodules is presented.

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.