Purification, physical characterization, and NH2-terminal sequence of glutamine synthetase from the cyanobacterium Anabaena 7120

Calm on the surface, dynamic on the inside. Molecular homeostasis of Anabaena sp. PCC 7120 nitrogen metabolism

Nitrogen sources are all converted into ammonium/ia as a first step of assimilation. It is reasonable to expect that molecular components involved in the transport of ammonium/ia across biological membranes connect with the regulation of both nitrogen and central metabolism. We applied both genetic (i.e., Δamt mutation) and environmental treatments to a target biological system, the cyanobacterium Anabaena sp PCC 7120. The aim was to both perturb nitrogen metabolism and induce multiple inner nitrogen states, respectively, followed by targeted quantification of key proteins, metabolites and enzyme activities. The absence of AMT transporters triggered a substantial whole-system response, affecting enzyme activities and quantity of proteins and metabolites, spanning nitrogen and carbon metabolisms. Moreover, the Δamt strain displayed a molecular fingerprint indicating nitrogen deficiency even under nitrogen replete conditions. Contrasting with such dynamic adaptations was the striking near-complete lack of an externally measurable altered phenotype. We conclude that this species evolved a highly robust and adaptable molecular network to maintain homeostasis, resulting in substantial internal but minimal external perturbations. This analysis provides evidence for a potential role of AMT transporters in the regulatory/signalling network of nitrogen metabolism and the existence of a novel fourth regulatory mechanism controlling glutamine synthetase activity.

Coevolution of metal availability and nitrogen assimilation in cyanobacteria and algae

Marine primary producers adapted over eons to the changing chemistry of the oceans. Because a number of metalloenzymes are necessary for N assimilation, changes in the availability of transition metals posed a particular challenge to the supply of this critical nutrient that regulates marine biomass and productivity. Integrating recently developed geochemical, biochemical, and genetic evidence, we infer that the use of metals in N assimilation - particularly Fe and Mo - can be understood in terms of the history of metal availability through time. Anoxic, Fe-rich Archean oceans were conducive to the evolution of Fe-using enzymes that assimilate abiogenic NH(4)(+) and NO(2)(-). The N demands of an expanding biosphere were satisfied by the evolution of biological N(2) fixation, possibly utilizing only Fe. Trace O(2) in late Archean environments, and the eventual 'Great Oxidation Event' c. 2.3 Ga, mobilized metals such as Mo, enabling the evolution of Mo (or V)-based N(2) fixation and the Mo-dependent enzymes for NO(3)(-) assimilation and denitrification by prokaryotes. However, the subsequent onset of deep-sea euxinia, an increasingly-accepted idea, may have kept ocean Mo inventories low and depressed Fe, limiting the rate of N(2) fixation and the supply of fixed N. Eukaryotic ecosystems may have been particularly disadvantaged by N scarcity and the high Mo requirement of eukaryotic NO(3)(-) assimilation. Thorough ocean oxygenation in the Neoproterozoic led to Mo-rich oceans, possibly contributing to the proliferation of eukaryotes and thus the Cambrian explosion of metazoan life. These ideas can be tested by more intensive study of the metal requirements in N assimilation and the biological strategies for metal uptake, regulation, and storage.

PII Is Important in Regulation of Nitrogen Metabolism but Not Required for Heterocyst Formation in the Cyanobacterium Anabaena sp. PCC 7120

PII is an important signal protein for regulation of nitrogen metabolism in bacteria and plants. We constructed a mutant of glnB, encoding PII, in a heterocystous cyanobacterium, Anabaena sp. PCC 7120, with a cre-loxP system. The mutant (MP2alpha) grew more slowly than the wild type under all nitrogen regimens. It excreted a large amount of ammonium when grown on nitrate due to altered activities of glutamine synthetase and nitrate reductase. MP2alpha had a low nitrogenase activity but was able to form heterocysts under diazotrophic conditions, suggesting that PII is not required for heterocyst differentiation. Analysis of the PII with mass spectroscopy found tyrosine nitration at Tyr-51 under diazotrophic conditions while no phosphorylation at Ser-49 was detected. The strains 51F and 49A, which have PII with mutations of Y51F and S49A, respectively, were constructed to analyze the functions of the two key residues on the T-loop. Like MP2alpha, they had low nitrogenase activity and grew slowly under diazotrophic conditions. 49A was also impaired in nitrate uptake and formed heterocysts in the presence of nitrate. The up-regulation of ntcA after nitrogen step-down, which was present in the wild type, was not observed in 51F and 49A. While our results showed that the Ser-49 residue is important to the function of PII in Anabaena sp. PCC 7120, evidence from the PII pattern of the wild type and 49A in non-denaturing gel electrophoresis suggested that Ser-49 is not modified. The possible physiological roles of tyrosine nitration of PII are discussed.

Ammonium assimilation in cyanobacteria

In cyanobacteria, after transport by specific permeases, ammonium is incorporated into carbon skeletons by the sequential action of glutamine synthetase (GS) and glutamate synthase (GOGAT). Two types of GS (GSI and GSIII) and two types of GOGAT (ferredoxin-GOGAT and NADH-GOGAT) have been characterized in cyanobacteria. The carbon skeleton substrate of the GS-GOGAT pathway is 2-oxoglutarate that is synthesized by the isocitrate dehydrogenase (IDH). In order to maintain the C-N balance and the amino acid pools homeostasis, ammonium assimilation is tightly regulated. The key regulatory point is the GS, which is controlled at transcriptional and posttranscriptional levels. The transcription factor NtcA plays a critical role regulating the expression of the GS and the IDH encoding genes. In the unicellular cyanobacterium Synechocystis sp. PCC 6803, NtcA controls also the expression of two small proteins (IF7 and IF17) that inhibit the activity of GS by direct protein-protein interaction. Cyanobacteria perceive nitrogen status by sensing the intracellular concentration of 2-oxoglutarate, a signaling metabolite that is able to modulate allosterically the function of NtcA, in vitro. In vivo, a functional dependence between NtcA and the signal transduction protein PII in controlling NtcA-dependent genes has been also shown.

Glutamine synthetase from the marine cyanobacteria Prochlorococcus spp: characterization, phylogeny and response to nutrient limitation

The regulation of glutamine synthetase (EC 6.3.1.2) from Prochlorococcus was previously shown to exhibit unusual features: it is not upregulated by nitrogen starvation and it is not inactivated by darkness (El Alaoui et al. (2001) Appl Environ Microbiol 67: 2202-2207). These are probably caused by adaptations to oligotrophic environments, as confirmed in this work by the marked decrease in the enzymatic activity when cultures were subjected to iron or phosphorus starvation. In order to further understand the adaptive features of ammonium assimilation in this cyanobacterium, glutamine synthetase was purified from two Prochlorococcus strains: PCC 9511 (high-light adapted) and SS120 (low-light adapted). We obtained approximately 100-fold purified samples of glutamine synthetase electrophoretically homogeneous, with a yield of approximately 30%. The estimated molecular mass of the subunits was roughly the same for both strains: 48.3 kDa. The apparent Km constants for the biosynthetic activity were 0.30 mM for ammonium, 1.29 mM for glutamate and 1.35 mM for ATP; the optimum pH was 8.0. Optimal temperature was surprisingly high (55 degrees C). Phylogenetic analysis of glnA from three Prochlorococcus strains (MED4, MIT9313 and SS120) showed they group closely with marine Synechococcus isolates, in good agreement with other studies based on 16 S RNA sequences. All of our results suggest that the structure and kinetics of glutamine synthetase in Prochlorococcus have not been significantly modified during the evolution within the cyanobacterial radiation, in sharp contrast with its regulatory properties.

Mutational analysis of Asp51 of Anabaena azollae glutamine synthetase. D51E mutation confers resistance to the active site inhibitors L-methionine-DL-sulfoximine and phosphinothricin

The role of Asp51 in the catalytic activity of glutamine synthetase from the cyanobacterium Anabaena azollae has been analyzed. Five mutant enzymes, D51S, D51A, D51E, D51N and D51R, were constructed by site-directed mutagenesis and characterized. Asp51 appears to participate in the binding of ammonium ion, as affinity for this substrate was affected in all cases, although it varied according to the charge and/or size of the amino-acid residue, decreasing in the order Glu > Asn > Ser > Ala. The replacement of Asp51 by Glu (D51E) conferred besides a high resistance to the herbicides L-methionine-DL-sulfoximine and phosphinothricin, as a result of a decreased phosphorylation ability.

Nitrogen control of the glnN gene that codes for GS type III, the only glutamine synthetase in the cyanobacterium Pseudanabaena sp. PCC 6903

Pseudanabaena sp. strain PCC 6903 is the first cyanobacteria lacking the typical prokaryotic glutamine synthetase type I encoded by the glnA gene. The glnN gene product, glutamine synthetase type III, is the only glutamine synthetase activity present in this cyanobacterium. Analysis of glnN expression clearly indicated a nitrogen-dependent regulation. Pseudanabaena glnN gene expression and GSIII activity were upregulated under nitrogen starvation or using nitrate as a nitrogen source, while low levels of transcript and activity were found in ammonium-containing medium. Primer extension analysis showed that the glnN gene promoter structure resembled that of the NtcA-related promoters. Mobility shift assays demonstrated that Synechocystis sp. PCC 6803 NtcA protein, expressed and purified from Escherichia coli, bound to the promoter of the Pseudanabaena 6903 glnN gene. The NtcA control of the glnN gene in this cyanobacterium suggested that, in the absence of a glnA gene, NtcA took control of the only glutamine synthetase gene in a fashion similar to the way the glnA gene is governed in those cyanobacteria harbouring a glnA gene.

Purification and characterization of a new type of glutamine synthetase from cyanobacteria

The cyanobacterium Synechocystis sp. PCC 6803 contains two genes encoding two different types of glutamine synthetases (GS), glnA and glnN. The first codes for a typical prokaryotic GS type I and the second one codes for a GS type III, different in amino acid sequence to the prokaryotic GSI and the eukaryotic GSII. The glnN gene has been expressed in Escherichia coli and the corresponding protein purified almost to homogeneity (92%). The native enzyme (500 kDa) was composed of six identical subunits with an apparent molecular mass of 80 kDa. The protein was strongly stabilized in the presence of Mn2+ but not with other divalent cations. Biosynthetic activity of GSIII required the same substrates and cofactors as GSI and GSII enzymes. Apparent Km values for ATP, glutamate and ammonium were 0.43 mM, 0.9 mM and 0.19 mM, respectively. The enzyme was weakly inhibited by several amino acids and strongly inhibited by ADP. Synechocystis GSIII was also inhibited by L-methionine sulfoximine and DL-phosphinotricin, two transition-state analogs of the GS reaction mechanism. GSIII has also been purified from nitrogen-starved Synechocystis 6803 glnA mutant cells, demonstrating that the GS activity, strongly induced under nitrogen starvation in these cells, corresponds to the glnN gene product. In addition, a Synechocystis 6803 glnN mutant lacks the corresponding 80-kDa protein (GSIII). Polyclonal antibodies specific for GSIII cross-react with GSIII from other cyanobacteria. In all the strains analysed, levels of GSIII protein increased under nitrogen deficiency. These data suggest that GSIII is specifically required under conditions of nitrogen starvation.

Oxidative inactivation of glutamine synthetase from the cyanobacterium Anabaena variabilis

In crude extracts of the cyanobacterium Anabaena variabilis, glutamine synthetase (GS) could be effectively inactivated by the addition of NADH. GS inactivation was completed within 30 min. Both the inactivated GS and the active enzyme were isolated. No difference between the two enzyme forms was seen in sodium dodecyl sulfate-gels, and only minor differences were detectable by UV spectra, which excludes modification by a nucleotide. Mass spectrometry revealed that the molecular masses of active and inactive GS are equal. While the Km values of the substrates were unchanged, the Vmax values of the inactive GS were lower, reflecting the inactivation factor in the crude extract. This result indicates that the active site was affected. From the crude extract, a fraction mediating GS inactivation could be enriched by ammonium sulfate precipitation and gel filtration. GS inactivation by this fraction required the presence of NAD(P)H, Fe3+, and oxygen. In the absence of the GS-inactivating fraction, GS could be inactivated by Fe2+ and H2O2. The GS-inactivating fraction produced Fe2+ and H2O2, using NADPH, Fe3+, and oxygen. Accordingly, the inactivating fraction was inhibited by catalase and EDTA. This GS-inactivating system of Anabaena is similar to that described for oxidative GS inactivation in Escherichia coli. We conclude that GS inactivation by NAD(P)H is caused by irreversible oxidative damage and is not due to a regulatory mechanism of nitrogen assimilation.

A new type of glutamine synthetase in cyanobacteria: the protein encoded by the glnN gene supports nitrogen assimilation in Synechocystis sp. strain PCC 6803

A new glutamine synthetase gene, glnN, which encodes a polypeptide of 724 amino acid residues (M(r), 79,416), has been identified in the unicellular cyanobacterium Synechocystis sp. strain PCC 6803; this is the second gene that encodes a glutamine synthetase (GS) in this cyanobacterium. The functionality of this gene was evidenced by its ability to complement an Escherichia coli glnA mutant and to support Synechocystis growth in a strain whose glnA gene was inactivated by insertional mutagenesis. In this mutant (strain SJCR3), as well as in the wild-type strain, the second GS activity was subject to regulation by the nitrogen source, being strongly enhanced in nitrogen-free medium. Transcriptional fusion of a chloramphenicol acetyltransferase (cat) gene with the 5'-upstream region of glnN suggested that synthesis of the second Synechocystis GS is regulated at the transcriptional level. Furthermore, the level of glnN mRNA, a transcript of about 2,300 bases, was found to be strongly increased in nitrogen-free medium. The glnN product is similar to the GS subunits of Bacteroides fragilis and Butyrivibrio fibrisolvens, two obligate anaerobic bacteria whose GSs are markedly different from other prokaryotic and eukaryotic GSs. However, significant similarity is evident in the five regions which are homologous in all of the GSs so far described. The new GS gene was also found in other cyanobacteria but not in N2-fixing filamentous species.

Molecular characterization of the gene encoding glutamine synthetase in the cyanobacterium Calothrix sp. PCC 7601

In order to study the regulation of the synthesis of glutamine synthetase in response to changes in environmental parameters (light and nitrogen sources), we have cloned and sequenced the glnA gene from the filamentous cyanobacterium Calothrix PCC 7601. This gene consists of 472 codons and encodes a polypeptide of M(r) 52,290 highly homologous to that from Anabaena PCC 7120, but more distant from those identified from other procaryotes. The relative abundance of the two glnA transcripts (1.6 and 1.8 kb) is equivalent in cells grown under either red or green light, but the 1.6-kb species predominates in nitrate-grown cells and the 1.8-kb species in ammonia-grown cells. The very high identity (74%) observed between the 374-bp long nucleotide sequence upstream from the Calothrix and Anabaena glnA genes suggests the existence of similar regulatory signals for the control of glnA expression in both cyanobacteria.

Purification and properties of glutamine synthetases from the cyanobacteria Synechocystis sp. strain PCC 6803 and Calothrix sp. strain PCC 7601

Glutamine synthetases (GSs) from two cyanobacteria, one unicellular (Synechocystis sp. strain PCC 6803) and the other filamentous (Calothrix sp. strain PCC 7601 [Fremyella diplosiphon]), were purified to homogeneity. The biosynthetic activities of both enzymes were strongly inhibited by ADP, indicating that the energy charge of the cell might regulate the GS activity. Both cyanobacteria exhibited an ammonium-mediated repression of GS synthesis. In addition, the Synechocystis sp. showed an inactivation of GS promoted by ammonium that had not been demonstrated previously in cyanobacteria.

Purification and properties of glutamine synthetase from the non-N2-fixing cyanobacterium Phormidium laminosum

Soluble glutamine synthetase activity (L-glutamate:ammonia ligase, ADP forming, EC 6.3.1.2) was purified to electrophoretic homogeneity from the filamentous non-N2-fixing cyanobacterium Phormidium laminosum (OH-1-p.Cl1) by using conventional purification procedures in the absence of stabilizing ligands. The pure enzyme showed a specific activity of 152 mumol of gamma-glutamylhydroxamate formed.min-1 (transferase activity), which corresponded to 4.4 mumol of Pi released.min-1 (biosynthetic activity). The relative molecular mass of the native enzyme was 602 kilodaltons and was composed of 12 identically sized subunits of 52 kilodaltons. Biosynthetic activity required the presence of Mg2+ as an essential activator, although Co2+ and Zn2+ were partially effective. The kinetics of activation by Mg2+, Co2+, and Zn2+ were sigmoidal, and concentrations required for half-maximal activity were 18 mM (h = 2.2), 6.3 mM (h = 5.6), and 6.3 mM (h = 2.45), respectively. However, transferase activity required Mn2+ (Ka = 3.5 microM), Cu2+, Co2+, or Mg2+ being less effective. The substrate affinities calculated for L-Glu, ammonium, ATP, L-Gln, and hydroxylamine were 15, 0.4, 1.9 (h = 0.75), 14, and 4.1 mM, respectively. Optimal pH and temperature were 7.2 and 55 degrees C for biosynthetic activity and 7.5 and 45 degrees C for transferase activity. The biosynthetic reaction mechanism proceeded according to an ordered three-reactant system, the binding order being ammonium, L-Glu, and ATP. The presence of Mn2+ or Mg2+ drastically affected the thermostability of transferase and biosynthetic activities. Heat inactivation of biosynthetic activity in the presence of Mn2+ obeyed first-order kinetics, with an Ea of 76.8 kcal (ca. 321 kJ) mol-1. Gly, L-Asp, L-Ala, L-Ser and, with lower efficiency, L-Lys and L-Met, L-Lys, and L-Glu inhibited only transferase activity. No cumulative inhibition was observed when mixtures of amino acids were used. Biosynthetic activity was inhibited by AMP (Ki= 7 mM), ADP (Ki= 2.3 mM), p-hydroxymercuribenzoate (Ki= 25 microM), and L-methionine-D, L-sulfoximine (Ki= 2 microM). The enzyme was not activated in vitro by chemically reduced Anabaena thioredoxin. This is the first report of glutamine synthetase activity purified from a filamentous non-N2-fixing cyanobacterium.

Characterization of two early promoters of cyanophage N-1

Restriction fragments of cyanophage N-1 DNA, containing genes transcribed early in infection of Anabaena, were identified by hybridization with RNA prepared from infected cells. Two of these fragments were isolated by binding to Anabaena RNA polymerase on filters and were subsequently cloned. The start sites for transcription by Anabaena RNA polymerase were determined by run-off assays. The sequences of the two promoter regions thus localized were similar to each other in the -35 and -10 regions and nearly identical to the consensus sequence for promoters in Escherichia coli.

RNA polymerase subunit homology among cyanobacteria, other eubacteria and archaebacteria

RNA polymerase purified from vegetative cells of the cyanobacterium Anabaena sp. strain PCC 7120 contains a dissociable sigma factor and a core of five subunits: the beta', beta, and two alpha subunits characteristic of all eubacteria and an additional 66,000-molecular-weight polypeptide called gamma. Fifteen of fifteen strains of unicellular and filamentous cyanobacteria tested contained a serologically related gamma protein. Antiserum to gamma reacted with Escherichia coli beta' and the A subunit of RNA polymerase of the archaebacterium Sulfolobus acidocaldarius. Thus the evolution of the RNA polymerase beta' subunit has followed different paths in three groups of procaryotes: cyanobacteria, other eubacteria, and archaebacteria.

Glutamine synthetase specific activity and protein concentration in symbiotic Anabaena associated with Azolla caroliniana

Glutamine synthetase (GS) is the primary NH4+ assimilating enzyme of cyanobacteria. The specific activities and cellular protein concentration of GS in symbiotic cyanobacteria associated with the water fern Azolla caroliniana were determined and compared to free-living cultures of Nostoc sp. strain 7801, a strain originally isolated from symbiotic association with the bryophyte Anthoceros punctatus. Both the in vitro specific activity and concentration of GS in symbiotic cyanobacteria separated from A. caroliniana were approximately 3-fold lower than the free-living Nostoc sp. strain 7801 culture. These results imply depressed synthesis of GS by the symbiont associated with A. caroliniana.

Chick brain glutamine synthetase and Mn2+-Mg2+ interactions

Glutamine synthetase (GS) from the chick brain was purified to apparent homogeneity by ammonium sulfate fractionation followed by affinity chromatography, electrofocusing and Sephadex G-150 chromatography. The purified enzyme showed a single band on sodium dodecyl sulfate analysis in polyacrylamide gel. By sedimentation equilibrium analysis and gel electrophoresis analysis, it was shown that the enzyme has a subunit molecular weight of 45,000 and a native molecular weight of 364,000, which is consistent with an octameric structure. Sedimentation analysis in the presence of Mg2+ revealed three different forms of macromolecules corresponding respectively to a monomer, a tetramer and an octamer. Among eight cations tested (Ca2+, Co2+, Fe2+, Li+, Mg2+, Mn2+, Ni2+, Zn2+) only Co2+, Mg2+ and Mn2+ supported GS activity; the order of activatory ability was Mg2+ greater than Co2+ greater than Mn2+. The maximum activating effect of Mn2+ occurs only within a very narrow range of concentration: with an excess of cation causing strong inhibition of GS activity. For each cation, maximal GS activity occurs at a defined cation/ATP ratio. A regulatory system in which Mn2+, modulates the Mg2+ dependent GS activity, is proposed; such cation interactions may be of significance in the intracellular control of glutamine synthesis.

Regulation of expression of glutamine synthetase in a symbiotic Nostoc strain associated with Anthoceros punctatus

A characteristic of N2-fixing cyanobacteria in symbiotic associations appears to be release of N2-derived NH4+. The specific activity of the primary ammonium-assimilating enzyme, glutamine synthetase (GS), was found to be three- to fourfold lower in Nostoc sp. strain 7801 grown in symbiotic association with the bryophyte Anthoceros punctatus than in free-living Nostoc sp. strain 7801. Quantitative immunological assays with antisera against GS purified from Nostoc sp. strain 7801 and from Escherichia coli indicated that similar amounts of the GS protein were present in symbiotic (50 micrograms mg-1) and free-living (68 micrograms mg-1) cultures. The conclusion from these experiments is that GS is regulated by a posttranslational mechanism in Anthoceros-associated Nostoc sp. strain 7801. However, the results of comparative catalytic and immunological experiments between N2- and NH4+-grown free-living Nostoc sp. strain 7801 implied control of GS synthesis. A correlation was not observed between the level of GS expression and the extent of symbiotic heterocyst differentiation in Nostoc sp. strain 7801 associated with A. punctatus.

Glutamine synthetase: activity and localization in cyanobacteria of the cycadsCycas revoluta andZamia skinneri

Nitrogen-fixing cyanobacteria inhabit the zone between the inner and outer cortex of cycad coralloid roots. In the growing tip of such roots the cyanobacterial heterocyst frequency, nitrogenase activity (C2H2-reduction) and glutamine synthetase activity (both transferase and biosynthetic) were comparable to those found in freeliving cyanobacteria. The relative level of glutamine synthetase protein and its pattern of cellular/subcellular localization in heterocysts and vegetative cells were also similar to those of free-living cyanobacteria. However, there was a progressive decline in nitrogenase activity along the coralloid root with maximum reduction occurring in the regions farthest from the growing tip. A similar but less pronounced pattern was observed for glutamine synthetase activity. Distribution of glutamine synthetase protein in cyanobacteria in the first 2-3 mm of the root tip indicated a slight decrease in the heterocysts and vegetative cells. However, the overall level of cyanobacterial glutamine synthetase protein did not change because of a drastic increase in the numbers of heterocysts, which contain a proportionally higher level of glutamine synthetase than the vegetative cells.

Isolation and characterization of nitrogenase-derepressed mutant strains of cyanobacterium Anabaena variabilis

A positive selection method for isolation of nitrogenase-derepressed mutant strains of a filamentous cyanobacterium, Anabaena variabilis, is described. Mutant strains that are resistant to a glutamate analog, L-methionine-D,L-sulfoximine, were screened for their ability to produce and excrete NH4+ into medium. Mutant strains capable of producing nitrogenase in the presence of NH4+ were selected from a population of NH4+-excreting mutants. One of the mutant strains (SA-1) studied in detail was found to be a conditional glutamine auxotroph requiring glutamine for growth in media containing N2, NO3-, or low concentrations of NH4+ (less than 0.5 mM). This glutamine requirement is a consequence of a block in the assimilation of NH4+ produced by an enzyme system like nitrogenase. Glutamate and aspartate failed to substitute for glutamine because of a defect in the transport and utilization of these amino acids. Strain SA-1 assimilated NH4+ when the concentration in the medium reached about 0.5 mM, and under these conditions the growth rate was similar to that of the parent. Mutant strain SA-1 produced L-methionine-D,L-sulfoximine-resistant glutamine synthetase activity. Kinetic properties of the enzyme from the parent and mutant were similar. Mutant strain SA-1 can potentially serve as a source of fertilizer nitrogen to support growth of crop plants, since the NH4+ produced by nitrogenase, utilizing sunlight and water as sources of energy and reductant, respectively, is excreted into the environment.

Purification and characterization of glutamine synthetase from the archaebacterium Methanobacterium ivanovi

Glutamine synthetase (GS) was purified to electrophoretic homogeneity from the obligate anaerobic archaebacterium Methanobacterium ivanovi. The 130-fold-purified enzyme was obtained by heat treatment, ion-exchange chromatography, and gel filtration. Like all other eubacterial GSs known so far, the GS of M. ivanovi was found to be a dodecamer of about 600,000 daltons composed of a single type of subunit. The enzyme was stable at 63 degrees C for 10 min and was not sensitive to oxygen. The isoelectric point was 4.6, and the optimum pH of gamma-glutamyltransferase activity was 8.0. The Km values for hydroxylamine, glutamine, and ADP in the transferase reaction were 6.8, 22.7, and 0.35 mM, respectively. L-Methionine-DL-sulfoximine strongly inhibited the activity. Like the GS from gram-positive bacteria, Anabaena sp., several yeasts, and mammals, the enzyme from M. ivanovi was not regulated by adenylylation as demonstrated by snake venom phosphodiesterase treatment. Inhibition of the transferase activity by L-alanine, glycine, L-histidine, and L-tryptophan was observed. L-Glutamine alone or in the presence of AMP did not inhibit the GS synthetic activity. The GS of Methanobacterium ivanovi did not cross-react with a variety of antisera against GS from Escherichia coli, Anabaena strain 7120, or Bacillus megaterium. Archaebacterial GS appears to be structurally and functionally similar to eubacterial GS in gram-positive bacteria.

Primary structure of peptides from bovine brain glutamine synthetase. Comparison with sequences of glutamine synthetases from other organisms

An analysis of the covalent structure of bovine brain glutamine synthetase has been initiated. Cyanogen bromide and tryptic digests have yielded peptides accounting for most of the polypeptide subunit, and sequence analysis has placed in order over half of the amino acids within these peptides. The amino terminus is acetylated and has the following partial sequence: Ac(H, S3, A2, T)-L-B-K-G-I-K-Z-V-Y-M. The carboxyl-terminal sequence is: A-L-P-Q-G-D-K-V-Q-A-M. The peptides isolated from bovine glutamine synthetase show a high degree of homology with peptides isolated from ovine and porcine brain glutamine synthetases. In contrast to the sequence homologies of the proteins from eukaryotic sources, there are no obvious amino acid sequence homologies between bovine brain glutamine synthetase and any prokaryotic glutamine synthetase. Bovine brain glutamine synthetase is inactivated by phenylglyoxal and N-ethylmaleimide. In both cases catalytic activity is protected by the presence of ATP, suggesting the presence of arginine and cysteine residues at or near the ATP binding site.

Regulation of glutamine synthetase activity and synthesis in free-living and symbiotic Anabaena spp

Regulation of the synthesis and activity of glutamine synthetase (GS) in the cyanobacterium Anabaena sp. strain 7120 was studied by determining GS transferase activity and GS antigen concentration under a variety of conditions. Extracts prepared from cells growing exponentially on a medium supplemented with combined nitrogen had a GS activity of 17 mumol of gamma-glutamyl transferase activity per min per mg of protein at 37 degrees C. This activity doubled in 12 h after transfer of cells to a nitrogen-free medium, corresponding to the time required for heterocyst differentiation and the start of nitrogen fixation. Addition of NH3 to a culture 11 h after an inducing transfer immediately blocked the increase in GS activity. In the Enterobacteriaceae, addition of NH3 after induction results in the covalent modification of GS by adenylylation. The GS of Anabaena is not adenylylated by such a protocol, as shown by the resistance of the transferase activity of the enzyme to inhibition by Mg2+ and by the failure of the enzyme to incorporate 32P after NH3 upshift. Methionine sulfoximine inhibited Anabaena GS activity rapidly and irreversibly in vivo. After the addition of methionine sulfoximine to Anabaena, the level of GS antigen neither increased nor decreased, indicating that Glutamine cannot be the only small molecule capable of regulating GS synthesis. Methionine sulfoximine permitted heterocyst differentiation and nitrogenase induction to escape repression by NH3. Nitrogen-fixing cultures treated with methionine sulfoximine excreted NH3. The fern Azolla caroliniana contains an Anabaena species living in symbiotic association. The Anabaena species carries out nitrogen fixation sufficient to satisfy all of the combined nitrogen requirements of the host fern. Experiments by other workers have shown that the activity of GS in the symbiont is significantly lower than the activity of GS in free-living Anabaena. Using a sensitive radioimmune assay and a normalization procedure based on the content of diaminopimelic acid, a component unique to the symbiont, we found that the level of GS antigen in the symbiont was about 5% of the level in free-living Anabaena cells. Thus, the host fern appears to repress synthesis of Anabaena GS in the symbiotic association.