Glutamine metabolism and cycling in Neurospora crassa

ω-Amidase and Its Substrate α-Ketoglutaramate (the α-Keto Acid Analogue of Glutamine) as Biomarkers in Health and Disease

A large literature exists on the biochemistry, chemistry, metabolism, and clinical importance of the α-keto acid analogues of many amino acids. However, although glutamine is the most abundant amino acid in human tissues, and transamination of glutamine to its α-keto acid analogue (α-ketoglutaramate; KGM) was described more than seventy years ago, little information is available on the biological importance of KGM. Herein, we summarize the metabolic importance of KGM as an intermediate in the glutamine transaminase - ω-amidase (GTωA) pathway for the conversion of glutamine to anaplerotic α-ketoglutarate. We describe some properties of KGM, notably its occurrence as a lactam (2-hydroxy-5-oxoproline; 99.7% at pH 7.2), and its presence in normal tissues and body fluids. We note that the concentration of KGM is elevated in the cerebrospinal fluid of liver disease patients and that the urinary KGM/creatinine ratio is elevated in patients with an inborn error of the urea cycle and in patients with citrin deficiency. Recently, of the 607 urinary metabolites measured in a kidney disease study, KGM was noted to be one of five metabolites that was most significantly associated with uromodulin (a potential biomarker for tubular functional mass). Finally, we note that KGM is an intermediate in the breakdown of nicotine in certain organisms and is an important factor in nitrogen homeostasis in some microorganisms and plants. In conclusion, we suggest that biochemists and clinicians should consider KGM as (i) a key intermediate in nitrogen metabolism in all branches of life, and (ii) a biomarker, along with ω-amidase, in several diseases.

Assembly status transition offers an avenue for activity modulation of a supramolecular enzyme

Nature has evolved many supramolecular proteins assembled in certain, sometimes even seemingly oversophisticated, morphological manners. The rationale behind such evolutionary efforts is often poorly understood. Here, we provide atomic-resolution insights into how the dynamic building of a structurally complex enzyme with higher order symmetry offers amenability to intricate regulation. We have established the functional coupling between enzymatic activity and protein morphological states of glutamine synthetase (GS), an old multi-subunit enzyme essential for cellular nitrogen metabolism. Cryo-EM structure determination of GS in both the catalytically active and inactive assembly states allows us to reveal an unanticipated self-assembly-induced disorder-order transition paradigm, in which the remote interactions between two subcomplex entities significantly rigidify the otherwise structurally fluctuating active sites, thereby regulating activity. We further show in vivo evidences that how the enzyme morphology transitions could be modulated by cellular factors on demand. Collectively, our data present an example of how assembly status transition offers an avenue for activity modulation, and sharpens our mechanistic understanding of the complex functional and regulatory properties of supramolecular enzymes.

How to enter the state of dormancy? A suggestion by Trichoderma atroviride conidia

It is generally accepted that conidia, propagules of filamentous fungi, exist in the state of dormancy. This state is defined mostly phenomenologically, e.g., by germination requirements. Its molecular characteristics are scarce and are concentrated on the water or osmolyte content, and/or respiration. However, a question of whether conidia are metabolic or ametabolic forms of life cannot be answered on the basis of available experimental data. In other words, are mature conidia open thermodynamic systems as are mycelia, or do they become closed upon the transition to the dormant state? In this article, we present observations which may help to define the transition of freshly formed conidia to the putative dormant forms using measurements of selected enzyme activities, ¹H- and ¹³C-NMR and LC-MS-metabolomes, and ¹⁴C-bicarbonate or ⁴⁵Ca²⁺ inward transport. We have found that Trichoderma atroviride and Aspergillus niger conidia arrest the ⁴⁵Ca²⁺ uptake during the development stopping thereby the cyclic (i.e., bidirectional) Ca²⁺ flow existing in vegetative mycelia and conidia of T. atroviride across the cytoplasmic membrane. Furthermore, we have found that the activity of α-ketoglutarate dehydrogenase was rendered completely inactive after 3 weeks from the conidia formation unlike of other central carbon metabolism enzymes. This may explain the loss of conidial respiration. Finally, we found that conidia take up the H¹⁴CO₃^- and convert it into few stable compounds within 80 d of maturation, with minor quantitative differences in the extent of this process. The uptake of H¹³CO₃^- confirmed these observation and demonstrated the incorporation of H¹³CO₃^- even in the absence of exogenous substrates. These results suggest that T. atroviride conidia remain metabolically active during first ten weeks of maturation. Under these circumstances, their metabolism displays features similar to those of chemoautotrophic microorganisms. However, the Ca²⁺ homeostasis changed from the open to the closed thermodynamic state during the early period of conidial maturation. These results may be helpful for studying the conidial ageing and/or maturation, and for defining the conidial dormant state in biochemical terms.

MoGLN2 Is Important for Vegetative Growth, Conidiogenesis, Maintenance of Cell Wall Integrity and Pathogenesis of Magnaporthe oryzae

Glutamine is a non-essential amino acid that acts as a principal source of nitrogen and nucleic acid biosynthesis in living organisms. In Saccharomyces cerevisiae, glutamine synthetase catalyzes the synthesis of glutamine. To determine the role of glutamine synthetase in the development and pathogenicity of plant fungal pathogens, we used S. cerevisiae Gln1 amino acid sequence to identify its orthologs in Magnaporthe oryzae and named them MoGln1, MoGln2, and MoGln3. Deletion of MoGLN1 and MoGLN3 showed that they are not involved in the development and pathogenesis of M. oryzae. Conversely, ΔMogln2 was reduced in vegetative growth, experienced attenuated growth on Minimal Medium (MM), and exhibited hyphal autolysis on oatmeal and straw decoction and corn media. Exogenous l-glutamine rescued the growth of ΔMogln2 on MM. The ΔMogln2 mutant failed to produce spores and was nonpathogenic on barley leaves, as it was unable to form an appressorium-like structure from its hyphal tips. Furthermore, deletion of MoGLN2 altered the fungal cell wall integrity, with the ΔMogln2 mutant being hypersensitive to H2O2. MoGln1, MoGln2, and MoGln3 are located in the cytoplasm. Taken together, our results shows that MoGLN2 is important for vegetative growth, conidiation, appressorium formation, maintenance of cell wall integrity, oxidative stress tolerance and pathogenesis of M. oryzae.

Study on the dynamic changes and formation pathways of metabolites during the fermentation of black waxy rice wine

Black waxy rice wine fermentation metabolites are closely related to the product's final quality. However, little is known about dynamic metabolite changes during fermentation. Here, we used gas chromatography time-of-flight mass spectrometry (GC-TOF-MS) metabolomics and multivariate statistical analysis to explore the relationship between metabolites and fermentation time. A total of 159 metabolites were identified during the entire fermentation process. The PCA analysis revealed a clear separation between the samples after 4 days and 2 days, and the samples after 4-24 days clustered together. This indicated that BGRW fermentation progresses rapidly in the first 48 hr of fermentation. A total of 40 metabolites were identified as differential during fermentation (VIP > 1 and p < .05), including 12 organic acids, four amino acids, one fatty acid, 17 sugars and sugar alcohols, one alcohol, and five other metabolites. Pathway analysis showed that the differential metabolites were involved in 28 metabolic pathways, and the most commonly influenced pathways (impact value > 0.1 and p < .05) were galactose metabolism, pyruvate metabolism; starch and sucrose metabolism; alanine, aspartic acid, and glutamate metabolism; the tricarboxylic acid cycle, glyoxylic acid, and dicarboxylic acid metabolism; and amino sugar and nucleotide sugar metabolism. Moreover, the integrated metabolic pathway was generated to understand the transformation and accumulation of differential metabolites. Overall, these results provide a comprehensive overview of metabolite changes during black waxy rice wine fermentation.

Glutamate synthase MoGlt1-mediated glutamate homeostasis is important for autophagy, virulence and conidiation in the rice blast fungus

Glutamate homeostasis plays a vital role in central nitrogen metabolism and coordinates several key metabolic functions. However, its function in fungal pathogenesis and development has not been investigated in detail. In this study, we identified and characterized a glutamate synthase gene MoGLT1 in the rice blast fungus Magnaporthe oryzae that was important to glutamate homeostasis. MoGLT1 was constitutively expressed, but showed the highest expression level in appressoria. Deletion of MoGLT1 resulted in a significant reduction in conidiation and virulence. The ΔMoglt1 mutants were defective in appressorial penetration and the differentiation and spread of invasive hyphae in penetrated plant cells. The addition of exogenous glutamic acid partially rescued the defects of the ΔMoglt1 mutants in conidiation and plant infection. Assays for MoAtg8 expression and localization showed that the ΔMoglt1 mutants were defective in autophagy. The ΔMoglt1 mutants were delayed in the mobilization of glycogens and lipid bodies from conidia to developing appressoria. Taken together, our results show that glutamate synthase MoGlt1-mediated glutamate homeostasis is important for pathogenesis and development in the rice blast fungus, possibly via the regulation of autophagy.

The Genomes of Three Uneven Siblings: Footprints of the Lifestyles of Three Trichoderma Species

The genus Trichoderma contains fungi with high relevance for humans, with applications in enzyme production for plant cell wall degradation and use in biocontrol. Here, we provide a broad, comprehensive overview of the genomic content of these species for "hot topic" research aspects, including CAZymes, transport, transcription factors, and development, along with a detailed analysis and annotation of less-studied topics, such as signal transduction, genome integrity, chromatin, photobiology, or lipid, sulfur, and nitrogen metabolism in T. reesei, T. atroviride, and T. virens, and we open up new perspectives to those topics discussed previously. In total, we covered more than 2,000 of the predicted 9,000 to 11,000 genes of each Trichoderma species discussed, which is >20% of the respective gene content. Additionally, we considered available transcriptome data for the annotated genes. Highlights of our analyses include overall carbohydrate cleavage preferences due to the different genomic contents and regulation of the respective genes. We found light regulation of many sulfur metabolic genes. Additionally, a new Golgi 1,2-mannosidase likely involved in N-linked glycosylation was detected, as were indications for the ability of Trichoderma spp. to generate hybrid galactose-containing N-linked glycans. The genomic inventory of effector proteins revealed numerous compounds unique to Trichoderma, and these warrant further investigation. We found interesting expansions in the Trichoderma genus in several signaling pathways, such as G-protein-coupled receptors, RAS GTPases, and casein kinases. A particularly interesting feature absolutely unique to T. atroviride is the duplication of the alternative sulfur amino acid synthesis pathway.

GATA-Dependent Glutaminolysis Drives Appressorium Formation in Magnaporthe oryzae by Suppressing TOR Inhibition of cAMP/PKA Signaling

Fungal plant pathogens are persistent and global food security threats. To invade their hosts they often form highly specialized infection structures, known as appressoria. The cAMP/ PKA- and MAP kinase-signaling cascades have been functionally delineated as positive-acting pathways required for appressorium development. Negative-acting regulatory pathways that block appressorial development are not known. Here, we present the first detailed evidence that the conserved Target of Rapamycin (TOR) signaling pathway is a powerful inhibitor of appressorium formation by the rice blast fungus Magnaporthe oryzae. We determined TOR signaling was activated in an M. oryzae mutant strain lacking a functional copy of the GATA transcription factor-encoding gene ASD4. Δasd4 mutant strains could not form appressoria and expressed GLN1, a glutamine synthetase-encoding orthologue silenced in wild type. Inappropriate expression of GLN1 increased the intracellular steady-state levels of glutamine in Δasd4 mutant strains during axenic growth when compared to wild type. Deleting GLN1 lowered glutamine levels and promoted appressorium formation by Δasd4 strains. Furthermore, glutamine is an agonist of TOR. Treating Δasd4 mutant strains with the specific TOR kinase inhibitor rapamycin restored appressorium development. Rapamycin was also shown to induce appressorium formation by wild type and Δcpka mutant strains on non-inductive hydrophilic surfaces but had no effect on the MAP kinase mutant Δpmk1. When taken together, we implicate Asd4 in regulating intracellular glutamine levels in order to modulate TOR inhibition of appressorium formation downstream of cPKA. This study thus provides novel insight into the metabolic mechanisms that underpin the highly regulated process of appressorium development.

Many fungal pathogens destroy important crops by first gaining entrance to the host using specialized appressorial cells. Understanding the molecular mechanisms that control appressorium formation could provide new routes for managing severe plant diseases. Here, we describe a previously unknown regulatory pathway that suppresses appressorium formation by the rice pathogen Magnaporthe oryzae. We provide evidence that a mutant M. oryzae strain, unable to form appressoria, accumulates intracellular glutamine that, in turn, inappropriately activates a conserved signaling pathway called TOR. Reducing intracellular glutamine levels, or inactivating TOR, restored appressorium formation to the mutant strain. TOR activation is thus a powerful inhibitor of appressorium formation and could be leveraged to develop sustainable mitigation practices against recalcitrant fungal pathogens.

Nitrogen regulation of fungal secondary metabolism in fungi

Fungi occupy diverse environments where they are constantly challenged by stressors such as extreme pH, temperature, UV exposure, and nutrient deprivation. Nitrogen is an essential requirement for growth, and the ability to metabolize a wide variety of nitrogen sources enables fungi to colonize different environmental niches and survive nutrient limitations. Favored nitrogen sources, particularly ammonium and glutamine, are used preferentially, while the expression of genes required for the use of various secondary nitrogen sources is subject to a regulatory mechanism called nitrogen metabolite repression. Studies on gene regulation in response to nitrogen availability were carried out first in Saccharomyces cerevisiae, Aspergillus nidulans, and Neurospora crassa. These studies revealed that fungi respond to changes in nitrogen availability with physiological and morphological alterations and activation of differentiation processes. In all fungal species studied, the major GATA transcription factor AreA and its co-repressor Nmr are central players of the nitrogen regulatory network. In addition to growth and development, the quality and quantity of nitrogen also affects the formation of a broad range of secondary metabolites (SMs). Recent studies, mainly on species of the genus Fusarium, revealed that AreA does not only regulate a large set of nitrogen catabolic genes, but can also be involved in regulating production of SMs. Furthermore, several other regulators, e.g., a second GATA transcription factor, AreB, that was proposed to negatively control nitrogen catabolic genes by competing with AreA for binding to GATA elements, was shown to act as activator of some nitrogen-repressed as well as nitrogen-induced SM gene clusters. This review highlights our latest understanding of canonical (AreA-dependent) and non-canonical nitrogen regulation mechanisms by which fungi may regulate biosynthesis of certain SMs in response to nitrogen availability.

Identification and characterization of omega-amidase as an enzyme metabolically linked to asparagine transamination in Arabidopsis

In higher plants, asparagine (Asn) is a major form of organic nitrogen used for transport and storage. There are two pathways of Asn metabolism, involving asparaginase and Asn aminotransferase. The enzyme serine:glyoxylate aminotransferase encoded by AGT1 has been identified as an asparagine aminotransferase in Arabidopsis. The product of asparagine transamination, alpha-ketosuccinamate, can be hydrolyzed by the enzyme omega-amidase to form oxaloacetate and ammonia. A candidate gene was identified in Arabidopsis based on its sequence similarity with mouse omega-amidase. Recombinant omega-amidase exhibited comparable catalytic activities with alpha-hydroxysuccinamate, alpha-ketosuccinamate and alpha-ketoglutaramate, the product of glutamine transamination. A mutant with a T-DNA inserted in the first exon accumulated alpha-ketosuccinamate and alpha-hydroxysuccinamate as compared with wild-type, both under control conditions and after treatment with Asn. Treatment with Asn led to decreased transcript levels of omega-amidase in root, while transcript levels of AGT1 are increased under these conditions, suggesting that excess Asn may lead to the accumulation of alpha-ketosuccinamate and alpha-hydroxysuccinamate.

The logic of metabolism and its fuzzy consequences

Intermediary metabolism molecules are orchestrated into logical pathways stemming from history (L-amino acids, D-sugars) and dynamic constraints (hydrolysis of pyrophosphate or amide groups is the driving force of anabolism). Beside essential metabolites, numerous variants derive from programmed or accidental changes. Broken down, variants enter standard pathways, producing further variants. Macromolecule modification alters enzyme reactions specificity. Metabolism conform thermodynamic laws, precluding strict accuracy. Hence, for each regular pathway, a wealth of variants inputs and produces metabolites that are similar to but not the exact replicas of core metabolites. As corollary, a shadow, paralogous metabolism, is associated to standard metabolism. We focus on a logic of paralogous metabolism based on diversion of the core metabolic mimics into pathways where they are modified to minimize their input in the core pathways where they create havoc. We propose that a significant proportion of paralogues of well-characterized enzymes have evolved as the natural way to cope with paralogous metabolites. A second type of denouement uses a process where protecting/deprotecting unwanted metabolites - conceptually similar to the procedure used in the laboratory of an organic chemist - is used to enter a completely new catabolic pathway.

Reconstruction and validation of a genome-scale metabolic model for the filamentous fungus Neurospora crassa using FARM

The filamentous fungus Neurospora crassa played a central role in the development of twentieth-century genetics, biochemistry and molecular biology, and continues to serve as a model organism for eukaryotic biology. Here, we have reconstructed a genome-scale model of its metabolism. This model consists of 836 metabolic genes, 257 pathways, 6 cellular compartments, and is supported by extensive manual curation of 491 literature citations. To aid our reconstruction, we developed three optimization-based algorithms, which together comprise Fast Automated Reconstruction of Metabolism (FARM). These algorithms are: LInear MEtabolite Dilution Flux Balance Analysis (limed-FBA), which predicts flux while linearly accounting for metabolite dilution; One-step functional Pruning (OnePrune), which removes blocked reactions with a single compact linear program; and Consistent Reproduction Of growth/no-growth Phenotype (CROP), which reconciles differences between in silico and experimental gene essentiality faster than previous approaches. Against an independent test set of more than 300 essential/non-essential genes that were not used to train the model, the model displays 93% sensitivity and specificity. We also used the model to simulate the biochemical genetics experiments originally performed on Neurospora by comprehensively predicting nutrient rescue of essential genes and synthetic lethal interactions, and we provide detailed pathway-based mechanistic explanations of our predictions. Our model provides a reliable computational framework for the integration and interpretation of ongoing experimental efforts in Neurospora, and we anticipate that our methods will substantially reduce the manual effort required to develop high-quality genome-scale metabolic models for other organisms.

Few organisms have been as foundational to the development of modern genetics and cellular metabolism as Neurospora crassa. Given the wealth of knowledge available for this filamentous fungus, the effort required to manually curate a high-quality genome-scale metabolic reconstruction would be daunting. To aid the reconstruction process, we developed three optimization-based algorithms. The first algorithm predicts flux while linearly accounting for metabolite dilution; the second algorithm removes blocked reactions with one compact linear program; and the third algorithm reconciles differences between in silico predictions and experimental observations of mutant viability. We have used these algorithms to develop the first genome-scale metabolic model for Neurospora. We have validated the accuracy of our model against an independent test set of more than 300 growth/no-growth phenotypes, and our model displays 93% sensitivity and specificity. Simulating the biochemical genetics experiments originally performed on Neurospora, we comprehensively predicted essential genes, nutrient rescues of auxotroph mutants and synthetic lethal interactions. With these predictions, we provide potential mechanistic insight into known mutant phenotypes, and testable hypotheses for novel mutant phenotypes. The model, the algorithms and the testable hypotheses provide a computational foundation for the study of Neurospora crassa metabolism.

Glutathione is required by Rhizobium etli for glutamine utilization and symbiotic effectiveness

Here, we provide genetic and biochemical evidence indicating that the ability of Rhizobium etli bacteria to efficiently catabolize glutamine depends on its ability to produce reduced glutathione (l-γ-glutamyl-l-cysteinylglycine [GSH]). We find that GSH-deficient strains, namely a gshB (GSH synthetase) and a gor (GSH reductase) mutant, can use different amino acids, including histidine, alanine, and asparagine but not glutamine, as sole source of carbon, energy, and nitrogen. Moreover, l-buthionine(S,R)-sulfoximine, a GSH synthesis inhibitor, or diamide that oxidizes GSH, induced the same phenotype in the wild-type strain. Among the steps required for its utilization, glutamine uptake, occurring through the two well-characterized carriers (Aap and Bra systems) but not glutamine degradation or respiration, was largely reduced in GSH-deficient strains. Furthermore, GSH-deficient mutants of R. etli showed a reduced symbiotic efficiency. Exogenous GSH was sufficient to rescue glutamine uptake or degradation ability, as well as the symbiotic effectiveness of GSH mutants. Our results suggest a previously unknown GSH-glutamine metabolic relationship in bacteria.

Identification of the putative tumor suppressor Nit2 as omega-amidase, an enzyme metabolically linked to glutamine and asparagine transamination

The present report identifies the enzymatic substrates of a member of the mammalian nitrilase-like (Nit) family. Nit2, which is widely distributed in nature, has been suggested to be a tumor suppressor protein. The protein was assumed to be an amidase based on sequence homology to other amidases and on the presence of a putative amidase-like active site. This assumption was recently confirmed by the publication of the crystal structure of mouse Nit2. However, the in vivo substrates were not previously identified. Here we report that rat liver Nit2 is omega-amidodicarboxylate amidohydrolase (E.C. 3.5.1.3; abbreviated omega-amidase), a ubiquitously expressed enzyme that catalyzes a variety of amidase, transamidase, esterase and transesterification reactions. The in vivo amidase substrates are alpha-ketoglutaramate and alpha-ketosuccinamate, generated by transamination of glutamine and asparagine, respectively. Glutamine transaminases serve to salvage a number of alpha-keto acids generated through non-specific transamination reactions (particularly those of the essential amino acids). Asparagine transamination appears to be useful in mitochondrial metabolism and in photorespiration. Glutamine transaminases play a particularly important role in transaminating alpha-keto-gamma-methiolbutyrate, a key component of the methionine salvage pathway. Some evidence suggests that excess alpha-ketoglutaramate may be neurotoxic. Moreover, alpha-ketosuccinamate is unstable and is readily converted to a number of hetero-aromatic compounds that may be toxic. Thus, an important role of omega-amidase is to remove potentially toxic intermediates by converting alpha-ketoglutaramate and alpha-ketosuccinamate to biologically useful alpha-ketoglutarate and oxaloacetate, respectively. Despite its importance in nitrogen and sulfur metabolism, the biochemical significance of omega-amidase has been largely overlooked. Our report may provide clues regarding the nature of the biological amidase substrate(s) of Nit1 (another member of the Nit family), which is a well-established tumor suppressor protein), and emphasizes a) the crucial role of Nit2 in nitrogen and sulfur metabolism, and b) the possible link of Nit2 to cancer biology.

Assay and purification of omega-amidase/Nit2, a ubiquitously expressed putative tumor suppressor, that catalyzes the deamidation of the alpha-keto acid analogues of glutamine and asparagine

omega-Amidase (omega-amidodicarboxylate amidohydrolase, EC 3.5.1.3) isolated from rat liver cytosol is a versatile enzyme that catalyzes a large number of amidase, transamidase, and ester hydrolysis reactions. omega-Amidase activity toward alpha-ketoglutaramate and alpha-ketosuccinamate (the alpha-keto acid analogues of glutamine and asparagine, respectively) is present in mammalian tissues, tumors, plants, bacteria, and fungi. Despite its versatility, widespread occurrence, and high specific activity, the enzyme has been little studied, possibly because the assay procedure previously required a substrate (alpha-ketoglutaramate) that is not commercially available. Here we report a simplified method for preparing alpha-ketoglutaramate and an assay procedure that measures alpha-ketoglutarate formation from alpha-ketoglutaramate in a 96-well plate format. We also describe a 96-well plate assay procedure that measures omega-amidase-catalyzed hydroxaminolysis of commercially available succinamic acid. The product, succinyl hydroxamate, yields a stable brown color in the presence of acidic ferric chloride that can be quantitated spectrophotometrically with negligible background interference. The two assay procedures (hydrolysis of alpha-ketoglutaramate and hydroxaminolysis of succinamate) were employed in purifying omega-amidase approximately 3600-fold from rat liver cytosol. The ratio of alpha-ketoglutaramate hydrolysis to succinamate hydroxaminolysis remained constant during the purification. omega-Amidase has recently been shown to be identical to Nit2, a putative tumor suppressor protein. It is anticipated that these new assay procedures will help to characterize the function of omega-amidase/Nit2 in tumor suppression, will provide the basis of high-throughput procedures to search for potent inhibitors and enhancers of omega-amidase, and will assist in identifying biological interactions between nitrogen metabolism and tumor biology.

Allosteric NADP-glutamate dehydrogenase from aspergilli: purification, characterization and implications for metabolic regulation at the carbon-nitrogen interface

NADP-dependent glutamate dehydrogenase (NADP-GDH) mediates fungal ammonium assimilation through reductive synthesis of glutamate from 2-oxoglutarate. By virtue of its position at the interface of carbon and nitrogen metabolism, biosynthetic NADP-GDH is a potential candidate for metabolic control. In order to facilitate characterization, a new and effective dye-affinity method was devised to purify NADP-GDH from two aspergilli, Aspergillus niger and Aspergillus nidulans. The A. niger NADP-GDH was characterized at length and its kinetic interaction constants with glutamate (Km 34.7 mM) and ammonium (Km 1.05 mM; Ki 0.4 mM) were consistent with an anabolic role. Isophthalate, 2-methyleneglutarate and 2,4-pyridinedicarboxylate were significant inhibitors, with respective Ki values of 6.9, 9.2 and 202.0 microM. The A. niger enzyme showed allosteric properties and a sigmoid response (nH=2.5) towards 2-oxoglutarate saturation. The co-operative behaviour was a feature common to NADP-GDH from Aspergillus awamori, A. nidulans and Aspergillus oryzae. NADP-GDH may therefore be a crucial determinant in adjusting 2-oxoglutarate flux between the tricarboxylic acid cycle and glutamate biosynthesis in aspergilli.

Suillus bovinus glutamine synthetase gene organization, transcription and enzyme activities in the Scots pine mycorrhizosphere developed on forest humus

•  Glutamine synthetase (GS) expression and activity is of central importance for cellular ammonium assimilation and recycling. Thus, a full characterization of this enzyme at the molecular level is of critical importance for a better understanding of nitrogen (N) assimilation in the mycorrhizal symbiosis. •  Genomic and cDNA libraries of Suillus bovinus were constructed to isolate the GS gene, glnA, and corresponding cDNAs. The transcription initiation site was identified and transcription and enzyme activities were characterized in pure culture mycelium and mycorrhiza, and extramatrical mycelium samples harvested from Scots pine-Suillus bovinus microcosms grown on forest humus. •  Pure culture mycelium, mycorrhiza and extramatrical mycelium all exhibited equivalent levels of GS transcription, translation and enzyme activities. However, levels of transcription and enzyme activity did not correlate as a large majority of detectable transcripts showed specific 5'-end truncation. •  Our data suggest that GS is constitutively expressed and not directly affected by environmental conditions of the symbiotic N uptake. Any changes in the intracellular ammonium level are most likely handled by regulatory flexibility of GS at enzyme level.

Glutamine utilization by Rhizobium etli

We undertook the study of the use of glutamine (Gln) as the source of carbon and energy by Rhizobium etli. Tn5-induced mutagenesis allowed us to identify several genes required for Gln utilization, including those coding for two broad-range amino acid transporters and a glutamate dehydrogenase. The isolated mutants were characterized by the analysis of their capacity i) to grow on different media, ii) to transport Gln (uptake assays), and iii) to utilize Gln as the C energy source (CO2 production from Gln). We show that Gln is degraded through the citric acid cycle and that its utilization as the sole C source is related to a change in the bacterial cell shape (from bacillary to coccoid form) and a high susceptibility to a thiol oxidative insult. Both these data and the analysis of ntr-dependent promoters suggested that Gln-grown bacteria are under a condition of C starvation and N sufficiency, and as expected, the addition of glucose counteracted the morphological change and increased both the bacterial growth rate and their resistance to oxidative stress. Finally, a nodulation analysis indicates that the genes involved in Gln transport and degradation are dispensable for the bacterial ability to induce and invade developing nodules, whereas those involved in gluconeogenesis and nucleotide biosynthesis are strictly required.

Role of glutamine synthetase in nitrogen metabolite repression in Aspergillus nidulans

Glutamine synthetase (GS), EC 6.3.1.2, is a central enzyme in the assimilation of nitrogen and the biosynthesis of glutamine. We have isolated the Aspergillus nidulans glnA gene encoding GS and have shown that glnA encodes a highly expressed but not highly regulated mRNA. Inactivation of glnA results in an absolute glutamine requirement, indicating that GS is responsible for the synthesis of this essential amino acid. Even when supplemented with high levels of glutamine, strains lacking a functional glnA gene have an inhibited morphology, and a wide range of compounds have been shown to interfere with repair of the glutamine auxotrophy. Heterologous expression of the prokaryotic Anabaena glnA gene from the A. nidulans alcA promoter allowed full complementation of the A. nidulans glnADelta mutation. However, the A. nidulans fluG gene, which encodes a protein with similarity to prokaryotic GS, did not replace A. nidulans glnA function when similarly expressed. Our studies with the glnADelta mutant confirm that glutamine, and not GS, is the key effector of nitrogen metabolite repression. Additionally, ammonium and its immediate product glutamate may also act directly to signal nitrogen sufficiency.

Regulation of glutamine synthetase from the white button mushroom Agaricus bisporus

The regulation of glutamine synthetase (GS) from Agaricus bisporus was studied at the posttranscriptional level using a specific antibody fraction directed against purified GS. The cross-reactivity of the antiserum against various Agaricus species and other fungi was tested and low reactivity with the Ascomycetes was found. GS protein and activity levels were measured in cell-free extracts of mycelium grown on different N sources. In mycelium grown on glutamine or ammonium as N source, the biosynthetic GS activity is higher than the transferase activity. Moreover, the results show a correlation between GS biosynthetic activity, GS protein, and previously reported mRNA levels. Also, after addition of ammonium or glutamine to glutamate-utilizing cultures, transferase activity decreased more rapidly than biosynthetic activity and GS protein level. This suggests a conformational modification which only affects transferase activity.

Glutamine biosynthesis and the utilization of succinate and glutamine by Rhizobium etli and Sinorhizobium meliloti

Sinorhizobium meliloti 1021 and Rhizobium etli CE3 turn over nitrogen and carbon from glutamine to ammonium and CO2, respectively. Some of the ammonium released is assimilated back into glutamine, indicating that a glutamine cycle similar to that in Neurospora operates in Rhizobium. In addition, a previously unrecognized metabolic pathway in Rhizobium was discovered--namely, conversion of glutamine-carbon to gamma-hydroxybutyric acid and beta-hydroxybutyric acid. Additionally, some of the 2-oxoglutarate derived from glutamine catabolism in Rhizobium is converted to succinate in glutamine-containing medium. Both S. meliloti 1021 and R. etli CE3 oxidize succinate preferentially over glutamine when provided with both carbon sources. In contrast to Sinorhizobium meliloti 1021 and Rhizobium etli CE3, an S. meliloti double mutant that lacks both glutamine synthetase (GS) I and II preferentially oxidizes glutamine over succinate when supplied with both substrates. GSII activity is induced in wild-type S. meliloti 1021 and R. etli CE3 grown in succinate-glutamine medium, and this enzyme participates in the cycling of glutamine-carbon and -nitrogen. On the other hand, GSII activity is repressed in both micro-organisms when glutamine is the only carbon source. These findings show that, in medium containing both glutamine and succinate, glutamine synthesis helps drive the utilization of succinate. When glutamine is in excess as an energy-providing substrate its synthesis is restricted, allowing for more effective utilization of glutamine as an energy source.

Isolation and characterization of Rhizobium etli mutants altered in degradation of asparagine

Rhizobium etli mutants unable to grow on asparagine as the nitrogen and carbon source were isolated. Two kinds of mutants were obtained: AHZ1, with very low levels of aspartase activity, and AHZ7, with low levels of asparaginase and very low levels of aspartase compared to the wild-type strain. R. etli had two asparaginases differentiated by their thermostabilities, electrophoretic mobilities, and modes of regulation. The AHZ mutants nodulated as did the wild-type strain and had nitrogenase levels similar to that of the wild-type strain.

Identification of two glutaminases in Rhizobium etli

We present evidence that Rhizobium etli has two glutaminases differentiated by their thermostability and electrophoretic mobility. The thermostable glutaminase (B) is constitutive, in contrast with the thermolabile glutaminase (A), which is positively regulated by glutamine and negatively regulated by ammonium and by the carbon source. In distinction to glutaminase A, glutaminase B plays a minor role in the utilization of glutamine as a carbon source, but it may play a role in maintaining the balance of glutamine and glutamate. By complementation of the Rhizobium etli LM16 mutant that lacks glutaminase A, we have cloned the gene that codes for this enzyme.

Glutamine synthesis is a regulatory signal controlling glucose catabolism in Saccharomyces cerevisiae

The effect of glutamine biosynthesis and degradation on glucose catabolism in Saccharomyces cerevisiae was studied. A wild-type strain and mutants altered in glutamine biosynthesis and degradation were analyzed. Cells having low levels of glutamine synthetase activity showed high ATP/ADP ratios and a diminished rate of glucose metabolism. It is proposed that glutamine biosynthesis plays a role in the regulation of glucose catabolism.