Regulation of glutamine metabolism during the development of Bombyx mori larvae

Ammonia transport in the excretory system of mosquito larvae (Aedes aegypti): Rh protein expression and the transcriptome of the rectum

The role of the mosquito excretory organs (Malpighian tubules, MT and hindgut, HG) in ammonia transport as well as expression and function of the Rhesus (Rh protein) ammonia transporters within these organs was examined in Aedes aegypti larvae and adult females. Immunohistological examination revealed that the Rh proteins are co-localized with V-type H+-ATPase (VA) to the apical membranes of MT and HG epithelia of both larvae and adult females. Of the two Rh transporter genes present in A. aegypti, AeRh50-1 and AeRh50-2, we show using quantitative real-time PCR (qPCR) and an RNA in-situ hybridization (ISH) assay that AeRh50-1 is the predominant Rh protein expressed in the excretory organs of larvae and adult females. Further assessment of AeRh50-1 function in larvae and adults using RNAi (i.e. dsRNA-mediated knockdown) revealed significantly decreased [NH4+] (mmol l-1) levels in the secreted fluid of larval MT which does not affect overall NH4+ transport rates, as well as significantly decreased NH4+ flux rates across the HG (haemolymph to lumen) of adult females. We also used RNA sequencing to identify the expression of ion transporters and enzymes within the rectum of larvae, of which limited information currently exists for this important osmoregulatory organ. Of the ammonia transporters in A. aegypti, AeRh50-1 transcript is most abundant in the rectum thus validating our immunohistochemical and RNA ISH findings. In addition to enriched VA transcript (subunits A and d1) in the rectum, we also identified high Na+-K+-ATPase transcript (α subunit) expression which becomes significantly elevated in response to HEA, and we also found enriched carbonic anhydrase 9, inwardly rectifying K+ channel Kir2a, and Na+-coupled cation-chloride (Cl-) co-transporter CCC2 transcripts. Finally, the modulation in excretory organ function and/or Rh protein expression was examined in relation to high ammonia challenge, specifically high environmental ammonia (HEA) rearing of larvae. NH4+ flux measurements using the scanning-ion selective electrode (SIET) technique revealed no significant differences in NH4+ transport across organs comprising the alimentary canal of larvae reared in HEA vs freshwater. Further, significantly increased VA activity, but not NKA, was observed in the MT of HEA-reared larvae. Relatively high Rh protein immunostaining persists within the hindgut epithelium, as well as the ovary, of females at 24-48 h post blood meal corresponding with previously demonstrated peak levels of ammonia formation. These data provide new insight into the role of the excretory organs in ammonia transport physiology and the contribution of Rh proteins in mediating ammonia movement across the epithelia of the MT and HG, and the first comprehensive examination of ion transporter and channel expression in the mosquito rectum.

Juvenile hormone regulates silk gene expression by m6A RNA methylation

Juvenile hormone (JH) is an indispensable insect hormone that is critical in regulating insect development and physiology. N6-methyladenosine (m6A) is the most abundant modification of RNA that regulates RNA fate in eukaryotic organisms. However, the relationship between m6A and JH remains largely unknown. Here, we found that the application of a Juvenile hormone analog (JHA) extended the larval period of Bombyx mori and increased the weight and thickness of the cocoon. Interestingly, global transcriptional patterns revealed that m6A-related genes are specifically regulated by JHA in the posterior silk gland (PSG) that synthesizes the major component of cocoon silk. By transcriptome and m6A sequencing data conjointly, we discovered that JHA significantly regulated the m6A modification in the PSG of B. mori and many m6A-containing genes are related to nucleic acid binding, nucleus, and nucleobase-containing compound metabolism. Notably, 547 genes were significantly regulated by JHA at both the m6A modification and expression levels, especially 16 silk-associated genes, including sericin2, seroin1, Serine protease inhibitors 4 (BmSPI4), Serine protease inhibitors 5 (BmSPI5), and LIM domain-binding protein 2 (Ldb). Among them, 11 silk associated genes were significantly affected by METTL3 knockdown, validating that these genes are targets of m6A modification. Furthermore, we confirm that JHA directly regulates the expression of BmSPI4 and BmSPI5 through m6A modification of CDS regions. These results demonstrate the essential role of m6A methylation regulated by JH in PSG, and elucidate a novel mechanism by which JH affects silk gland development via m6A methylation. This study uncovers that m6A modification is a critical factor mediating the effect of JH in insects.

Effects of Midazolam on Antioxidant Levels, Biochemical and Metabolic Parameters in Eurygaster integriceps Puton (Hemiptera: Scutelleridae) Eggs Parasitized by Trissolcus semistriatus Nees (Hymenoptera: Scelionidae)

Eurygaster integriceps Puton (Hemiptera: Scutelleridae) is among the most important insect pests of wheat (Triticum sativum L.) and barley (Hordeum vulgare L.) grown in the Middle East. Biological and chemical methods are insufficient to control E. integriceps populations below economic thresholds. In this study, we investigated the effects of midazolam, a clinical drug, on selected metabolic enzyme activity, antioxidant levels, and biochemical parameters in E. integriceps eggs parasitized by Trissolcus semistriatus Nees (Hymenoptera: Scelionidae). Increasing concentrations of midazolam caused cell damage in the parasitized eggs due to its oxidative effects. Transferase enzymes, such as, aspartate transferase, alanine transferase, and gamma glutamyl transferase activities were altered following exposure. Metabolic enzymes, such as, creatine kinase, alkaline phosphatase, amylase, and lactate dehydrogenase also were adversely affected. Levels of the non-enzymatic antioxidants uric acid, bilirubin, and albumin also were altered.

Probiotic potentials of the silkworm gut symbiont Enterococcus casseliflavus ECB140, a promising L-tryptophan producer living inside the host

AIMS

L-tryptophan is an essential aromatic amino acid for the growth and development of animals. Studies about enteric L-tryptophan-producing bacteria are scarce. In this report, we characterized the probiotic potential of Enterococcus casseliflavus ECB140, focusing on its L-tryptophan production abilities.

METHODS AND RESULTS

ECB140 strain was isolated from the silkworm gut and can survive under strong alkaline environmental conditions. Bacterial colonization traits (motility and biofilm) were examined and showed that only ECB140 produced flagellum and strong biofilms compared with other Enterococcus strains. Comparative genome sequence analyses showed that only ECB140 possessed a complete route for L-tryptophan synthesis among all 15 strains. High-performance liquid chromatography and qRT-PCR confirmed the capability of ECB140 to produce L-tryptophan. Besides, the genome also contains the biosynthesis pathways of several other essential amino acids, such as phenylalanine, threonine, valine, leucine, isoleucine and lysine. These results indicate that ECB140 has the ability to survive passage through the gut and could act as a candidate probiotic.

CONCLUSIONS

The study describes a novel, natural silkworm gut symbiont capable of producing L-tryptophan. Enterococcus casseliflavus ECB140 physical and genomic attributes offer possibilities for its colonization and provide L-tryptophan for lepidopteran insects.

Effects of Copper Oxide Nanoparticles on Immune and Metabolic Parameters of Galleria mellonella L

In this study, the effects of dietary CuO nanoparticles (NPs) on metabolic enzyme activity, biochemical parameters, and total (THC) and differential hemocyte counts (DHC) were determined in Galleria mellonella larvae. Using concentrations of 10, 100, 1000 mg/L and the LC10 and LC30 levels of CuO NPs, we determined that the NPs negatively impacted metabolic enzyme activity and biochemical parameters in larval hemolymph. Compared with the control, the greatest increase in THC was observed in larvae fed on diets with 100 mg L^-1 of CuO NPs. Plasmatocytes and granulocytes were among the most numerous hemocytes in all treatments. These results suggest that dietary CuO NPs effects the metabolic metabolism and immune system of G. mellonella and provide indirect information regarding the toxic effects of CuO NPs in mammalian immune system given similarities between mammalian blood cells and insect hemocytes.

A defect in purine nucleotide metabolism in the silkworm, Bombyx mori, causes a translucent larval integument and male infertility

p-oily (op) is a novel mutant of Bombyx mori exhibiting translucent larval integument and male infertility. Elucidation of the causative gene of the op mutant will help understand the genetic mechanism underlying larval integument coloration and male fertility. Using polymorphisms between B. mori and B. mandarina, the op locus was narrowed down to a 375-kb region. Using RNA-seq analysis, we found that op mutants have a frameshift mutation in the KWMTBOMO13770 gene located in the 375-kb region. A database search indicated that this gene is the human cytosolic 5'-nucleotidase II gene (cN-II) homolog in Bombyx, which mediates the conversion of inosine monophosphate (IMP) to inosine, a precursor of uric acid. CRISPR/Cas9-mediated knockout mutants of the Bm-cN-II gene showed translucent integuments, and there appeared translucent larvae in the crosses between knockout moths and +/op moths. Moreover, the translucent phenotype of, and decreased uric acid content in the larval integument caused by the mutations in the Bm-cN-II gene were rescued by oral administration of inosine. These results indicated that the Bm-cN-II gene is responsible for the op phenotype and that the molecular function of the Bm-cN-II gene is the conversion of IMP to inosine. We also discuss the genetic relationship between the Bm-cN-II gene and male fertility.

Identification of a novel function of the silkworm integument in nitrogen metabolism: Uric acid is synthesized within the epidermal cells in B. mori

During nitrogen metabolism, animals convert toxic ammonia to less toxic forms. Uric acid (UA) is an end product of this process in terrestrial insects. In lepidopteran larvae, a large amount of UA is stored in the integument via a phenomenon known as storage excretion. Physiologically, integumental UA plays crucial roles as a barrier against sunlight and as a white pigment for larval pigmentation patterns. Conventionally, UA is thought to be synthesized in the fat body, the insect equivalent of the liver of vertebrates, and to be transported to the epidermis via the hemolymph. Here, we reconsidered the conventional theory by a mosaic analysis targeting genes governing UA synthesis, using CRISPR/Cas9 mutagenesis and a traditional genetic method in Bombyx mori. Notably, we observed mosaic larvae in which the integument comprised both UA-containing white and UA-lacking translucent areas, indicating that UA synthesis in the epidermis is indispensable to the accumulation of a large amount of highly insoluble UA in the epidermis. Our results thus provide a genetic basis for storage excretion wherein lepidopteran insects use nitrogenous waste to adapt to their environment.

Artificial selection on storage protein 1 possibly contributes to increase of hatchability during silkworm domestication

Like other domesticates, the efficient utilization of nitrogen resources is also important for the only fully domesticated insect, the silkworm. Deciphering the way in which artificial selection acts on the silkworm genome to improve the utilization of nitrogen resources and to advance human-favored domestication traits, will provide clues from a unique insect model for understanding the general rules of Darwin's evolutionary theory on domestication. Storage proteins (SPs), which belong to a hemocyanin superfamily, basically serve as a source of amino acids and nitrogen during metamorphosis and reproduction in insects. In this study, through blast searching on the silkworm genome and further screening of the artificial selection signature on silkworm SPs, we discovered a candidate domestication gene, i.e., the methionine-rich storage protein 1 (SP1), which is clearly divergent from other storage proteins and exhibits increased expression in the ova of domestic silkworms. Knockout of SP1 via the CRISPR/Cas9 technique resulted in a dramatic decrease in egg hatchability, without obvious impact on egg production, which was similar to the effect in the wild silkworm compared with the domestic type. Larval development and metamorphosis were not affected by SP1 knockout. Comprehensive ova comparative transcriptomes indicated significant higher expression of genes encoding vitellogenin, chorions, and structural components in the extracellular matrix (ECM)-interaction pathway, enzymes in folate biosynthesis, and notably hormone synthesis in the domestic silkworm, compared to both the SP1 mutant and the wild silkworm. Moreover, compared with the wild silkworms, the domestic one also showed generally up-regulated expression of genes enriched in the structural constituent of ribosome and amide, as well as peptide biosynthesis. This study exemplified a novel case in which artificial selection could act directly on nitrogen resource proteins, further affecting egg nutrients and eggshell formation possibly through a hormone signaling mediated regulatory network and the activation of ribosomes, resulting in improved biosynthesis and increased hatchability during domestication. These findings shed new light on both the understanding of artificial selection and silkworm breeding from the perspective of nitrogen and amino acid resources.

Like other domesticates, nitrogen resources are also important for the only fully domesticated insect, the silkworm. Deciphering the way in which artificial selection acts on the silkworm genome to improve the utilization of nitrogen resources, thereby advancing human-favored domestication traits, will provide clues from a unique insect model for understanding the general rules of Darwin's theory on artificial selection. However, the mechanisms of domestication in the silkworm remain largely unknown. In this study, we focused on one important nitrogen resource, the storage protein (SP). We discovered that the methionine-rich storage protein 1 (SP1), which is divergent from other SPs, is the only target of artificial selection. Based on functional evidence, together with key findings from the comprehensive comparative transcriptome, we propose that artificial selection favored higher expression of SP1 in the domestic silkworm, which would influence the genes or pathways vital for egg development and eggshell formation. Artificial selection also consistently favored activated ribosome activities and improved amide and peptide biosynthesis in the ova, like what they may act in the silk gland to increase silk-cocoon yield. We highlighted a novel case in which artificial selection could directly act on a nitrogen resource protein associated with a human-desired domestication trait.

The evolutionary road from wild moth to domestic silkworm

The Silk Road, which derives its name from the trade of silk produced by the domestic silkworm Bombyx mori, was an important episode in the development and interaction of human civilizations. However, the detailed history behind silkworm domestication remains ambiguous, and little is known about the underlying genetics with respect to important aspects of its domestication. Here, we reconstruct the domestication processes and identify selective sweeps by sequencing 137 representative silkworm strains. The results present an evolutionary scenario in which silkworms may have been initially domesticated in China as trimoulting lines, then subjected to independent spreads along the Silk Road that gave rise to the development of most local strains, and further improved for modern silk production in Japan and China, having descended from diverse ancestral sources. We find that genes with key roles in nitrogen and amino acid metabolism may have contributed to the promotion of silk production, and that circadian-related genes are generally selected for their adaptation. We additionally identify associations between several candidate genes and important breeding traits, thereby advancing the applicable value of our resources.

Metabolomic profiling of 13C-labelled cellulose digestion in a lower termite: insights into gut symbiont function

Termites consume an estimated 3–7 billion tonnes of lignocellulose annually, a role in nature which is unique for a single order of invertebrates. Their food is digested with the help of microbial symbionts, a relationship that has been recognized for 200 years and actively researched for at least a century. Although DNA- and RNA-based approaches have greatly refined the details of the process and the identities of the participants, the allocation of roles in space and time remains unclear. To resolve this issue, a pioneer study is reported using metabolomics to chart the in situ catabolism of ¹³C-cellulose fed to the dampwood species Hodotermopsis sjostedti. The results confirm that the secretion of endogenous cellulases by the host may be significant to the digestive process and indicate that a major contribution by hindgut bacteria is phosphorolysis of cellodextrins or cellobiose. This study provides evidence that essential amino acid acquisition by termites occurs following the lysis of microbial tissue obtained via proctodaeal trophallaxis.

Whitefly genome expression reveals host-symbiont interaction in amino acid biosynthesis

Background

Whitefly (Bemisia tabaci) complex is a serious insect pest of several crop plants worldwide. It comprises several morphologically indistinguishable species, however very little is known about their genetic divergence and biosynthetic pathways. In the present study, we performed transcriptome sequencing of Asia 1 species of B. tabaci complex and analyzed the interaction of host-symbiont genes in amino acid biosynthetic pathways.

Methodology/Principal Findings

We obtained about 83 million reads using Illumina sequencing that assembled into 72716 unitigs. A total of 21129 unitigs were annotated at stringent parameters. Annotated unitigs were mapped to 52847 gene ontology (GO) terms and 131 Kyoto encyclopedia of genes and genomes (KEGG) pathways. Expression analysis of the genes involved in amino acid biosynthesis pathways revealed the complementation between whitefly and its symbiont partner Candidatus Portiera aleyrodidarum. Most of the non-essential amino acids and intermediates of essential amino acid pathways were supplied by the host insect to its symbiont. The symbiont expressed the pathways for the essential amino acids arginine, threonine and tryptophan and the immediate precursors of valine, leucine, isoleucine and phenyl-alanine. High level expression of the amino acid transporters in the whitefly suggested the molecular mechanisms for the exchange of amino acids between the host and the symbiont.

Conclusions/Significance

Our study provides a comprehensive transcriptome data for Asia 1 species of B. tabaci complex that focusses light on integration of host and symbiont genes in amino acid biosynthesis pathways.

GC/MS-based metabolomic studies reveal key roles of glycine in regulating silk synthesis in silkworm, Bombyx mori

Metabolic profiling of silkworm, especially the factors that affect silk synthesis at the metabolic level, is little known. Herein, metabolomic method based on gas chromatography-mass spectrometry was applied to identify key metabolic changes in silk synthesis deficient silkworms. Forty-six differential metabolites were identified in Nd group with the defect of silk synthesis. Significant changes in the levels of glycine and uric acid (up-regulation), carbohydrates and free fatty acids (down-regulation) were observed. The further metabolomics of silk synthesis deficient silkworms by decreasing silk proteins synthesis using knocking out fibroin heavy chain gene or extirpating silk glands operation showed that the changes of the metabolites were almost consistent with those of the Nd group. Furthermore, the increased silk yields by supplying more glycine or its related metabolite confirmed that glycine is a key metabolite to regulate silk synthesis. These findings provide important insights into the regulation between metabolic profiling and silk synthesis.

A putative Δ1-pyrroline-5-carboxylate synthetase involved in the biosynthesis of proline and arginine in Leptinotarsa decemlineata

Delta 1-pyrroline-5-carboxylate synthetase (P5CS) catalyzes the conversion of glutamate (Glu) to Glu semialdehyde (GSA). GSA spontaneously cyclizes to form P5C. P5C is then reduced to proline (Pro) or is converted to ornithine, the intermediate for arginine (Arg) biosynthesis. In the present study, a full-length Ldp5cs complementary DNA was cloned from the Colorado potato beetle Leptinotarsa decemlineata, a notorious insect defoliator of potato in most potato-growing regions of the world. Ldp5cs encodes a 792-amino-acid protein which shares high identity to homologues from other insect species. Quantitative reverse transcription polymerase chain reaction revealed that Ldp5cs was ubiquitously expressed in the eggs, first to fourth-instar larvae, wandering larvae, pupae and sexually mature adults. In the adults, Ldp5cs mRNA levels were higher in the fat body, foregut, midgut and hindgut, moderate in the ventral ganglion, lower in the thorax muscles, epidermis and Malpighian tubules. Two double-stranded RNAs (dsRNAs) (dsLdp5cs1 and dsLdp5cs2) targeting Ldp5cs were constructed and bacterially expressed. Ingestion during 3 consecutive days of dsLdp5cs1 or dsLdp5cs2 successfully silenced Ldp5cs, significantly reduced the contents of Pro and Arg in the hemolymph, decreased flight speed and shortened flight distance of the resulting adults. Furthermore, knocking down Ldp5cs significantly increased adult mortality. Thus, our results suggest that identified Ldp5cs encodes a functional P5CS enzyme that is involved in the biosynthesis of Pro and Arg in L. decemlineata.

Constructing the major biosynthesis pathways for amino acids in the brown planthopper, Nilaparvata lugens Stål (Hemiptera: Delphacidae), based on the transcriptome data

Nilaparvata lugens is a serious phloem-feeding pest of rice throughout Asia. Rice phloem sap can meet its nutrition requirement for sugars but not for some essential amino acids such as isoleucine, leucine, methionine, phenylalanine, tryptophan, lysine, arginine and histidine. N. lugens harbours yeast-like symbionts in mycetocytes formed by abdominal fat body cells. Removal of the symbionts results in negative physiological effects, suggesting that the symbionts play a pivotal role in the nitrogen metabolism. In the present paper, 521 mRNA expressed sequence tags (ESTs) encoding 126 enzymes that were involved in amino acid biosynthesis were identified based on a transcriptome data, reverse transcription (RT)-PCR and rapid amplification of cDNA ends. Similarity analysis, codon usage bias, along with tissue-biased expression and phylogenetic analysis of a subset of ESTs, suggest that 437 ESTs out of the 521 originate from symbionts, and the remaining 84 mRNA fragments come from N. lugens. Accordingly, the biosynthesis pathways for 20 amino acids were manually constructed. It is postulated that both N. lugens and its symbiont can independently assimilate ammonia and biosynthesize seven non-essential amino acids: glutamate; glutamine; aspartate; asparagine; alanine; serine; and glycine. N. lugens and symbiont enzymes may work collaboratively to catalyse the biosynthesis of proline, methionine, valine, leucine, isoleucine, phenylalanine and tyrosine. We infer from this that symbionts function in the biosynthesis of lysine, arginine, tryptophan, threonine, histidine and cysteine. Our data support the previously proposed hypothesis, i.e. the yeast-like symbionts compensate for, at least partially, the amino acid needs of N. lugens.

Aphid genome expression reveals host-symbiont cooperation in the production of amino acids

The evolution of intimate symbiosis requires the coordination of gene expression and content between the distinct partner genomes; this coordination allows the fusion of capabilities of each organism into a single integrated metabolism. In aphids, the 10 essential amino acids are scarce in the phloem sap diet and are supplied by the obligate bacterial endosymbiont (Buchnera), which lives inside specialized cells called bacteriocytes. Although Buchnera's genome encodes most genes for essential amino acid biosynthesis, several genes in essential amino acid pathways are missing, as are most genes for production of nonessential amino acids. Additionally, it is unresolved whether the supply of nitrogen for amino acid biosynthesis is supplemented by recycling of waste ammonia. We compared pea aphid gene expression between bacteriocytes and other body tissues using RNA sequencing and pathway analysis and exploiting the genome sequences available for both partners. We found that 26 genes underlying amino acid biosynthesis were up-regulated in bacteriocytes. Seven of these up-regulated genes fill the gaps of Buchnera's essential amino acid pathways. In addition, genes underlying five nonessential amino acid pathways lost from Buchnera are up-regulated in bacteriocytes. Finally, our results reveal that two genes, glutamine synthetase and glutamate synthase, which potentially work together in the incorporation of ammonium nitrogen into glutamate (GOGAT) cycle to assimilate ammonia into glutamate, are up-regulated in bacteriocytes. Thus, host gene expression and symbiont capabilities are closely integrated within bacteriocytes, which function as specialized organs of amino acid production. Furthermore, the GOGAT cycle may be a key source of nitrogen fueling the integrated amino acid metabolism of the aphid-Buchnera partnership.

Systematic NMR analysis of stable isotope labeled metabolite mixtures in plant and animal systems: coarse grained views of metabolic pathways

Background

Metabolic phenotyping has become an important ‘bird's-eye-view’ technology which can be applied to higher organisms, such as model plant and animal systems in the post-genomics and proteomics era. Although genotyping technology has expanded greatly over the past decade, metabolic phenotyping has languished due to the difficulty of ‘top-down’ chemical analyses. Here, we describe a systematic NMR methodology for stable isotope-labeling and analysis of metabolite mixtures in plant and animal systems.

Methodology/Principal Findings

The analysis method includes a stable isotope labeling technique for use in living organisms; a systematic method for simultaneously identifying a large number of metabolites by using a newly developed HSQC-based metabolite chemical shift database combined with heteronuclear multidimensional NMR spectroscopy; Principal Components Analysis; and a visualization method using a coarse-grained overview of the metabolic system. The database contains more than 1000 ¹H and ¹³C chemical shifts corresponding to 142 metabolites measured under identical physicochemical conditions. Using the stable isotope labeling technique in Arabidopsis T87 cultured cells and Bombyx mori, we systematically detected >450 HSQC peaks in each ¹³C-HSQC spectrum derived from model plant, Arabidopsis T87 cultured cells and the invertebrate animal model Bombyx mori. Furthermore, for the first time, efficient ¹³C labeling has allowed reliable signal assignment using analytical separation techniques such as 3D HCCH-COSY spectra in higher organism extracts.

Conclusions/Significance

Overall physiological changes could be detected and categorized in relation to a critical developmental phase change in B. mori by coarse-grained representations in which the organization of metabolic pathways related to a specific developmental phase was visualized on the basis of constituent changes of 56 identified metabolites. Based on the observed intensities of ¹³C atoms of given metabolites on development-dependent changes in the 56 identified ¹³C-HSQC signals, we have determined the changes in metabolic networks that are associated with energy and nitrogen metabolism.

Ammonia metabolism in Aedes aegypti

We investigated the mechanisms by which Aedes aegypti mosquitoes are able to metabolize ammonia. When females were given access to solutions containing NH(4)Cl or to a blood meal, hemolymph glutamine and proline concentrations increased markedly, indicating that ammonium/ammonia can be removed from the body through the synthesis of these two amino acids. The importance of glutamine synthetase was shown when an inhibitor of the enzyme was added to the meal causing the glutamine concentration in hemolymph to decrease significantly, while the proline concentration increased dramatically. Unexpectedly, we found an important role for glutamate synthase. When mosquitoes were fed azaserine, an inhibitor of glutamate synthase, the glutamine concentration increased and the proline concentration decreased significantly. This confirms the presence of glutamate synthase in mosquitoes and suggests that this enzyme contributes to the production of glutamate for proline synthesis. Several key enzymes related to ammonium/ammonia metabolism showed activity in homogenates of mosquito fat body and midgut. The mosquito genes encoding glutamate dehydrogenase, glutamine synthetase, glutamate synthase, pyrroline-5-carboxylate synthase were cloned and sequenced. The mRNA expression patterns of these genes were examined by a real-time RT-PCR in fat body and midgut. The results show that female mosquitoes have evolved efficient mechanisms to detoxify large loads of ammonium/ammonia.

Structure--function studies on the iron-sulfur flavoenzyme glutamate synthase: an unexpectedly complex self-regulated enzyme

Glutamate synthase (GltS) is, with glutamine synthetase, the key enzyme of ammonia assimilation in bacteria, microorganisms and plants. GltS isoforms result from the assembly and co-evolution of conserved functional domains. They share a common mechanism of reductive glutamine-dependent glutamate synthesis from 2-oxoglutarate, which takes place within the alpha subunit ( approximately 150 kDa) of the NADPH-dependent bacterial enzyme and the corresponding polypeptides of other GltS forms, and involves: (i) an Ntn-type amidotransferase domain and (ii) a flavin mononucleotide-containing (beta/alpha)(8) barrel synthase domain connected by (iii) a approximately 30 A-long intramolecular ammonia tunnel. The synthase domain harbors the [3Fe/4S](0,+1) cluster of the enzyme, which participates in the electron transfer process from the physiological reductant: reduced ferredoxin in the plant-type enzyme or NAD(P)H in the bacterial and the non-photosynthetic eukaryotic form. The NAD(P)H-dependent GltS requires a tightly bound flavin adenine dinucleotide-dependent reductase (beta subunit, approximately 50 kDa), also determining the presence of two low-potential [4Fe-4S](+1,+2) clusters. Structural, functional and computational data available on GltS and related enzymes show how the enzyme may control and coordinate the reactions taking place at the glutaminase and synthase sites by sensing substrate binding and cofactor redox state.

Molecular characterization of the glutamine synthetase gene in the Pacific oyster Crassostrea gigas: expression study in response to xenobiotic exposure and developmental stage

In this study, we characterized the full-length cDNA and genomic sequence of the gene encoding cytosolic glutamine synthetase (CgGSII) in the Pacific oyster, Crassostrea gigas. A phylogenetic analysis of GS sequences showed that CgGS clustered with the invertebrate group as expected. We analyzed the expression of mRNA CgGSII using RT-PCR to follow the expression of this gene in gills and digestive gland of oysters exposed, under experimental conditions, to hypoxia and to several contaminants (hydrocarbons and two pesticide treatments, glyphosate and a mixture of atrazine, diuron and isoproturon). We also investigated the expression of CgGSII in different developmental stages of C. gigas. Our results show that CgGSII expression was highly regulated in xenobiotic-exposed oysters compared to the control for all the treatments. Likewise, CgGSII expression was highly regulated according to the developmental stage of C. gigas. Finally, use of CgGSII as a possible marker to monitor xenobiotic exposure in disturbed ecosystems is discussed.