Isolation of human cDNAs for asparagine synthetase and expression in Jensen rat sarcoma cells

The relationship between plasma amino acids and circulating albumin and haemoglobin in postabsorptive stroke patients

Background

This retrospective study had two main aims: (1) to document possible correlations between plasma Amino Acids (AAs) and circulating Albumin (Alb) and Haemoglobin (Hb); and (2) to identify which AAs were predictors of Alb and Hb.

Methods

The study considered 125 stroke subjects (ST) (61.6% males; 65.6 +/- 14.9 years) who met the eligibility criteria (absence of co morbidities associated with altered plasma AAs and presence of plasma AAs determined after overnight fasting). Fifteen matched healthy subjects with measured plasma AAs served as controls.

Results

The best correlations of Alb were with tryptophan (Trp) and histidine (His) (r = + 0.53; p < 0.0001), and those of Hb were with histidine (r = +0.47) and Essential AAs (r = +0.47) (both p<0.0001). In multivariate analysis, Trp (p< 0.0001) and His (p = 0.01) were shown to be the best positive predictors of Alb, whereas glutamine (p = 0.006) was the best positive predictor of Hb.

Conclusions

The study shows that the majority of plasma AAs were positively correlated with Alb and Hb. The best predictors of circulating Alb and Hb were the levels of tryptophan and glutamine, respectively.

Leishmania infantum Asparagine Synthetase A Is Dispensable for Parasites Survival and Infectivity

A growing interest in asparagine (Asn) metabolism has currently been observed in cancer and infection fields. Asparagine synthetase (AS) is responsible for the conversion of aspartate into Asn in an ATP-dependent manner, using ammonia or glutamine as a nitrogen source. There are two structurally distinct AS: the strictly ammonia dependent, type A, and the type B, which preferably uses glutamine. Absent in humans and present in trypanosomatids, AS-A was worthy of exploring as a potential drug target candidate. Appealingly, it was reported that AS-A was essential in Leishmania donovani, making it a promising drug target. In the work herein we demonstrate that Leishmania infantum AS-A, similarly to Trypanosoma spp. and L. donovani, is able to use both ammonia and glutamine as nitrogen donors. Moreover, we have successfully generated LiASA null mutants by targeted gene replacement in L. infantum, and these parasites do not display any significant growth or infectivity defect. Indeed, a severe impairment of in vitro growth was only observed when null mutants were cultured in asparagine limiting conditions. Altogether our results demonstrate that despite being important under asparagine limitation, LiAS-A is not essential for parasite survival, growth or infectivity in normal in vitro and in vivo conditions. Therefore we exclude AS-A as a suitable drug target against L. infantum parasites.

Knockdown of asparagine synthetase A renders Trypanosoma brucei auxotrophic to asparagine

Asparagine synthetase (AS) catalyzes the ATP-dependent conversion of aspartate into asparagine using ammonia or glutamine as nitrogen source. There are two distinct types of AS, asparagine synthetase A (AS-A), known as strictly ammonia-dependent, and asparagine synthetase B (AS-B), which can use either ammonia or glutamine. The absence of AS-A in humans, and its presence in trypanosomes, suggested AS-A as a potential drug target that deserved further investigation. We report the presence of functional AS-A in Trypanosoma cruzi (TcAS-A) and Trypanosoma brucei (TbAS-A): the purified enzymes convert L-aspartate into L-asparagine in the presence of ATP, ammonia and Mg²⁺. TcAS-A and TbAS-A use preferentially ammonia as a nitrogen donor, but surprisingly, can also use glutamine, a characteristic so far never described for any AS-A. TbAS-A knockdown by RNAi didn't affect in vitro growth of bloodstream forms of the parasite. However, growth was significantly impaired when TbAS-A knockdown parasites were cultured in medium with reduced levels of asparagine. As expected, mice infections with induced and non-induced T. brucei RNAi clones were similar to those from wild-type parasites. However, when induced T. brucei RNAi clones were injected in mice undergoing asparaginase treatment, which depletes blood asparagine, the mice exhibited lower parasitemia and a prolonged survival in comparison to similarly-treated mice infected with control parasites. Our results show that TbAS-A can be important under in vivo conditions when asparagine is limiting, but is unlikely to be suitable as a drug target.

The amino acid asparagine is important not only for protein biosynthesis, but also for nitrogen homeostasis. Asparagine synthetase catalyzes the synthesis of this amino acid. There are two forms of asparagine synthetase, A and B. The presence of type A in trypanosomes, and its absence in humans, makes this protein a potential drug target. Trypanosomes are responsible for serious parasitic diseases that rely on limited drug therapeutic options for control. In our study we present a functional characterization of trypanosomes asparagine synthetase A. We describe that Trypanosoma brucei and Trypanosoma cruzi type A enzymes are able to use either ammonia or glutamine as a nitrogen donor, within the conversion of aspartate into asparagine. Furthermore, we show that asparagine synthetase A knockdown renders Trypanosoma brucei auxotrophic to asparagine. Overall, this study demonstrates that interfering with asparagine metabolism represents a way to control parasite growth and infectivity.

An investigation into the mechanism ofL-asparaginase resistance in L5178Y murine leukemia cells

Resistance of leukemia cells toL-asparaginase is presumed to be due to increased expression of asparagine synthetase activity by resistant cells, so they are no longer dependent on an exogenous source ofL-asparagine for growth. The mechanism by which cells acquire the ability for increased enzyme expression, however, has not been clearly defined. Evidence presented here indicates that genomic alterations in the form of translocations, gene amplification, or increased P-glycoprotein expression, do not account for the phenotypic transformation fromL-asparaginase sensitivity toL-asparaginase resistance. Instead, both sensitive and resistant L5178Y cells contain immunoreactive material detected by Western blotting with an antiserum prepared against bovine pancreatic asparagine synthetase. This suggests that the mechanism of resistance might involve modification of asparagine synthetase inL-asparaginase-resistant cells by an as-yet-unidentified mechanism or by inhibition of enzyme activity in theL-asparaginase-sensitive cells.

Asparagine synthetase: regulation by cell stress and involvement in tumor biology

Asparagine synthetase (ASNS) catalyzes the conversion of aspartate and glutamine to asparagine and glutamate in an ATP-dependent reaction. The enzyme is ubiquitous in its organ distribution in mammals, but basal expression is relatively low in tissues other than the exocrine pancreas. Human ASNS activity is highly regulated in response to cell stress, primarily by increased transcription from a single gene located on chromosome 7. Among the genomic elements that control ASNS transcription is the C/EBP-ATF response element (CARE) within the promoter. Protein limitation or an imbalanced dietary amino acid composition activate the ASNS gene through the amino acid response (AAR), a process that is replicated in cell culture through limitation for any single essential amino acid. Endoplasmic reticulum stress also increases ASNS transcription through the PERK-eIF2-ATF4 arm of the unfolded protein response (UPR). Both the AAR and UPR lead to increased synthesis of ATF4, which binds to the CARE and induces ASNS transcription. Elevated expression of ASNS protein is associated with resistance to asparaginase therapy in childhood acute lymphoblastic leukemia and may be a predictive factor in drug sensitivity for certain solid tumors as well. Activation of the GCN2-eIF2-ATF4 signaling pathway, leading to increased ASNS expression appears to be a component of solid tumor adaptation to nutrient deprivation and/or hypoxia. Identifying the roles of ASNS in fetal development, tissue differentiation, and tumor growth may reveal that ASNS function extends beyond asparagine biosynthesis.

Molecular mechanisms involved in the adaptation to amino acid limitation in mammals

In mammals, metabolic adaptations are required to cope with episodes of protein deprivation and malnutrition. Consequently, mammals have to adjust physiological functions involved in the adaptation to amino acid availability. Part of this regulation involves the modulation of the expression of numerous genes. In particular, it has been shown that amino acids by themselves can modify the expression of target genes. This review describes the regulation of amino acids homeostasis and the their role as signal molecules. The recent advances in the understanding of the molecular mechanisms involved in the control of mammalian gene expression in response to amino acid limitation will be described.

Amino acids as regulators of gene expression in mammals: molecular mechanisms

In mammals, the impact of nutrients on gene expression has become an important area of research. Because amino acids have multiple and important functions, their homeostasis has to be finely maintained. However, amino acidemia can be affected in some nutritional conditions and by various forms of stress. Consequently, mammals have to adjust physiological functions involved in the adaptation to amino acid availability. Part of this regulation involves the modulation of numerous gene expression. It has been shown that amino acids by themselves can modify the expression of target genes. This review focuses on the recent advances in the understanding of the mechanisms involved in the control of mammalian gene expression in response to amino acid limitation.

Differential soluble protein expression between Trichomonas vaginalis isolates exhibiting low and high virulence phenotypes

A comparative analysis of proteomic maps of long-term grown and fresh clinical Trichomonas vaginalis isolates exhibiting low and high virulence phenotypes, respectively, was performed using two-dimensional gel electrophoresis and mass spectrometry. Of 29 protein spots differentially expressed between the isolates, 19 were over-expressed in the isolate exhibiting high virulence phenotype: proteins associated with cytoskeletal dynamics, such as coronin and several isoforms of actin, as well as proteins involved in signal transduction, protein turnover, proteolysis, and energetic and polyamine metabolisms were identified. Some malate dehydrogenase, fructose-1,6-bisphosphate aldolase and ornithine cyclodeamidase isoforms were exclusively expressed by the highly virulent isolate. During interaction assays with VEC, parasites exhibiting high virulence phenotype rapidly adhered and switched to amoeboid forms. In contrast, low adhesion and no morphological transformation were observed in parasites displaying low virulence phenotype. Our findings demonstrate that expression of specific proteins by high and low virulence parasites could be associated with the ability of each isolate to undergo morphological transformation and interact with host cells. Such data represent an important step towards understanding of the complex interaction network of proteins that participate in the mechanism of pathogenesis of this protozoan.

Protein expression ofpectoralis majormuscle in chickens in response to dietary methionine status

The present study evaluated the effect of dietary methionine on breast-meat accretion and protein expression in skeletal muscle of broiler chickensin vivo. All broilers received a common pre-test diet up to 21d of age, and were subsequently fed either a methionine-deficient (MD) or -adequate (MA) diet (3·1V. 4·5g/kg diet) from age 21 to 42d. Dietary cystine levels were 3·7V. 3·6g/kg diet for the MD and MA diet, respectively. Detrimental effects on carcass yield (P=0·004), abdominal fat percentage (P=0·001), and breast-meat weight (P=0·001), yield (P=0·001), and uniformity (P=0·002) were observed and validated in birds fed MD diets. Via tandem MS, a total of 190 individual proteins were identified frompectoralis major(PM) muscle tissue. From the former composite, peptides from three proteins were observed to be present exclusively in breast muscle from those chickens fed the MD diet (pyruvate kinase, myosin alkali light chain-1, ribosomal-protein-L-29). No proteins were observed to be uniquely expressed in chickens fed MA diets. Research is warranted to further explore the possibility of the proteins pyruate kinase, myosin alkali light chain-1, or ribosomal protein L-29, as potential biological indicators of differences in protein expression of PM of chickens in response to a dietary methionine deficiency.

Knock-down of Sorcin Induces Up-regulation of MDR1 in HeLa Cells

In our previous study, the MDR1/Pglycoprotein-overexpressing multidrug resistant subline, Hvr100-6, was established from the human cervical carcinoma cell line HeLa-Ohio (HeLa) by stepwise exposure to an anti-microtubule agent, vinblastine sulfate, a typical substrate of MDR1. Their gene and protein expression profiles were analyzed herein, and 148 genes were identified to be differentially expressed by cDNA microarray analysis. The up-regulation of sorcin, a soluble resistance-related calcium-binding protein of 22 kDa, was confirmed in Hvr100-6 cells by the proteome analysis. To clarify the relationship between MDR1 and sorcin, HeLa cells were treated with small interfering RNAs (siRNAs) targeted for theirs mRNAs. The siRNA for MDR1 mRNA resulted in its decrease by 86% and 61% on the days 1 and 2 after the treatment, whereas the expression level of sorcin mRNA was not changed. On the other hand, the siRNA for sorcin mRNA suppressed its expression by 80-90% on days 1-3 after the treatment. Interestingly; suppression of sorcin induced a more than 3-fold increase in the expression level for MDR1 mRNA. An efflux function of MDR1 evaluated with using rhodamine 123 as a probe showed a tendency to be increased in HeLa cells treated with siRNA for sorcin, compared with that in the cells treated with scramble siRNA. The activity and the expression of caspase-3 in the sorcin knock-down HeLa cells were relatively higher than those in the cells treated with scramble siRNA. Thus, we demonstrated that sorcin might be a partial suppressor of MDR1 expression. Furthermore, the present study suggested that sorcin repressed apoptosis via dysfunction of caspase-3.

Epigenetic changes in the repression and induction of asparagine synthetase in human leukemic cell lines

In common with certain other lymphoid neoplasms, cells of the human lymphocytic leukemia lines 1873 and 1929 are asparagine (ASN) auxotrophs. Asparagine synthetase (ASY), which is a housekeeping gene, is repressed and the promoting region of the gene is highly methylated. We now demonstrate in these cells multiple levels in control of the expression of this gene, in a system of cocultivation with macrophages and other cell types. In this system, mediated by cell-to-cell contact, ASY becomes expressed by the leukemic cells and they become prototrophic. Demethylation of ASY occurs; it follows expression and is permanent over multiple cell generations, but the cells return to auxotrophy with rapid repression of ASY on removal from cell contact. With ASY expression, the associated histone H3 at lysine position 9 (H3K9) becomes acetylated and H3K4, methylated. In contrast to other systems, H3K9 methylation does not characterize the repressed state. The changes leading from repression to induction of ASY and demethylation parallel the physiological changes specific to functional maturation of normal lymphoid precursors. The lability of expression of ASY has potential significance in determining the sensitivity of leukemic cells to L-asparaginase.

Amino acids as regulators of gene expression: molecular mechanisms

Regulation of gene expression by nutrients in mammals is an important mechanism allowing them to adapt their physiological functions according to the supply of nutrient in the diet. It has been shown recently that amino acids are able to regulate by themselves the expression of numerous genes. CHOP, asparagine synthetase, and IGFBP-1 regulation following AA starvation will be described in this review with special interest in the molecular mechanisms involved.

Recent advances in the understanding of amino acid regulation of gene expression

In mammals, the impact of nutrients on gene expression has become an important area of research. Because amino acids have multiple and important functions, their homeostasis has to be finely maintained. However, amino acidemia can be affected by certain nutritional conditions or various forms of stress. Consequently, mammals must adjust several of the physiological functions involved in the adaptation to amino acid availability by regulating expression of numerous genes. It has been shown that amino acids alone can modify the expression of target genes. However, understanding of amino acid-dependent control of gene expression has just started to emerge. This review focuses on recent advances in the understanding of mechanisms involved in the amino acid control of gene expression.

An alternative mechanism for the nitrogen transfer reaction in asparagine synthetase

In the absence of crystallographic data, the mechanism of nitrogen transfer from glutamine in asparagine synthetase (AS) remains under active investigation. Surprisingly, the glutamine-dependent AS from Escherichia coli (AsnB) appears to lack a conserved histidine residue, necessary for nitrogen transfer if the reaction proceeds by the accepted pathway in other glutamine amidotransferases, but retains the ability to synthesize asparagine. We propose an alternative mechanism for nitrogen transfer in AsnB which obviates the requirement for participation of histidine in this step. Our hypothesis may also be more generally applicable to other glutamine-dependent amidotransferases.

Hypermethylation of CpG islands in the mouse asparagine synthetase gene: relationship to asparaginase sensitivity in lymphoma cells. Partial methylation in normal cells

We have sequenced the promoter region of the murine asparagine synthetase gene and examined its methylation profile in the CpG islands of L-asparaginase-sensitive 6C3HED cells (asparagine auxotrophs) and resistant variants (prototrophs). In the former, complete methylation of the CpG island is correlated with failure of expression of mRNA: cells of the latter possess both methylated and unmethylated alleles, as do cells of the intrinsically asparagine-independent lines L1210 and EL4. A similar phenomenon was seen in normal splenic cells of adult mice. This was age related: no methylation was found in weanlings, but up to 45% of gene copies in animals 18 weeks or older were methylated. It was also tissue related, with methylation occurring rarely in liver cells. The relationship of these changes to oncogenesis is considered.

Recent advances on molecular mechanisms involved in amino acid control of gene expression

In mammals, the impact of nutrients on gene expression has become an important area of research. Because amino acids have multiple and important functions, their homeostasis has to be finely maintained. However, amino acidaemia can be affected by certain nutritional conditions or various forms of aggression. It follows that mammals have to adjust several of their physiological functions involved in the adaptation to amino acid availability by regulating the expression of numerous genes. It has been shown that amino acids by themselves can modify the expression of target genes. However, the current understanding of amino acid-dependent control of gene expression has just started to emerge. This review focuses on the recent advances on mechanisms involved in the amino acids control of gene expression. Several examples discussed in this paper demonstrate that amino acids regulate gene expression at the level of transcription, messenger RNA stability and translation.

Do the 5'untranslated domains of human cDNAs challenge the rules for initiation of translation (or is it vice versa)?

The validity of the scanning mechanism for initiation of translation has been questioned based on a compilation of human cDNA sequences that showed a high frequency of upstream ATG codons. However, closer scrutiny of those cDNAs upholds the opposite view: the 5'UTRs on most cDNAs are compatible with standard rules for initiation of translation, and those rules can be used to flag anomalous cDNAs that, upon checking, turn out to have been misinterpreted. Some of the problematic 5'UTR sequences that persist, after obvious errors in the cDNA library have been corrected, might derive from transcripts that are not intended to be translated. Examples are given of genes that, for regulatory reasons, produce transcripts that are truncated, or retain an intron, or are otherwise configured in a way that precludes translation. The existence of a cDNA proves that a gene is transcribed, but only that; not every cDNA derives from a functional mRNA. Along with providing practical guidelines for interpreting cDNA sequences, the scanning model provides a theoretical framework for understanding the effects of certain mutations in the 5'UTR that alter the translatability of mRNAs, thereby contributing to cancer and other human diseases.

Sequence and genomic organization of the human G-protein Golfalpha gene (GNAL) on chromosome 18p11, a susceptibility region for bipolar disorder and schizophrenia

The sequence and genomic organization of the human Golfalpha (GNAL) gene were determined. The human GNAL gene was found to contain 12 coding exons, and it spans over 80 kb on chromosome 18p11. 5' RACE analysis suggested an additional transcription initiation start site. Sequence analysis of the putative promoter region revealed conserved binding sites for several transcription factors. Sequence analysis of the 3'-untranslated region revealed the presence of two Alu sequences and two polyadenylation signals. 3' RACE analysis confirmed the functionality of the most downstream poly-a signal. The human GNAL was found to be expressed as a single transcript of about 5.9 kb in the brain. One highly informative dinucleotide repeat was found in intron 5. Additionally, a processed pseudogene for asparagine synthetase was found about 6 kb upstream of the GNAL gene. Knowledge of the sequence and structure of the human GNAL gene provides essential information for further analysis of the GNAL locus at chromosome 18p11 which has been linked to bipolar disorder and schizophrenia.

Inhibition of plant asparagine synthetase by monoterpene cineoles

Asparagine (Asn) synthetase (AS) is the key enzyme in Asn biosynthesis and plays an important role in nitrogen mobilization. Despite its important physiological function, little research has been done documenting inhibitors of plant AS. Plant growth inhibition caused by the natural monoterpene 1,4-cineole and its structurally related herbicide cinmethylin was reversed 65% and 55%, respectively, by providing 100 microM Asn exogenously. Reversion of the phytotoxic effect was dependent on the concentration of Asn. The presence of either 1,4-cineole or cinmethylin stimulated root uptake of [(14)C]Asn by lettuce (Lactuca sativa) seedlings. Although the physiological responses suggested that both compounds affected Asn biosynthesis, biochemical analysis of AS activity showed that the natural monoterpene was a potent inhibitor (I(50) = approximately 0. 5 microM) of the enzyme, whereas the commercial product was not inhibitory up to levels of 10 mM. Analysis of the putative metabolite, 2-hydroxy-1,4-cineole, showed that the cis-enantiomer was much more active than the trans-enantiomer, suggesting that the hydroxyl group was involved in the specific ligand/active site interaction. This is the first report that AS is a suitable herbicide target site, and that cinmethylin is apparently a proherbicide that requires metabolic bioactivation via cleavage of the benzyl-ether side chain.

RT-PCR cloning, characterization and mRNA expression analysis of a cDNA encoding a type II asparagine synthetase in common bean

Following a RT-PCR strategy based on the design of degenerate oligonucleotides resembling conserved domains of asparagine synthetase (AS; EC 6.3.5.4), we isolated a 2 kb cDNA clone (PVAS2) from root tissue of the common bean (Phaseolus vulgaris). PVAS2 encodes a protein of 584 amino acids with a predicted relative molecular mass of 65810 Da, an isoelectric point of 6.4, and a net charge of -7.2 at pH 7.0. The amino acid sequence of the protein encoded by PVAS2 is very similar to that encoded by the soybean SAS2 asparagine synthetase gene. The amino-terminal residues of the predicted PVAS2 protein are identical to the amino acids that constitute the glutamine-binding (GAT) domain of AS from other plant species, which suggests that the PVAS2 cDNA encodes a type II glutamine-dependent form of asparagine synthetase. Southern blot analysis indicates that the common bean AS is part of a small family composed of at least two genes. Expression analysis by Northern blot revealed that the PVAS2 transcript accumulates to a high level in roots and, to a lesser extent, in nodules and developing pods. Accumulation of the PVAS2 transcript in the root seems to be negatively regulated by light and sucrose, and positively regulated by nitrate.

Transcriptional regulation of the human asparagine synthetase gene by carbohydrate availability

Transcription of the asparagine synthetase (AS) gene is induced by amino acid deprivation. The present data illustrate that this gene is also under transcriptional control by carbohydrate availability. Incubation of human HepG2 hepatoma cells in glucose-free medium resulted in an increased AS mRNA content, reaching a maximum of about 14-fold over control cells after approx. 12 h. Extracellular glucose caused the repression of the content of AS mRNA in a concentration-dependent manner, with a k1/2 (concentration causing a half-maximal repression) of 1 mM. Fructose, galactose, mannose, 2-deoxyglucose and xylitol were found to maintain the mRNA content of both AS and the glucose-regulated protein GRP78 in a state of repression, whereas 3-O-methylglucose did not. Incubation in either histidine-free or glucose-free medium also resulted in adaptive regulation of the AS gene in BNL-CL.2 mouse hepatocytes, rat C6 glioma cells and human MOLT4 lymphocytes, in addition to HepG2 cells. In contrast, the steady-state mRNA content of GRP78 was unaffected by amino acid availability. Transient transfection assays using a reporter gene construct documented that glucose deprivation increases AS gene transcription via elements within the proximal 3 kbp of the AS promoter. These results illustrate that human AS gene transcription is induced following glucose limitation of the cells.

Mechanistic issues in asparagine synthetase catalysis

The enzymatic synthesis of asparagine is an ATP-dependent process that utilizes the nitrogen atom derived from either glutamine or ammonia. Despite a long history of kinetic and mechanistic investigation, there is no universally accepted catalytic mechanism for this seemingly straightforward carboxyl group activating enzyme, especially as regards those steps immediately preceding amide bond formation. This chapter considers four issues dealing with the mechanism: (a) the structural organization of the active site(s) partaking in glutamine utilization and aspartate activation; (b) the relationship of asparagine synthetase to other amidotransferases; (c) the way in which ATP is used to activate the beta-carboxyl group; and (d) the detailed mechanism by which nitrogen is transferred.

Nucleotide sequence and deduced amino acid sequence of a putative asparagine synthetase in the mosquito Aedes aegypti (L.)

A cDNA was cloned from a Aedes aegypti head cDNA library, containing the complete coding sequence for an asparagine synthetase homolog. The predicted polypeptide sequence exhibits high homology with different proteins of the 'purF' glutamine amidotransferase enzyme family. The aminoterminal region, containing Cys-1 which is crucial to perform the glutaminase reaction, was highly conserved among the asparagine synthetase family. Subsequent expression of the cDNA yielded a 54,000 Da protein corresponding to the molecular weight of other asparagine synthetases.

Molecular cloning and characterization of a cDNA encoding asparagine synthetase from soybean (Glycine max L.) cell cultures

A cDNA encoding glutamine-dependent asparagine synthetase was isolated from dark-adapted Glycine max cell culture. The deduced amino acid sequence showed 76-89% identity with other plant sequences. The gene for asparagine synthetase is expressed predominantly in shoots as compared to roots of etiolated plants and the level of expression decreases following light treatment, suggesting that the gene expression is down-regulated by light.

Nitrogen assimilation in alfalfa: isolation and characterization of an asparagine synthetase gene showing enhanced expression in root nodules and dark-adapted leaves

Asparagine, the primary assimilation product from N2 fixation in temperate legumes and the predominant nitrogen transport product in many plant species, is synthesized via asparagine synthetase (AS; EC 6.3.5.4). Here, we report the isolation and characterization of a cDNA and a gene encoding the nodule-enhanced form of AS from alfalfa. The AS gene is comprised of 13 exons separated by 12 introns. The 5' flanking region of the AS gene confers nodule-enhanced reporter gene activity in transformed alfalfa. This region also confers enhanced reporter gene activity in dark-treated leaves. These results indicate that the 5' upstream region of the AS gene contains elements that affect expression in root nodules and leaves. Both AS mRNA and enzyme activity increased approximately 10- to 20-fold during the development of effective nodules. Ineffective nodules have strikingly reduced amounts of AS transcript. Alfalfa leaves have quite low levels of AS mRNA and protein; however, exposure to darkness resulted in a considerable increase in both. In situ hybridization with effective nodules and beta-glucuronidase staining of nodules from transgenic plants showed that AS is expressed in both infected and uninfected cells of the nodule symbiotic zone and in the nodule parenchyma. RNA gel blot analysis and in situ hybridization results are consistent with the hypothesis that initial AS expression in nodules is independent of nitrogenase activity.

Molecular cloning and expression of two cDNAs encoding asparagine synthetase in soybean

Two cDNA clones (SAS1 and SAS2) encoding different isoforms of asparagine synthetase (AS; EC 6.3.5.4) were isolated. Their DNA sequences were determined and compared. The amino-terminal residues of the predicted SAS1 and SAS2 proteins were identical to those of the glutamine binding domain of AS from pea, asparagus, Arabidopsis and human, suggesting that SAS1 and SAS2 cDNAs encode the glutamine-dependent form of AS. The open reading frames of SAS1 and SAS2 encode a protein of 579 and 581 amino acids with predicted molecular weights of 65182 and 65608 Da respectively. Similarity of the deduced amino acid sequences of SAS1 and SAS2 with other known AS sequences were 92% and 93% for pea AS1; 91% and 96% for pea AS2; 88% and 91% for asparagus; 88% and 90.5% for Arabidopsis; 70.5% and 72.5% for E. coli asnB and 61% and 63% for man. A plasmid, pSAS2E, was constructed to express the soybean AS protein in Escherichia coli. Complementation experiments revealed that the soybean AS protein was functional in E. coli. Southern blot analysis indicated that the soybean AS is part of a small gene family. AS transcript was expressed in all tissues examined, but higher levels were seen in stem and root of light-grown tissue and leaves of dark-treated tissue.

Lactobacillus bulgaricus asparagine synthetase and asparaginyl-tRNA synthetase: coregulation by transcription antitermination?

Genes encoding the ammonia-dependent asparagine synthetase (asnA) and asparaginyl-tRNA synthetase (asnS) have been cloned from Lactobacillus bulgaricus ATCC 11842. The nucleotide sequence suggests that asnA and asnS are organized as one operon and regulated by the tRNA-directed transcription antitermination mechanism (T. M. Henkin, Mol. Microbiol. 13:381-387, 1994).

Eukaryotic gene expression: metabolite control by amino acids

Our understanding of the metabolite control in mammalian cells lags far behind that in prokaryotes. This is particularly true for amino-acid-dependent gene expression. Few proteins have been identified for which synthesis is selectively regulated by amino-acid availability, and the mechanisms for control of transcription and translation in response to changes in amino-acid availability have not yet been elucidated. The intimate relationship between amino-acid supply and the fundamental cellular process of protein synthesis makes amino-acid-dependent control of gene expression particularly important. Future studies should provide important insight into amino-acid and other nutrient signaling pathways, and their impact on cellular growth and metabolism.

Metabolic regulation of the gene encoding glutamine-dependent asparagine synthetase in Arabidopsis thaliana

Here, we characterize a cDNA encoding a glutamine-dependent asparagine synthetase (ASN1) from Arabidopsis thaliana and assess the effects of metabolic regulation on ASN1 mRNA levels. Sequence analysis shows that the predicted ASN1 peptide contains a purF-type glutamine-binding domain. Southern blot experiments and cDNA clone analysis suggest that ASN1 is the only gene encoding glutamine-dependent asparagine synthetase in A. thaliana. The ASN1 gene is expressed predominantly in shoot tissues, where light has a negative effect on its mRNA accumulation. This negative effect of light on ASN1 mRNA levels was shown to be mediated, at least in part, via the photoreceptor phytochrome. We also investigated whether light-induced changes in nitrogen to carbon ratios might exert a metabolic regulation of the ASN1 mRNA accumulation. These experiments demonstrated that the accumulation of ASN1 mRNA in dark-grown plants is strongly repressed by the presence of exogenous sucrose. Moreover, this sucrose repression of ASN1 expression can be partially rescued by supplementation with exogenous amino acids such as asparagine, glutamine, and glutamate. These findings suggest that the expression of the ASN1 gene is under the metabolic control of the nitrogen to carbon ratio in cells. This is consistent with the fact that asparagine, synthesized by the ASN1 gene product, is a favored compound for nitrogen storage and nitrogen transport in dark-grown plants. We have put forth a working model suggesting that when nitrogen to carbon ratios are high, the gene product of ASN1 functions to re-direct the flow of nitrogen into asparagine, which acts as a shunt for storage and/or long-distance transport of nitrogen.

Fine mapping of the autosomal dominant split hand/split foot locus on chromosome 7, band q21.3-q22.1

Split hand/split foot (SHFD) is a human developmental defect characterized by missing digits, fusion of remaining digits, and a deep median cleft in the hands and feet. Cytogenetic studies of deletions and translocations associated with this disorder have indicated that an autosomal dominant split hand/split foot locus (gene SHFD1) maps to 7q21-q22. To characterize the SHFD1 locus, somatic cell hybrid lines were constructed from cytogenetically abnormal individuals with SHFD. Molecular analysis resulted in the localization of 93 DNA markers to one of 10 intervals surrounding the SHFD1 locus. The translocation breakpoints in four SHFD patients were encompassed by the smallest region of overlap among the SHfD-associated deletions. The order of DNA markers in the SHFD1 critical region has been defined as PON-D7S812-SHFD1-D7S811-ASNS. One DNA marker, D7S811, detected altered restriction enzyme fragments in three patients with translocations when examined by pulsed-field gel electrophoresis (PFGE). These data map SHFD1, a gene that is crucial for human limb differentiation, to a small interval in the q21.3-q22.1 region of human chromosome 7.

Isolation and characterization of a cDNA clone for a harvest-induced asparagine synthetase from Asparagus officinalis L

A full-length cDNA clone (pTIP27) encoding asparagine synthetase (AS; EC 6.3.5.4) was isolated from a cDNA library prepared from the tip section (apex to 30 mm) of Asparagus officinalis L. spears. The cDNA clone encodes an mRNA of 1978 bp, giving a derived protein of 66.5 kD molecular mass. The derived amino acid sequence is 81% homologous to AS from Pisum sativum. Only low levels of transcript for AS could be detected in growing spears, roots, or ferns. However, AS mRNA levels began to increase in the tips of harvested spears after 2 h at 20 degrees C, and in the other sections of the spear after 4 h, suggesting that all sections of the spear were responding to the same postharvest signal. The results are discussed in relation to metabolic changes occurring in harvested spears.

Cloning and sequence analysis of a cDNA for barley ferredoxin-dependent glutamate synthase and molecular analysis of photorespiratory mutants deficient in the enzyme

The NH2-terminal sequences of ferredoxin-dependent glutamate synthase (Fd-GOGAT; EC 1.4.7.1) purified from barley (Hordeum vulgare L.) and Chlamydomonas reinhardtii (Dangeard), and of a barley peptide, were determined and the barley sequences were used to design oligonucleotide primers for the polymerase chain reaction. A specific 1.3-kilobase (kb) cDNA fragment specifying the NH2-terminal one-third of the mature barley polypeptide, was amplified, cloned and sequenced. The NH2-terminus of plant Fd-GOGAT is highly conserved and homologous to the NH2-terminus of the heavy subunit of Escherichia coli NADPH-GOGAT. Based on sequence homologies, we tentatively identified the NH2-terminal region of Fd-GOGAT as the glutamine-amidotransferase domain, which is related to the corresponding domain of the purF-type amidotransferases. The Fd-GOGAT cDNA clone, and polyclonal antibodies raised against the barley enzyme, were used to analyse four Fd-GOGAT-deficient photorespiratory mutants. Three mutants (RPr 82/1, RPr 82/9 and RPr 84/82) had no detectable Fd-GOGAT protein in leaves, while the fourth (RPr 84/42) had a small amount of cross-reacting material. Hybridization to Northern blots of total leaf RNA revealed that both RPr 82/9 and RPr 84/82 were indistinguishable from the parental line (Maris Mink), having normal amounts of a 5.7-kb mRNA species. On the other hand, RPr 82/2 and RPr 84/42 each contained two distinct hybridizing RNA species, one of which was larger than 5.7 kb, the other smaller. Using a set of wheat-barley telosomic addition lines we have assigned the Fd-GOGAT structural locus to the short arm of chromosome 2.

Immunochemical characterization of rat testicular asparagine synthetase

We studied immunochemical properties of rat testicular asparagine synthetase. Western blot analysis of testis extract with polyclonal antibody raised against purified asparagine synthetase revealed an immunoreactive band at 62 kDa. The pancreas, brain, thymus, and spleen also showed 62-kDa bands. The intensities of these bands were roughly proportional to the specific activities of the enzyme in these tissues. The antibody showed some degree of cross-reactivity to asparagine synthetases from human, beef, pig, mouse, guinea pig, chicken, and frog, but not carp. But the enzyme from human HL-60 cells and lower vertebrates reacted with the antibody less strongly than enzyme from rats. The N-terminal amino acid sequence of the enzyme, determined by the Edman degradation method, in 10 recovered residues was identical to that of human asparagine synthetase deduced from corresponding cDNA (I.L. Andrulis et al., 1987, Mol. Cell. Biol. 7, 2435-2443). Immunohistochemical staining of the testis showed the presence of asparagine synthetase mainly in Sertoli cells in the seminiferous tubules.

Chemical modification of glucosamine-6-phosphate synthase by diethyl pyrocarbonate: evidence of histidine requirement for enzymatic activity

Glucosamine-6-phosphate synthase from Escherichia coli was inactivated by diethylpyrocarbonate at pH 7.3 and 4 degrees C with a second-order rate constant of 1220 M-1 min-1. The difference spectrum of inactivated vs native enzyme had a maximum absorption at 242 nm, which is characteristic of N-carbethoxyhistidine. No trough at around 280 nm due to O-carbethoxytyrosine was observed and the sulfhydryl content of the enzyme was unchanged. Studies with [14C]diethylpyrocarbonate provided evidence that derivatization of a single histidine residue of the amino-terminal glutamine-binding domain inactivated glucosamine-6P synthase. These results are consistent with the participation of an histidine residue in a catalytic triad, Cys/His/Asp, necessary to generate ammonia from glutamine.

Molecular and genetic characterization of human cell lines resistant to L-asparaginase and albizziin

Human cell lines resistant to L-asparaginase or albizziin were isolated by multistep selection of HT1080 fibrosarcoma and MIA PaCa-2 pancreatic carcinoma cells. Mutants were cross-resistant to both drugs, but more resistant to the drug used for selection. The drug-resistant cell lines expressed elevated levels of asparagine synthetase activity and protein, up to 17-fold over that of the parental cells. Enzyme overproduction was due to gene amplification in the albizziin-resistant cells, whereas increased expression without amplification was observed in L-asparaginase-resistant cells.