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1.
Low nitrate assimilation activity of the root nodules was demonstrated by assaying the activities of nitrate reductase, glutamate synthetase, glutamate dehydrogenase, and asparagine synthetase as well as the kinetics of 14C-labeled sucrose incorporation in the amino acids and amides of the cortex and the bacteroid-containing root nodule zones. Irrespective of the exogenous nitrogen concentration (0, 11.2, or 25 mM NO- 3), nitrate concentration in the nodules was low as compared to the plant roots, leaves, and stems. This allowed us to propose the presence of structural and/or metabolic barriers in the nodules limiting nitrate transport and assimilation in the nodule.  相似文献   

2.
The effect of amino acids on nitrate transport was studied in Zea mays cell suspension cultures and in Zea mays excised roots. The inclusion of aspartic acid, arginine, glutamine and glycine (15mM total amino acids) in a complete cell-culture media containing 1.0 mM NO3 - strongly inhibited nitrate uptake and the induction of accelerated uptake rates. The nitrate uptake rate increased sharply once solution amino acid levels fell below detection limits. Glutamine alone inhibited induction in the cell suspension culture. Maize seedlings germinated and grown for 7 days in a 15 mM mixture of amino acids also had lower nitrate uptake rates than seedlings grown in 0.5 mM Ca(NO3)2 or 1 mM CaCl2. As amino acids are the end product of nitrate assimilation, the results suggest an end-product feed-back mechanism for the regulation of nitrate uptake.  相似文献   

3.

Background

Chronic hypoxia is a major component of ischemic diseases such as stroke or myocardial infarction. Drosophila is more tolerant to hypoxia than most mammalian species. It is considered as a useful model organism to identify new mechanisms of hypoxic tolerance. The hypoxic tolerance of flies has previously been reported to be enhanced by low protein diets. This study analyses the mechanisms involved.

Results

Feeding adult Drosophila on a yeast diet dramatically reduced their longevities under chronic hypoxic conditions (5% O2). Mean and maximum longevities became close to the values observed for starving flies. The action of dietary yeast was mimicked by a whole casein hydrolysate and by anyone of the 20 natural amino acids that compose proteins. It was mimicked by amino acid intermediates of the urea cycle such as L-citrulline and L-ornithine, and by polyamines (putrescine, spermidine and spermine). α-difluoromethylornithine, a specific inhibitor of ornithine decarboxylase, partially protected hypoxic flies from amino acid toxicity but not from polyamine toxicity. N1-guanyl-1,7 diaminoheptane, a specific inhibitor of eIF5A hypusination, partially relieved the toxicities of both amino acids and polyamines.

Conclusion

Dietary amino acids reduced the longevity of chronically hypoxic flies fed on a sucrose diet. Pharmacological evidence suggests that the synthesis of polyamines and the hypusination of eIF5A contributed to the life-shortening effect of dietary amino acids.  相似文献   

4.
The mechanism of nitrate uptake for assimilation in procaryotes is not known. We used the radioactive isotope, 13N as NO3 -, to study this process in a prevalent soil bacterium, Pseudomonas fluorescens. Cultures grown on ammonium sulfate or ammonium nitrate failed to take up labeled nitrate, indicating ammonium repressed synthesis of the assimilatory enzymes. Cultures grown on nitrite or under ammonium limitation had measurable nitrate reductase activity, indicating that the assimilatory enzymes need not be induced by nitrate. In cultures with an active nitrate reductase, the form of 13N internally was ammonium and amino acids; the amino acid labeling pattern indicated that 13NO3 - was assimilated via glutamine synthetase and glutamate synthase. Cultures grown on tungstate to inactivate the reductase concentrated NO3 - at least sixfold. Chlorate had no effect on nitrate transport or assimilation, nor on reduction in cell-free extracts. Ammonium inhibited nitrate uptake in cells with and without active nitrate reductases, but had no effect on cell-free nitrate reduction, indicating the site of inhibition was nitrate transport into the cytoplasm. Nitrate assimilation in cells grown on nitrate and nitrate uptake into cells grown with tungstate on nitrite both followed Michaelis-Menten kinetics with similar K mvalues, 7 M. Both azide and cyanide inhibited nitrate assimilation. Our findings suggest that Pseudomonas fluorescens can take up nitrate via active transport and that nitrate assimilation is both inhibited and repressed by ammonium.  相似文献   

5.
Scott EE  Rothstein DE 《Oecologia》2011,167(2):547-557
The relationship between inorganic nitrogen (N) cycling and plant productivity is well established. However, recent research has demonstrated the ability of plants to take up low molecular weight organic N compounds (i.e., amino acids) at rates that often rival those of inorganic N forms. In this study, we hypothesize that temperate forest tree species characteristic of low-fertility habitats will prefer amino acids over species characteristic of high-fertility habitats. We measured the uptake of 15N-labeled amino acids (glycine, glutamine, arginine, serine), ammonium (NH4 +), and nitrate (NO3 ) by four tree species that commonly occur in eastern North America, where their abundances have been correlated with inorganic N availability. Specific uptake rates of amino acids were largely similar for all tree species; however, high-fertility species took up NH4 + at rates more than double those of low-fertility species, rendering amino acid N relatively more important to the N nutrition of low-fertility species. Low-fertility species acquired over four times more total N from arginine compared to NH4 + and NO3 ; high-fertility species acquired the most N from NH4 +. Arginine had the highest uptake rates of any amino acid by all species; there were no significant differences in uptake rates of the remaining amino acids. Our results support the idea that the dominant species in a particular habitat are those best able to utilize the most available N resources.  相似文献   

6.
The bindings of biogenic polyamines spermine (spm), spermidine (spmd) and synthetic polyamines 3,7,11,15-tetrazaheptadecane·4HCl (BE-333) and 3,7,11,15,19-pentazahenicosane·5HCl (BE-3333) with β-lactoglobulin (β-LG) were determined in aqueous solution. FTIR, UV-vis, CD and fluorescence spectroscopic methods as well as molecular modeling were used to determine the polyamine binding sites and the effect of polyamine complexation on protein stability and secondary structure. Structural analysis showed that polyamines bind β-LG via both hydrophilic and hydrophobic contacts. Stronger polyamine-protein complexes formed with synthetic polyamines than biogenic polyamines, with overall binding constants of Kspm-β-LG = 3.2(±0.6) × 104 M−1, Kspmd-β-LG = 1.8(±0.5) × 104 M−1, KBE-333-β-LG = 5.8(±0.3) × 104 M−1 and KBE-3333-β-LG = 6.2(±0.05) × 104 M−1. Molecular modeling showed the participation of several amino acids in the polyamine complexes with the following order of polyamine-protein binding affinity: BE-3333 > BE-333 > spermine > spermidine, which correlates with their positively charged amino group content. Alteration of protein conformation was observed with a reduction of β-sheet from 57% (free protein) to 55-51%, and a major increase of turn structure from 13% (free protein) to ∼21% in the polyamine-β-LG complexes, indicating a partial protein unfolding.  相似文献   

7.
Wild Type (WT) and transgenic tobacco plants expressing isopentenyltransferase (IPT), a gene encoding the enzyme regulating the rate-limiting step in cytokinins (CKs) synthesis, were grown under limited nitrogen (N) conditions. We analyzed nitrogen forms, nitrogen metabolism related-enzymes, amino acids and photorespiration related-enzymes in WT and PSARK∷IPT tobacco plants. Our results indicate that the WT plants subjected to N deficiency displayed reduced nitrate (NO3) assimilation. However, an increase in the production of ammonium (NH4+), by the degradation of proteins and photorespiration led to an increase in the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle in WT plants. In these plants, the amounts of amino acids decreased with N deficiency, although the relative amounts of glutamate and glutamine increased with N deficiency. Although the transgenic plants expressing PSARK∷IPT and growing under suboptimal N conditions displayed a significant decline in the N forms in the leaf, they maintained the GS/GOGAT cycle at control levels. Our results suggest that, under N deficiency, CKs prevented the generation and assimilation of NH4+ by increasing such processes as photorespiration, protein degradation, the GS/GOGAT cycle, and the formation of glutamine.  相似文献   

8.
A Mastigocladus species was isolated from the hot spring of Jakrem (Meghalaya) India. Uptake and utilization of nitrate, nitrite, ammonium and amino acids (glutamine, asparagine, arginine, alanine) were studied in this cyanobacterium grown at different temperatures (25°C, 45°C). There was 2–3 fold increase in the heterocyst formation and nitrogenase activity in N-free medium at higher temperature (45°C). Growth and uptake and assimilation of various nitrogen sources were also 2–3 fold higher at 45°C indicating that it is a thermophile. The extent of induction and repression of nitrate uptake by NO3 and NH4 +, respectively, differed from that of nitrite. It appeared that Mastigocladus had two independent nitrate/nitrite transport systems. Nitrate reductase and nitrite reductase activitiy was not NO3 -inducible and ammonium or amino acids caused only partial repression. Presence of various amino acids in the media partially repressed glutamine synthetase activity. Ammonium (methylammonium) and amino acid uptake showed a biphasic pattern, was energy-dependent and the induction of uptake required de novo protein synthesis. Ammonium transport was substrate (NH4 +)-repressible, while the amino acid uptake was substrate inducible. When grown at 25°C, the cyanobacterium formed maximum akinetes that remained viable upto 5 years under dry conditions.  相似文献   

9.
The effect of growth retarding amino acids (L-aspartic acid,L-leucine,L-methionine, andL-threonine) on nitrate reductase (NO3R), nitrite reductase (NO2R), and NADH2 dependent glutamate dehydrogenase (GDH) in excised pea roots was followed,L-methionine andL-threonine slightly depressed NO3R activity,L-aspartic acid enhanced NO2R and GDH activities.L-methionine andL-threonine also slightly decreased nitrate uptake. The results obtained are discussed in connection with the growth effects of the amino acids investigated.  相似文献   

10.
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