Mechanism of the mutagenic action of hydroxylamine. IX. The UV-induced cleavage of the N-O bond in N4-hydroxy-and N4-methoxycytidine and N6-methoxyadenosine.

The principal UV-induced (lambda = 2546nm) reaction of N4-hydroxy- and N4methoxycytidines and N6-methoxyadenosine in neutral aqueous solutions is cleavage of the exocyclic N-O bond with the respective formation of cytidine and adenosine. Quantum yields are 2.8x10(-3) and 2.2x10(-3) M/E for the first two compounds and 9.1x10(-3) M/E for N6-methoxyadenosine.     

Profiling Murine Tau with 0N, 1N and 2N Isoform-Specific Antibodies in Brain and Peripheral Organs Reveals Distinct Subcellular Localization,with the 1N Isoform Being Enriched in the Nucleus

In the adult murine brain, the microtubule-associated protein tau exists as three major isoforms, which have four microtubule-binding repeats (4R), with either no (0N), one (1N) or two (2N) amino-terminal inserts. The human brain expresses three additional isoforms with three microtubule-binding repeats (3R) each. However, little is known about the role of the amino-terminal inserts and how the 0N, 1N and 2N tau species differ. In order to investigate this, we generated a series of isoform-specific antibodies and performed a profiling by Western blotting and immunohistochemical analyses using wild-type mice in three age groups: two months, two weeks and postnatal day 0 (P0). This revealed that the brain is the only organ to express tau at significant levels, with 0N4R being the predominant isoform in the two month-old adult. Subcellular fractionation of the brain showed that the 1N isoform is over-represented in the soluble nuclear fraction. This is in agreement with the immunohistochemical analysis as the 1N isoform strongly localizes to the neuronal nucleus, although it is also found in cell bodies and dendrites, but not axons. The 0N isoform is mainly found in cell bodies and axons, whereas nuclei and dendrites are only slightly stained with the 0N antibody. The 2N isoform is highly expressed in axons and in cell bodies, with a detectable expression in dendrites and a very slight expression in nuclei. The 2N isoform that was undetectable at P0, in adult brain was mainly found localized to cell bodies and dendrites. Together these findings reveal significant differences between the three murine tau isoforms that are likely to reflect different neuronal functions.     

Bacterial immobilization and remineralization of N at different growth rates and N concentrations

An experiment was designed to resolve two largely unaddressed questions about the turnover of N in soils. One is the influence of microbial growth rate on mobilization and remineralization of cellular N. The other is to what extent heterotrophic immobilization of NO(3)(-) is controlled by the soil concentration of NH(4)(+). Bacteria were extracted from a deciduous forest soil and inoculated into an aqueous medium. Various N pool dilution/enrichment experiments were carried out to: (1) calculate the gross N immobilization and remineralization rates; (2) investigate their dependence on NH(4)(+)and NO(3)(-) concentrations; (3) establish the microbial preference for NH(4)(+)and NO(3)(-) depending on the NH(4)(+)/NO(3)(-) concentration ratio. Remineralization of microbial N occurred mainly at high growth rates and NH(4)(+) concentrations. There was a positive correlation between NH(4)(+) immobilization and remineralization rates, and intracellular recycling of N seemed to be an efficient way for bacteria to withstand low inorganic N concentrations. Thus, extensive remineralization of microbial N is likely to occur only when environmental conditions promote high growth rates. The results support previous observations of high NO(3)(-) immobilization rates, especially at low NH(4)(+) concentrations, but NO(3)(-) was also immobilized at high NH(4) concentrations. The latter can be understood if part of the microbial community has a preference for NO(3)(-) over NH(4)(+).     

减量施氮对冬小麦-夏玉米种植体系中氮利用与平衡的影响

研究了冬小麦-夏玉米种植体系中减量施N对作物N利用与平衡的影响,结果表明,与原有高量施N处理(N240和N360)相比,在冬小麦季减半施N未引起产量和吸N量的变化。但在原有低量施N处理(N120)下减半施N显著降低了小麦产量和吸N量;在夏玉米季,在上季减半施N的基础上停止施N后作物产量和吸N量均比原固定施N处理显著下降,N平衡计算结果表明,减量施N条件下0～1m土壤N残留和表观损失的数量均显著低于原有施N量处理,作物N利用率显著提高,但在1～2m层次中累积的硝态氮却不因减量施N而下降,说明这一土层的硝态氮可能难以被作物吸收利用,由此可见,在前茬高施N量下减少氮肥用量有利于提高作物的氮肥利用率、减少N残留与表观损失。     

古尔班通古特沙漠土壤酶活性和微生物量氮对模拟氮沉降的响应

本文以新疆古尔班通古特沙漠为研究区,原位设定0 (N0)、0.5 (N0.5)、1.0 (N1)、3.0 (N3)、6.0 (N6)和24.0 (N24) g N m?2 a-1 6个模拟施氮浓度,研究氮沉降对土壤酶活性和微生物量N的影响。结果表明：不同浓度的氮增加未改变土壤酶活性和微生物量N原有的垂直分布格局,0 ~ 5 cm土层土壤多酚氧化酶和过氧化物酶活性分别比5 ~ 10 cm土层低11.5 ~ 29.1%和1.4 ~ 14.2%,而该土层的蔗糖酶、脲酶、碱性磷酸酶活性和微生物量N则分别比5 ~ 10 cm土层高4.3 ~ 98.1%、45.3 ~ 119.0%、76.1 ~ 138.1%和77.5 ~ 162.3%。氮增加后,0 ~ 5 cm土层的土壤酶活性和微生物量N比5 ~ 10 cm土层受影响更大。低氮和中氮(N0.5～N3)增加对0 ~ 5 cm土层氧化酶活性影响较小,各处理间差异不显著;高氮(N6,N24)对该层氧化酶活性有明显抑制作用。与对照相比,N24处理下土壤多酚氧化物活性和过氧化物酶活性分别降低了22.4%和12.1%;5 ~ 10 cm土层氧化酶活性对氮增加响应不敏感,各施氮量之间差异不显著;两层土壤的蔗糖酶和碱性磷酸酶活性随氮的增加具有先增加再减少的趋势,而两层土壤的脲酶活性和土壤微生物量N随着施氮量增加分别降低和增加;随着土壤酶活性变化,土壤有效氮和微生物量N增加,有效磷先增加后减少。这些响应表明,氮增加可以改变该荒漠土壤系统的土壤酶活性和微生物量并影响土壤相关营养元素循环。     

Genetic Characterization of H1N1 and H1N2 Influenza A Viruses Circulating in Ontario Pigs in 2012

The objective of this study was to characterize H1N1 and H1N2 influenza A virus isolates detected during outbreaks of respiratory disease in pig herds in Ontario (Canada) in 2012. Six influenza viruses were included in analysis using full genome sequencing based on the 454 platform. In five H1N1 isolates, all eight segments were genetically related to 2009 pandemic virus (A(H1N1)pdm09). One H1N2 isolate had hemagglutinin (HA), polymerase A (PA) and non-structural (NS) genes closely related to A(H1N1)pdm09, and neuraminidase (NA), matrix (M), polymerase B1 (PB1), polymerase B2 (PB2), and nucleoprotein (NP) genes originating from a triple-reassortant H3N2 virus (tr H3N2). The HA gene of five Ontario H1 isolates exhibited high identity of 99% with the human A(H1N1)pdm09 [A/Mexico/InDRE4487/09] from Mexico, while one Ontario H1N1 isolate had only 96.9% identity with this Mexican virus. Each of the five Ontario H1N1 viruses had between one and four amino acid (aa) changes within five antigenic sites, while one Ontario H1N2 virus had two aa changes within two antigenic sites. Such aa changes in antigenic sites could have an effect on antibody recognition and ultimately have implications for immunization practices. According to aa sequence analysis of the M2 protein, Ontario H1N1 and H1N2 viruses can be expected to offer resistance to adamantane derivatives, but not to neuraminidase inhibitors.     

Effects of long-term free-air ozone fumigation on delta15N and total N in Fagus sylvatica and associated mycorrhizal fungi

Patterns of nitrogen (N) isotope composition (delta(15)N) and total N contents were determined in leaves, fine roots, root-associated ectomycorrhizal fungi (ECM) of adult beech trees (FAGUS SYLVATICA), and soil material under ambient (1 x O(3)) and double ambient (2 x O(3)) atmospheric ozone concentrations over a period of two years. From fine root to leaf material delta(15)N decreased consecutively. Under enhanced ozone concentrations total N was reduced in fine roots and delta(15)N showed a decrease in roots and leaves. In the soil and in most types of mycorrhizae, delta(15)N and total N were not altered due to ozone fumigation. The number of vital ectomycorrhizal root tips increased and the mycorrhizal community structure changed in 2 x O(3). Simultaneously, the specific rate of inorganic N-uptake by the roots was reduced under the double ozone regime. From these results it is assumed that 2 x O(3) changes N-nutrition of the trees at the level of N-acquisition, as indicated by enhanced mycorrhizal root tip density, altered mycorrhizal species composition, and reduced specific N-uptake rates.     

Hypnotic activity of N3-benzylthymidine on mice

The pharmacological effect in mice of N3-benzylthymidine (N3-ByTd) was examined by two routes of administration; intravenous (i.v.) and intracerebroventricular (i.c.v.), and compared with the effect of administration of N3-benzyluridine (N3-ByUd) previously reported. Hypnotic activity, pentobarbital (PB)-induced sleeping time, motor incoordination and spontaneous activity were used as indices of pharmacological effects. N3-ByTd (0.5-2.0 mumol/mouse, i.c.v.) and N3-ByUd (1.5-3.0 mumol/mouse, i.c.v.) were found to possess dose-dependent hypnotic activity, and N3-ByTd had more potent hypnotic activity than N3-ByUd. Both N3-ByTd and N3-ByUd (0.5 and 1.0 mumol/mouse, i.c.v., respectively) showed a synergistic effect on PB-induced sleep, although their parent compounds, thymidine (Td) and uridine (Ud), did not potentiate the activity at each dose. In motor incoordination, the effect of N3-ByTd (0.5 mumol/mouse) continued for 6 hr after i.c.v. injection. All compounds decreased the spontaneous activity of mice by i.c.v. administration. Furthermore, both N3-ByTd and N3-ByUd decreased the activity, when they were administered by i.v. These results reveal that both N3-benzylpyrimidine nucleosides have more direct depressant effects on the central nervous system (CNS) than the parent compounds. Among the pyrimidine nucleoside derivatives tested, N3-ByTd was found to be the most potent hypnotic substance.     

Amine inversion in proteins. A 13C-NMR study of proton exchange and nitrogen inversion rates in N epsilon,N epsilon,N alpha,N alpha-[13C]tetramethyllysine,N epsilon,N epsilon,N alpha,N alpha-[13C]tetramethyllysine methyl ester, and reductively methylated concanavalin A

Exchange rates were calculated as a function of pH from line widths of methylamine resonances in 13C-NMR spectra of N epsilon,N epsilon,N alpha,N alpha-[13C]tetramethyllysine (TML) and N epsilon,N epsilon,N alpha,N alpha-tetramethyllysine methyl ester (TMLME). The pH dependence of the dimethyl alpha-amine exchange rate could be adequately described by assuming base-catalyzed chemical exchange between two diastereotopic methyl populations related by nitrogen inversion. Deprotonation of the alpha-amine was assumed to occur by proton transfer to (1) OH-, (2) water, (3) a deprotonated amine or (4) RCO2-. Microscopic rate constants characterizing each of these transfer processes (k1, k2, k3 and k4, respectively) were determined by fitting the rates calculated from line width analysis to a steady-state kinetic model. Using this procedure it was determined that for both TML and TMLME k2 approximately equal to 1-10 M-1 s-1, k3 approximately equal to 10(6) M-1 s-1 and ki, the rate constant for nitrogen inversion was about 10(8)-10(9) s-1. Upper limits of 10(12) and 10(3) M-1 s-1 could be determined for k1 and k4, respectively. A similar kinetic analysis was used to explain pH-dependent line-broadening effects observed for the N-terminal dimethylalanyl resonance in 13C-NMR spectra of concanavalin A, reductively methylated using 90% [13C]formaldehyde. From exchange data below pH 4 it could be determined that amine inversion was limited by the proton transfer rate to the solvent, with a rate constant estimated at 20 M-1 s-1. Above pH 4, exchange was limited by proton transfer to other titrating groups in the protein structure. Based upon their proximity, the carboxylate side chains of Asp-2 and Asp-218 appear to be likely candidates. The apparent first-order microscopic rate constant characterizing proton transfer to these groups was estimated to be about 1 X 10(4) s-1. Rate constants characterizing nitrogen inversion (ki), proton transfer to OH- (k1) and proton transfer to the solvent (k2) were estimated to be of the same order of magnitude as those determined for the model compounds. On the basis of our results, it is proposed that chemical exchange processes associated with base-catalyzed nitrogen inversion may contribute to 15N or 13C spin-lattice relaxation times in reductively methylated peptides or proteins.     

N,N-dimethylcarbamyl derivatives of oxazepam

Three N,N-dimethylcarbamyl derivatives of oxazepam (1-(N,N-dimethylcarbamyl)oxazepam, 3-O-(N,N-dimethylcarbamyl)oxazepam, and 1,3-O-bis(N,N-dimethylcarbamyl) oxazepam) and a 3-O-acyl-1-(N,N-dimethylcarbamyl)-oxazepam were synthesized from either oxazepam or demoxepam. Enantiomeric pairs of these derivatives and of camazepam were resolved by high-performance liquid chromatography on at least two of three commercially available chiral stationary phase columns employed. Absolute configurations of resolved enantiomers were established by comparing their circular dichroism spectra to those of enantiomeric oxazepams with known absolute stereochemistry. Similar to those of oxazepam, enantiomers of 1-(N,N-dimethylcarbamyl)oxazepam undergo rapid racemization (t1/2 1.9 min at 23 degrees C and 0.9 min at 37 degrees C) in an aqueous solution at pH 7.5. The (R)-enantiomer of rac-3-O-acyl-1-(N,N-dimethylcarbamyl)oxazepam was hydrolyzed approximately 4.6-fold faster than the (S)-enantiomer by esterases in rat liver microsomes, whereas the (S)-enantiomer was hydrolyzed approximately 43-fold faster than the (R)-enantiomer by esterases in rat brain S9 fraction.     

A theory for 15N/14N fractionation in nitrate-grown vascular plants

We present a theory describing how the δ¹⁵N values of the nitrogen (N) pools in a vascular plant depend on that of its source N (nitrate), on ¹⁵N/¹⁴N fractionations during N assimilation, and on N transport within and N loss from the plant. The theory allows measured δ¹⁵N values to be interpreted in terms of physiological processes. The δ¹⁵N values of various N pools are calculated using three rules: (1) when a pool divides without transformation, there is no change in the δ¹⁵N values of the N entering the resulting pools; (2) when nitrate is assimilated by nitrate reductase, the δ¹⁵N values of the resulting pools (product and residual substrate) are described by a Rayleigh equation; (3) when two N pools mix, the δ¹⁵N value of the mixture is a weighted average of the δ¹⁵N values of the component pools. The theory is written as a spreadsheet and solved numerically. Potentially, it has multiple solutions. Some contravene physiological reality and are rejected. The remainder are distinguished, where possible, using additional physiological information. The theory simulated independent measurements of δ¹⁵N in N pools of Brassica campestris L. var. rapa (komatsuna) and Lycopersicon esculentum Mill. cv. T-5 (tomato). Received: 27 October 1997 / Accepted: 13 January 1998     

Simultaneous quantification of N2, NH3 and N2O emissions from a flooded paddy field under different N fertilization regimes

Gaseous nitrogen (N) emissions, especially emissions of dinitrogen (N₂) and ammonia (NH₃), have long been considered as the major pathways of N loss from flooded rice paddies. However, no studies have simultaneously evaluated the overall response of gaseous N losses to improved N fertilization practices due to the difficulties to directly measure N₂ emissions from paddy soils. We simultaneously quantified emissions of N₂ (using membrane inlet mass spectrometry), NH₃ and nitrous oxide (N₂O) from a flooded paddy field in southern China over an entire rice‐growing season. Our field experiment included three treatments: a control treatment (no N addition) and two N fertilizer (220 kg N/ha) application methods, the traditional surface application of N fertilizer and the incorporation of N fertilizer into the soil. Our results show that over the rice‐growing season, the cumulative gaseous N losses from the surface application treatment accounted for 13.5% (N₂), 19.1% (NH₃), 0.2% (N₂O) and 32.8% (total gaseous N loss) of the applied N fertilizer. Compared with the surface application treatment, the incorporation of N fertilizer into the soil decreased the emissions of NH₃, N₂ and N₂O by 14.2%, 13.3% and 42.5%, respectively. Overall, the incorporation of N fertilizer into the soil significantly reduced the total gaseous N loss by 13.8%, improved the fertilizer N use efficiency by 14.4%, increased the rice yield by 13.9% and reduced the gaseous N loss intensity (gaseous N loss/rice yield) by 24.3%. Our results indicate that the incorporation of N fertilizer into the soil is an effective agricultural management practice in ensuring food security and environmental sustainability in flooded paddy ecosystems.     

Two forms of N intermediate (N(open) and N(closed)) in the bacteriorhodopsin photocycle.

Glutaraldehyde, aluminum ions and glycerol (that inhibit the M intermediate decay in the wild-type bacteriorhodopsin and azide-induced M decay in the D96N mutant by stabilization of the M(closed)) accelerate the N decay in the D96N mutant. The aluminum ions, the most potent activator of the N decay, induce a blue shift of the N difference spectrum by approximately 10 nm. Protonated azide as well as acetate and formate inhibit the N decay in both the D96N mutant and the wild-type protein. It is concluded that the N intermediate represents, in fact, an equilibrium mixture of the two ('open' and 'closed') forms. These two forms, like M(closed) and M(open), come to an equilibrium in the microseconds range. The absorption spectrum of the N(open) is slightly shifted to red in comparison to that of the N(closed). Again, this resembles the M forms. 13-cis-all-trans re-isomerization is assumed to occur in the N(closed) form only. Binding of 1-2 molecules of protonated azide stabilizes the N(open) form. Existence of the 'open' and 'closed' forms of the M and N intermediates provides the appropriate explanation of the cooperative phenomenon as well as some other effects on the bacteriorhodopsin photocycle. Summarizing the available data, we suggest that M(open) is identical to the M(N) form, whereas M1 and M2 are different substates of M(closed).     

Coupled soil-availability and tree-limitation nutritional shifts induced by N deposition: insights from N to P relationships in Abies pinsapo forests

Background and aims

Interacting effects of atmospheric N deposition on the degree to which tree demand for other nutrients is met by soil supply has seldom been explored in Mediterranean-type ecosystems. We hypothesized that patterns for the relative availability of N and P in soils will be matched by variations in process rates related to soil organic P cycling and by shifts from N to P limitation of tree growth.

Methods

We examined N/P relationships in Mediterranean-fir (Abies pinsapo) forests from two nearby regions differing in N deposition levels.

Results

N pools and transformation rates and the contribution of organic fractions to the labile P pool in soils showed increasing trends toward the pollution source. Phosphomonoesterase activity (PME) in bulk soils, root PME per unit biomass (but not per unit soil volume) and biomass accumulation in P-fertilized root-in-growth cores incubated in situ were also the highest at the sites receiving elevated N deposition, indicating P limitation. In contrast, forest stands in the region farther from the pollutant source were N-limited (preferential root growth in N-rich soil microsites) and showed lower PME activities and higher total fine root biomass.

Conclusions

In the forests under elevated N deposition, higher values for an overall indicator of soil N status matched with indications of an accelerated soil organic P subcycle and P-limitation of tree growth.     

Sequence homology between Inc N group plasmids

DNA-DNA hybridization combined with "Southern blotting" was used to analyse the genetic organization and the nucleotide sequence homology between different regions of a previously characterized Inc N group plasmid pCUI and nine other Inc N group plasmids. The following conclusions could be reached: (1) N plasmids isolated from different parts of the world share substantial DNA sequence homology and also some similarity of overall genetic organization, (2) the majority of the N plasmids used in this study showed conservation of distribution of BglII and KpnI cleavage sites. Often, restriction endonuclease fragments of similar electrophoretic mobility encoded the same genetic function, (3) in one case, the N-specific properties appear to be integrated into the bacterial chromosome. (4) the plasmid DNA in strains carrying two Inc N plasmids, R199 and R113 were each composed of two molecular species only one of which constituted an N group plasmid.     

Vertical transport of dissolved organic C and N under long-term N amendments in pine and hardwood forests

At the Harvard Forest, Massachusetts, a long-term effort is under way to study responses in ecosystem biogeochemistry to chronic inputs of N in atmospheric deposition in the region. Since 1988, experimental additions of NH₄NO₃ (0, 5 and 15 g N m^–2 yr^–1) have been made in two forest stands:Pinus resinosa (red pine) and mixed hardwood. In the seventh year of the study, we measured solute concentrations and estimated solute fluxes in throughfall and at two soil depths, beneath the forest floors (Oa) and beneath the B horizons.Beneath the Oa, concentrations and fluxes of dissolved organic C and N (DOC and DON) were higher in the coniferous stand than in the hardwood stand. The mineral soil exerted a strong homogenizing effect on concentrations beneath the B horizons. In reference plots (no N additions), DON composed 56% (pine) and 67% (hardwood) of the total dissolved nitrogen (TDN) transported downward from the forest floor to the mineral soil, and 98% of the TDN exported from the solums. Under N amendments, fluxes of DON from the forest floor correlated positively with rates of N addition, but fluxes of inorganic N from the Oa exceeded those of DON. Export of DON from the solums appeared unaffected by 7 years of N amendments, but as in the Oa, DON composed smaller fractions of TDN exports under N amendments. DOC fluxes were not strongly related to N amendment rates, but ratios of DOC:DON often decreased.The hardwood forest floor exhibited a much stronger sink for inorganic N than did the pine forest floor, making the inputs of dissolved N to mineral soil much greater in the pine stand. Under the high-N treatment, exports of inorganic N from the solum of the pine stand were increased >500-fold over reference (5.2 vs. 0.01 g N m^–2 yr^–1), consistent with other manifestations of nitrogen saturation. Exports of N from the solum in the pine forest decreased in the order NO₃-N> NH₄-N> DON, with exports of inorganic N 14-fold higher than exports of DON. In the hardwood forest, in contrast, increased sinks for inorganic N under N amendments resulted in exports of inorganic N that remained lower than DON exports in N-amended plots as well as the reference plot.     

Pathogenicity and transmission in pigs of the novel A(H3N2)v influenza virus isolated from humans and characterization of swine H3N2 viruses isolated in 2010-2011

Swine influenza virus (SIV) H3N2 with triple reassorted internal genes (TRIG) has been enzootic in Unites States since 1998. Transmission of the 2009 pandemic H1N1 (pH1N1) virus to pigs in the United States was followed by reassortment with endemic SIV, resulting in reassorted viruses that include novel H3N2 genotypes (rH3N2p). Between July and December 2011, 12 cases of human infections with swine-lineage H3N2 viruses containing the pandemic matrix (pM) gene [A(H3N2)v] were detected. Whole-genome analysis of H3N2 viruses isolated from pigs from 2009 to 2011 sequenced in this study and other available H3N2 sequences showed six different rH3N2p genotypes present in the U.S. swine population since 2009. The presence of the pM gene was a common feature among all rH3N2p genotypes, but no specific genotype appeared to predominate in the swine population. We compared the pathogenic, transmission, genetic, and antigenic properties of a human A(H3N2)v isolate and two swine H3N2 isolates, H3N2-TRIG and rH3N2p. Our in vivo study detected no increased virulence in A(H3N2)v or rH3N2p viruses compared to endemic H3N2-TRIG virus. Antibodies to cluster IV H3N2-TRIG and rH3N2p viruses had reduced cross-reactivity to A(H3N2)v compared to other cluster IV H3N2-TRIG and rH3N2p viruses. Genetic analysis of the hemagglutinin gene indicated that although rH3N2p and A(H3N2)v are related to cluster IV of H3N2-TRIG, some recent rH3N2p isolates appeared to be forming a separate cluster along with the human isolates of A(H3N2)v. Continued monitoring of these H3N2 viruses is necessary to evaluate the evolution and potential loss of population immunity in swine and humans.     

Synthesis of N2, N2, 7-trimethylguanosine cap derivatives.

Several derivatives of N2,N2-7-trimethylguanosine (m3(2,2,7G)-cap, which was found at the 5' ends of small nuclear RNAs, were synthesized by use of S-phenyl N2,N2,7-trimethylguanosine 5'-phosphorothioate (PhSpm3(2,2,7)G) as a key intermediate. This compound was activated by iodine in the presence of phosphoric acid and diphosphoric acid to give N2,N2,7-trimethylguanosine-5'-diphosphate (ppm3(2,2,7)G) and 5'-triphosphate (ppm3(2,2,7)G), respectively. Similar reactions of PhSpm3(2,2,7)G with ADP and GDP gave capped dinucleoside triphosphates, m3(2,2,7)G5'pppA and m3(2,2,7)G5'pppG, respectively.     

Regulation of nitrogenase activity in Bradyrhizobium japonicum/soybean symbiosis by plant N status as determined by shoot C:N ratio

Regulation of nitrogenase is not sufficiently understood to engineer symbioses that achieve a high N₂ fixation rate under high levels of soil N. In the present hydroponic growth chamber study we evaluated the hypothesis that nitrogenase activity and the extent of its inhibition by NO₃⁻ may be related to both N and carbohydrate levels in plant tissues. A wide range of C:N ratios in various plant tissues (8.5 to 41.0, 1.9 to 3.7, and 0.8 to 1.8, respectively, in shoots, roots, and nodules) was generated through a combination of light and CO₂ levels, using two soybean genotypes differing in C and N acquisition rates. For both genotypes, N concentration in shoots was negatively correlated to nitrogenase activity and positively correlated to the extent of nitrogenase inhibition by NO₃⁻. Furthermore, nitrogenase activity was positively correlated to total nonstructural carbohydrates (TNC) and C:N ratio in shoot and nodules for both genotypes. Nitrogenase inhibition by NO₃⁻ was negatively correlated to TNC and C:N ratio in shoots, but not in nodules for both genotypes. At the onset of nitrogenase inhibition by NO₃⁻, C:N ratio declined in shoots but not in nodules. These results indicate that both C and N levels in plant tissues are involved in regulation of nitrogenase activity. We suggest that the level of nitrogenase activity may be determined by (1) N needs (as determined by shoot C:N) and (2) availability of carbohydrates in nodules. Modulation of the nitrogenase activity may occur through sensing changes in plant N, i.e. changes in shoot C:N ratio, possibly through some phloem translocatable compound(s).     

Regulation of nitrogenase activity in Bradyrhizobium japonicum/soybean symbiosis by plant N status as determined by shoot C:N ratio

Regulation of nitrogenase is not sufficiently understood to engineer symbioses that achieve a high N₂ fixation rate under high levels of soil N. In the present hydroponic growth chamber study we evaluated the hypothesis that nitrogenase activity and the extent of its inhibition by NO₃⁻ may be related to both N and carbohydrate levels in plant tissues. A wide range of C:N ratios in various plant tissues (8.5 to 41.0, 1.9 to 3.7, and 0.8 to 1.8, respectively, in shoots, roots, and nodules) was generated through a combination of light and CO₂ levels, using two soybean genotypes differing in C and N acquisition rates. For both genotypes, N concentration in shoots was negatively correlated to nitrogenase activity and positively correlated to the extent of nitrogenase inhibition by NO₃⁻. Furthermore, nitrogenase activity was positively correlated to total nonstructural carbohydrates (TNC) and C:N ratio in shoot and nodules for both genotypes. Nitrogenase inhibition by NO₃⁻ was negatively correlated to TNC and C:N ratio in shoots, but not in nodules for both genotypes. At the onset of nitrogenase inhibition by NO₃⁻, C:N ratio declined in shoots but not in nodules. These results indicate that both C and N levels in plant tissues are involved in regulation of nitrogenase activity. We suggest that the level of nitrogenase activity may be determined by (1) N needs (as determined by shoot C:N) and (2) availability of carbohydrates in nodules. Modulation of the nitrogenase activity may occur through sensing changes in plant N, i.e. changes in shoot C:N ratio, possibly through some phloem translocatable compound(s).