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971.
Transgenic tobacco (Nicotiana tabacum L.) plants with decreased and increased transport capacities of the chloroplast triose phosphate/phosphate translocator (TPT)
were used to study the control the TPT exerts on the flux of starch and sucrose biosynthesis, as well as CO2 assimilation, respiration and photosynthetic electron transport. For this purpose, tobacco lines with an antisense repression
of the endogenous TPT (αTPT) and tobacco lines overexpressing a TPT gene from Flaveria trinervia (FtTPT) were used. In ambient CO2, there was no or little effect of altered TPT transport activities on either rates of photosynthetic electron transport and/or
CO2 assimilation. However, in elevated CO2 (1500 μl · l−1) and low O2 (2%) the TPT exerted strong control on the rate of CO2 assimilation (control coefficient for the wild type; CJA
TPT=0.30) in saturating light. Similarly, the incorporation of 14C into starch in high CO2 was increased in tobacco plants with decreased TPT activity, but was reduced in plants overexpressing the TPT from F. trinervia. Thus, the TPT exerted negative control on the rate of starch biosynthesis with a CJStarch
TPT=−0.19 in the wild type estimated from a hyperbolic curve fitted to the data points. This was less than the positive control
strength on the rate of sucrose biosynthesis (CJSuc
TPT=0.35 in the wild type). Theoretically, the positive control exerted on sucrose biosynthesis should be numerically identical
to the negative control on starch biosynthesis unless additional metabolic pathways are affected. The rate of dark respiration
showed some correlation with the TPT activity in that it increased in FtTPT overexpressors, but decreased in αTPT plants with
an apparent control coefficient of CJRes
TPT=0.24. If the control on sucrose biosynthesis is referred to as “gain of carbon” (positive control) and the control on starch
biosynthesis as well as dark respiration as a “loss of carbon” (negative control) for sucrose biosynthesis and subsequent
export, the sum of the control coefficients on dark respiration and starch biosynthesis would be numerically similar to the
control coefficient on the rate of sucrose biosynthesis. There was also some control on the rate of photosynthetic electron
transport, but only at high light and in elevated CO2 combined with low O2. The control coefficient for the rate of photosynthetic electron transport was CJETR
TPT=0.16 in the wild type. Control coefficients were also calculated for plants with elevated and lowered TPT activity. Furthermore,
the extent to which starch degradation/glucose utilisation compensates for the lack of triose phosphate export was assessed.
The TPT also exerted control on metabolite contents in air.
Received: 26 March 1999 / Accepted: 21 August 1999 相似文献
972.
In malic enzyme-dependent crassulacean-acid-metabolism (ME-CAM) plants, malic acid is decarboxylated by NADP-ME and NAD-ME and generates pyruvate with CO2. Pyruvate is phosphorylated to phosphoenolpyruvate by pyruvate, Pi dikinase (PPDK) and is then conserved in gluconeogenesis. Although PPDK was considered to be located in chloroplasts (e.g., Mesembryanthemum crystallinum), it has recently been found to accumulate in both the chloroplasts and the cytosol in two Kalancho? species. In this study, the intracellular localization of PPDK was investigated in 22 ME-CAM species in 13 genera of 5 families by immunogold labeling and electron microscopy. This revealed that the pattern of intracellular localization of PPDK varies among the ME-CAM plants and is divided into three types: Chlt, in which PPDK accumulates only in the chloroplasts; Cyt-Chlt, in which PPDK accumulates in both chloroplasts and cytosol; and Cyt, in which PPDK accumulates predominantly in the cytosol. Members of a particular genus tend to have a common PPDK-localization type. In the Cactaceae, all species from seven genera were classified as Cyt. The photosynthetic tissues of all ME-CAM species, including the Cyt type, had substantial PPDK activity, suggesting that PPDK in the cytosol is active and probably plays a functional role. In the Chlt species, NADP-ME activity was relatively greater than NAD-ME activity. In the Cyt-Chlt and Cyt species, however, either the activity of NAD-ME was higher than that of NADP-ME or they were approximately the same. The species variation in the intracellular localization of PPDK is discussed in relation to CAM function and to molecular and phylogenetic aspects. 相似文献
973.
Mesnard F Azaroual N Marty D Fliniaux MA Robins RJ Vermeersch G Monti JP 《Planta》2000,210(3):446-453
Nitrogen metabolism was monitored in suspension cultured cells of Nicotiana plumbaginifolia Viv. using nuclear magnetic resonance (NMR) spectroscopy following the feeding of (15NH4)2SO4 and K15NO3. By using two-dimensional 15N-1H NMR with heteronuclear single-quantum-coherence spectroscopy and heteronuclear multiple-bond-coherence spectroscopy sequences,
an enhanced resolution of the incorporation of 15N label into a range of compounds could be detected. Thus, in addition to the amino acids normally observed in one-dimensional
15N NMR (glutamine, aspartate, alanine), several other amino acids could be resolved, notably serine, glycine and proline. Furthermore,
it was found that the peak normally assigned to the non-protein amino-acid γ-aminobutyric acid in the one-dimensional 15N NMR spectrum was resolved into a several components. A peak of N-acetylated compounds was resolved, probably composed of the intermediates in arginine biosynthesis, N-acetylglutamate and N-acetylornithine and, possibly, the intermediate of putrescine degradation into γ-aminobutyric acid, N-acetylputrescine. The occurrence of 15N-label in agmatine and the low detection of labelled putrescine indicate that crucial intermediates of the pathway from glutamate
to polyamines and/or the tobacco alkaloids could be monitored. For the first time, labelling of the peptide glutathione and
of the nucleotide uridine could be seen.
Received: 29 March 1999 / Accepted: 15 July 1999 相似文献
974.
Astroglia-rich primary cultures and brain slices rapidly metabolize branched-chain amino acids (BCAAs), in particular leucine, as energy substrates. To allocate the capacity to degrade leucine oxidatively in neural cells, we have purified beta-methylcrotonyl-CoA carboxylase (beta-MCC) from rat liver as one of the enzymes unique for the irreversible catabolic pathway of leucine. Polyclonal antibodies raised against beta-MCC specifically cross-reacted with both enzyme subunits in liver and brain homogenates. Immunocytochemical examination of astroglia-rich rat primary cultures demonstrated the presence of beta-MCC in astroglial cells, where the enzyme was found to be located in the mitochondria, the same organelle that the mitochondrial isoform of the BCA(A) aminotransferase (BCAT) is located in. This colocalization of the two enzymes supports the hypothesis that mitochondrial BCAT is the isoenzyme that in brain energy metabolism prepares the carbon skeleton of leucine for irreversible degradation in astrocytes. Analysis of neuron-rich primary cultures revealed also that the majority of neurons contained beta-MCC. The presence of beta-MCC in most neurons demonstrates their ability to degrade the alpha-ketoisocaproate that could be provided by neighboring astrocytes or could be generated locally from leucine by the action of the cytosolic isoform of BCAT that is known to occur in neurons. 相似文献
975.
The inhibitory or antibiotic action of four kinds of the selenomorpholine complex on a strain of Escherichia coli was studied by microcalorimetry. Differences in their capacities to inhibit the metabolism of this bacterium were observed.
The extent and duration of the inhibitory effect on the metabolism as judged from the rate constant, k, and the half-inhibitory concentration, IC50, varied with the different drugs. The rate constant (k) of Escherichia coli (in the log phase) in the presence of the drugs decreased with increasing concentrations of the drugs (C). The relationship of k and C is nearly linear for (1) selenomorpholine and (2) selenomorpholine hydrochloride, but for (3) N,N′-methylene bisselenomorpholine and (4) N-dodecyl selenomorpholine, it is not linear. The experimental results reveal that the sequence of antibiotic activity of selenomorpholines
is (3) and (4)>(1)>(2). 相似文献
976.
977.
Methodological developments in recent years have led to an increase in empirical databases on the abundance and functions of aquatic microbes, now allowing synthesis studies. Most of these studies have adopted a comparative approach, such that comparative analyses are now available for most aspects of aquatic microbial food webs (more than 50 papers published in the last 15 years). Some of these analyses apparently yield conflicting results, introducing confusion and unnecessary disputes in the field. We briefly review the comparative analyses so far produced and we highlight generalities, show that some of the perceived discrepancies largely derive from partial analyses of a general underlying trend and formulate predictions based on these general trends that provide new avenues for research. 相似文献
978.
979.
Riding the sulfur cycle--metabolism of sulfonates and sulfate esters in gram-negative bacteria 总被引:1,自引:0,他引:1
Kertesz MA 《FEMS microbiology reviews》2000,24(2):135-175
Sulfonates and sulfate esters are widespread in nature, and make up over 95% of the sulfur content of most aerobic soils. Many microorganisms can use sulfonates and sulfate esters as a source of sulfur for growth, even when they are unable to metabolize the carbon skeleton of the compounds. In these organisms, expression of sulfatases and sulfonatases is repressed in the presence of sulfate, in a process mediated by the LysR-type regulator protein CysB, and the corresponding genes therefore constitute an extension of the cys regulon. Additional regulator proteins required for sulfonate desulfonation have been identified in Escherichia coli (the Cbl protein) and Pseudomonas putida (the AsfR protein). Desulfonation of aromatic and aliphatic sulfonates as sulfur sources by aerobic bacteria is oxygen-dependent, carried out by the alpha-ketoglutarate-dependent taurine dioxygenase, or by one of several FMNH(2)-dependent monooxygenases. Desulfurization of condensed thiophenes is also FMNH(2)-dependent, both in the rhodococci and in two Gram-negative species. Bacterial utilization of aromatic sulfate esters is catalyzed by arylsulfatases, most of which are related to human lysosomal sulfatases and contain an active-site formylglycine group that is generated post-translationally. Sulfate-regulated alkylsulfatases, by contrast, are less well characterized. Our increasing knowledge of the sulfur-regulated metabolism of organosulfur compounds suggests applications in practical fields such as biodesulfurization, bioremediation, and optimization of crop sulfur nutrition. 相似文献
980.
Isolation and characterisation of ethoprophos-degrading bacteria 总被引:1,自引:0,他引:1
An enrichment culture technique was used to isolate bacteria responsible for the enhanced biodegradation of ethoprophos in a soil from Northern Greece. Restriction fragment length polymorphism patterns of the 16S rRNA gene, partial 16S rRNA sequence analysis, and sodium dodecylsulfate-polyacrylamide gel electrophoresis total protein profile analysis were used to characterise the isolated bacteria. Two of the three ethoprophos-degrading cultures were pure and both isolates were classified as strains of Pseudomonas putida (epI and epII). The third culture comprised three distinct components, a strain identical to P. putida epI and two strains with 16S rRNA sequence similarity to Enterobacter strains. Isolate epI effectively removed a fresh ethoprophos addition from both fumigated and non-fumigated soil when introduced at high inoculum density, but removed it only from fumigated soil at low inoculum density. Isolates epI and epII degraded cadusafos, isazofos, isofenphos and fenamiphos, but only at a slow rate. This high substrate specificity was attributed to minor (cadusafos), or major (isazofos, isofenphos, fenamiphos) structural differences from ethoprophos. Studies with (14)C-labelled ethoprophos indicated that isolates epI and epII degraded the nematicide by removing the S-propyl moiety. 相似文献