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1.
Acclimation of chlorophyll biosynthetic reactions to temperature stress in cucumber (Cucumis sativus L.) 总被引:1,自引:0,他引:1
The adaptive responses of the greening process of plants to temperature stress were studied in cucumber (Cucumis sativus L. cv. Poinsette) seedlings grown at ambient (25 °C), low (7 °C) and high (42 °C) temperatures. Plastids isolated from these
seedlings were incubated at different temperatures and the net syntheses of various tetrapyrroles were monitored. In plastids
isolated from control seedlings grown at 25 °C, the optimum temperature for synthesis of Mg-protoporphyrin IX monoester or
protochlorophyllide was 35 °C. Temperature maxima for Mg-protoporphyrin IX monoester and protochlorophyllide syntheses were
shifted to 30 °C in chill-stressed seedlings. The net synthesis of total tetrapyrroles was severely reduced in heat-stressed
seedlings and the optimum temperature for Mg-protoporphyrin IX monoester or protochlorophyllide synthesis shifted slightly
towards higher temperatures, i.e. a broader peak was observed. To further study the temperature acclimation of seedlings with
respect to the greening process, tetrapyrrole biosynthesis was monitored at 25 °C after pre-heating the plastids (28–70 °C)
isolated from control, chill- and heat-stressed seedlings. In comparison to 28 °C-pre-heated plastids the percent inhibition
of protochlorophyllide synthesis in 40 °C-pre-heated plastids was higher than for the control (25 °C-grown) in chill-stressed
seedlings and lower than for the control in heat-stressed seedlings. Maximum synthesis of total tetrapyrroles and protoporphyrin
IX was observed when chloroplasts were heated at 50 °C, which was probably due to heat-induced activation of the enzymes involved
in protoporphyrin IX synthesis. Prominent shoulders towards lower or higher temperatures were seen in chill-stressed or heat-stressed
seedlings, respectively. The shift in optimum temperature for tetrapyrrole biosynthesis in chill- and heat-stressed seedlings
was probably due to acclimation of membranes possibly undergoing desaturation or saturation of membrane lipids. Proteins synthesized
in response to temperature-stress may also play an important role in conferring stress-tolerance in plants.
Received: 8 October 1998 / Accepted: 19 November 1998 相似文献
2.
Robert K. Ellsworth Amy S. Hsing 《Biochimica et Biophysica Acta (BBA)/General Subjects》1973,313(1):119-129
Portions of crude homogenates of etiolated wheat seedlings incubated with Mg-protoporphyrin IX and and then added to other portions of the same crude homogenates that were pretreated with [1-3H]ethanol and yeast alcohol dehydrogenase provided, after a short reaction period, 3H-labeled Mg-protoporphyrin IX monomethyl ester. The 3H-labeled Mg-protoporphyrin IX monomethyl ester thus obtained was shown to contain the 3H in one reduced (to ethyl) vinyl side-chain. Subsequently, 3H-labeled Mg-monoethyl-(monodivinyl)-protoporphyrin IX monomethyl ester was obtained when Mg-protoporphyrin IX monomethyl ester and [3H]NADH were added to dialyzed crude homogenates of etiolated wheat seedlings. Insignificant amounts of 3H were incorporated into poprhyrin substrates when Mg-2,4-divinylpheoporphyrin a5 or [3H]NADPH were substituted in reaction mixtures for Mg-protoporphyrin IX monomethyl ester or [3H]NADPH, respectively. The results of these and further experiments suggest that an NADPH-dependent enzyme in the crude homogenates of etiolated wheat seedlings was capable of catalyzing the reduction to ethyl of one vinyl side-chain of Mg-protoporphyrin IX monomethyl ester. These findings suggest that the 4-vinyl side-chain reductive reaction likely occurs after the biosynthesis IX monomethyl ester, but before isocyclic ring formation in the pathway to chlorophyll a. 相似文献
3.
Chlamydomonas reinhardtii chloroplasts catalyzed two sequential steps of Chl biosynthesis, S-adenosyl-l-methionine:Mg-protoporphyrin IX methyltransferase and Mg-protoporphyrin IX monomethyl ester oxidative cyclase. A double mutant strain of C. reinhardtii was constructed which has a cell wall deficiency and is unable to form chlorophyll in the dark. Dark-grown cells were disrupted with a BioNeb nebulizer under conditions which lysed the plasma membrane but not the chloroplast envelope. Chloroplasts were purified by Percoll density gradient centrifugation. The purified chloroplasts were used to define components required for the biosynthesis of Mg-2,4-divinylpheoporphyrin a
5 (divinyl protochlorophyllide) from Mg-protoporphyrin IX. Product formation requires the addition of Mg-protoporphyrin IX, the substrate for S-adenosyl-l-methionine:Mg-protoporphyrin IX methyltransferase which produces Mg-protoporphyrin IX monomethyl ester. The Mg-protoporphyrin IX monomethyl ester that is generated in situ is the substrate for Mg-protoporphyrin IX monomethyl ester oxidative cyclase. The reaction product was identified as Mg-2,4-divinylpheoporphyrin a
5 (divinyl protochlorophyllide) by excitation and emission spectrofluorometry and HPLC on ion-paired reverse-phase and polyethylene columns. Mg-2,4-divinylpheoporphyrin a
5 formation by the coupled enzyme system required O2 and was stimulated by the addition of NADP+, an NADPH regenerating system, and S-adenosyl-l-methionine. Product was formed at a relatively steady rate for at least 60 min.Abbreviations MgDVP
Mg-2,4-divinylpheoporphyrin a
5 (divinyl protochlorophyllide)
- SAM
S-adenosyl-l-methionine 相似文献
4.
Free porphyrins and their magnesium complexes, including chlorophylls, are potent photo-sensitizers. Plants usually accumulate
these compounds bound to proteins together with protective compounds like carotenoids. Besides their protective role, carotenoids
can play a structural role in these complexes. To analyze the effect of impaired carotenogenesis on plastid membranes we applied
to barley seedlings the bleaching herbicide 2-(4-chlorophenylthio)triethylamine (CPTA) as a specific inhibitor for the cyclization
of lycopene. To avoid interference with photo-oxidation, the essential experiments were performed on seedlings grown in darkness.
While the amount of total carotenoids decreased, we found accumulation of more δ-carotene than lycopene in darkness clearly
showing that CPTA inhibits the lycopene β-cyclase more effectively than the lycopene ε-cyclase. The CPTA treatment resulted
in accumulation of non-photoactive protochlorophyllide a; the amount of photoactive protochlorophyllide and NADPH:protochlorophyllide oxidoreductase remained constant. Further, the
level of Mg protophorphyrin and its monomethyl ester increased to an extent similar to that obtained by application of 5-aminolevulinic
acid (ALA). The perturbation of the ultrastructure of etioplast inner membranes, observed after CPTA-treatment, was not found
after ALA-treatment; this excluded the accumulated tetrapyrroles as responsible for the perturbation. By contrast, the down-regulation
of Lhcb and RbcS genes found after CPTA-treatment was compatible with the presumed role of Mg protophorphyrin as “plastid signal” for regulation
of nuclear gene expression. Possible mechanisms for enhancement of tetrapyrrole accumulation by non-cyclic carotenoids are
discussed. 相似文献
5.
The influence of 2,2′-dipyridyl (2,2′-DP) on the activity of one of the enzymes at the initial stages of chlorophyll (Chl)
biosynthesis, δ-aminolevulinic acid dehydratase (ALAD; δ-aminolevulinate hydro-lyase, EC 4.2.1.24), as well as on δ-aminolevulinic
acid (ALA) accumulation was investigated in green barley (Hordeum vulgare L.) leaves. In seven-day-old green leaves treated with 3 mM 2,2′-DP for 17 h in darkness and subsequently irradiated with "white
light" (15 W m-2) for 4, 8, and 24 h the ALAD activity was 51 % as compared to that in untreated leaves. At the same time,
the ALA forming system was most sensitive to the photodynamic processes caused by 2,2′-DP. After 8 h of irradiation, ALA synthesis
was entirely inhibited. After the treatment the leaves accumulated exceptionally high amounts of Chl precursors such as protoporphyrin
IX (Proto), Mg-protoporphyrin IX (Mg-Proto), its monomethyl ester, and protochlorophyllide (Pchlide) that are photosensitizers
of photodynamic processes in plants. A comparatively low Chl and carotenoid (Car) destruction was registered during the subsequent
4 and 8 h of irradiation. At the same time, the content of Chl precursors was negligible. The low photodestruction of Chl
and Car included in pigment-protein complexes, against the background of fast porphyrin disappearance, and fast decrease of
enzymatic activities at the initial stages of Chl production could mean that the photodynamic effect induced by porphyrins
accumulated in the presence of 2,2′-DP affected first the Chl enzymatic system and did not change the pool of already synthesized
photosynthetic pigments.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
6.
Tetrapyrrole regulation of nuclear gene expression 总被引:5,自引:0,他引:5
Tetrapyrroles are the structural backbone of chlorophyll and heme, and are essential for primary photochemistry, light harvesting,
and electron transport. The biochemistry of their synthesis has been studied extensively, and it has been suggested that some
of the tetrapyrrole biochemical intermediates can affect nuclear gene expression. In this review, tetrapyrrole biosynthesis,
which occurs in the chloroplast, and its regulation will be covered. An analysis of the intracellular location of tetrapyrrole
intermediates will also be included. The focus will be on tetrapyrrole intermediates that have been suggested to affect gene
expression. These include Mg-protoporphyrin IX and Mg-protoporphyrin IX monomethyl ester. Recent evidence also suggests a
specific signaling role for the H subunit of Mg-chelatase, an enzyme that catalyzes the insertion of Mg into the tetrapyrrole
ring. Since gene expression studies have been done in plants and green algae, our discussion will be limited to these organisms.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
7.
Peter Mascia 《Molecular & general genetics : MGG》1978,161(3):237-244
Summary Several mutants of maize defective in chlorophyll synthesis are analysed. By feeding shoots of dark-grown seedlings -aminolevulinic acid, the regulatory step in chlorophyll biosynthesis is bypassed and chlorophyll precursors accumulate. In normal plants this results in a buildup of protoporphyrin IX and protochlorophyllide, while mutants accumulate precursors, depending on the site of the mutant-induced lesion. Mutants at three loci, l
*-Blandy4, 113, and oy, are defective in conversion of protoporphyrin IX to Mg-protoporphyrin. Mutants at the oro and oro2 loci are defective in conversion of Mg-protoporphyrin monomethyl ester to protochlorophyllide. A dominant modifier gene, Orom, which allows oro seedlings to bypass their lesion is also described.Journal Paper No J-9076 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa Project No. 2035 相似文献
8.
In the cyanobacterium Synechocystis sp. PCC 6803 five open reading frames (scpA–scpE) have been identified that code for single-helix proteins resembling helices I and III of chlorophyll a/b-binding (Cab) antenna proteins from higher plants. They have been named SCPs (small Cab-like proteins). Deletion of a single scp gene in a wild-type or in a photosystem I-less (PS I-less) strain has little effect. However, the effects of functional deletion of scpB or scpE were remarkable under conditions where chlorophyll availability was limited. When cells of a strain lacking PS I and chlL (coding for a polypeptide needed for light-independent protochlorophyllide reduction) were grown in darkness, the phycobilin and protochlorophyllide levels decreased upon deletion of scpB or scpE and the protoheme level was reduced in the strain lacking scpE. Addition of -aminolevulinic acid (ALA) in darkness drastically increased the level of Mg-protoporphyrin IX and Mg-protoporphyrin IX monomethyl ester in the PS I-less/chlL
–/scpE
– strain, whereas PChlide accumulated in the PS I-less/chlL
–/scpB
– strain. In the PS I-less/chlL
– control strain ALA supplementation did not lead to large changes in the levels of tetrapyrrole biosynthesis intermediates. We propose that ScpE and ScpB regulate tetrapyrrole biosynthesis as a function of pigment availability. This regulation occurs primarily at an early step of tetrapyrrole biosynthesis, prior to ALA. In view of the conserved nature of chlorophyll-binding sites in these proteins, it seems likely that regulation by SCPs occurs as a function of chlorophyll availability, with SCPs activating chlorophyll biosynthesis steps when they do not have pigments bound. 相似文献
9.
Formation of Mg-Containing Chlorophyll Precursors from Protoporphyrin IX, delta-Aminolevulinic Acid, and Glutamate in Isolated, Photosynthetically Competent, Developing Chloroplasts
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Intact developing chloroplasts isolated from greening cucumber (Cucumis sativus L. var Beit Alpha) cotyledons were found to contain all the enzymes necessary for the synthesis of chlorophyllide. Glutamate was converted to Mg-protoporphyrin IX (monomethyl ester) and protoclorophyllide. δ-Aminolevulinic acid and protoporphyrin IX were converted to Mg-protoporphyrin IX, Mg-protoporphyrin IX monomethyl ester, protochlorophyllide and chlorophyllide a. The conversion of δ-aminolevulinic acid or protoporphyrin IX to Mg-protoporphyrin IX (monomethyl ester) was inhibited by AMP and p-chloromercuribenzene sulfonate. Light stimulated the formation of Mg-protoporphyrin IX from all three substrates. In the case of δ-aminolevulinic acid and protoporphyrin IX, light could be replaced by exogenous ATP. In the case of glutamate, both ATP and reducing power were necessary to replace light. With all three substrates, glutamate, δ-aminolevulinic acid, and protoporphyrin IX, the stimulation of Mg-protoporphyrin IX accumulation in the light was abolished by DCMU, and this DCMU block was overcome by added ATP and reducing power. 相似文献
10.
Primary Site of Action of Amitrole in Arabidopsis thaliana Involves Inhibition of Root Elongation but Not of Histidine or Pigment Biosynthesis 总被引:2,自引:1,他引:1
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Interference with histidine metabolism, inhibition of pigment biosynthesis, or both have been the principal candidates for the primary site of action of 3-amino 1,2,4-triazole (amitrole). Arabidopsis thaliana is sensitive to 1,2,4-triazole-3-alanine, a feedback inhibitor of histidine biosynthesis, and this effect is reversed by histidine. The combination of triazolealanine and histidine, however, does not reverse the herbicidal effect of amitrole. This indicates that amitrole toxicity is not caused by histidine starvation, nor is it caused by the accumulation of a toxic intermediate of the histidine pathway. Amitrole inhibits root elongation at lower concentrations than it causes pigment bleaching in the leaves. In contrast, fluridone, a known inhibitor of the carotenoid biosynthetic pathway does not block root elongation. Fluridone also inhibits carotenoid accumulation in etiolated seedlings in the dark, but amitrole does not. Last, gabaculine and acifluorfen, but not amitrole, prevent chlorophyll accumulation in greening etiolated seedlings of Arabidopsis. These experiments cast doubt on pigment biosynthesis as the primary site of action of amitrole. 相似文献
11.
The RbcS multigene family of hexaploid (bread) wheat, Triticum aestivum (genome BBAADD), which encodes the small subunit of Rubisco, comprises at least 22 genes. Based on their 3′ non-coding sequences,
these genes have been classified into four subfamilies (SFs), of which three (SF-2, SF-3 and SF-4) are located on chromosomes
of homoeologous group 2 and one (SF-1) on homoeologous group 5. In the present study we hybridized three RbcS subfamily-specific probes (for SF-1, SF-2 and SF-3) to total DNA digested with four restriction enzymes and analyzed the
RFLP patterns of these subfamilies in eight diploid species of Aegilops and Triticum, and in two tetraploid and one hexaploid species of wheat (the diploid species are the putative progenitors of the polyploid
wheats). The three subfamilies varied in their level of polymorphism, with SF-2 being the most polymorphic in all species.
In the diploids, the order of polymorphism was SF-2 > SF-3 > SF-1, and in the polyploids SF-2 > SF-1 > SF-3. The RbcS genes of the conserved SF-1 were previously reported to have the highest expression levels in all the wheat tissues studied,
indicating a negative correlation between polymorphism and gene expression. Among the diploids, the species with the D and
the S genomes were the most polymorphic and the A-genome species were the least polymorphic. The polyploids were less polymorphic
than the diploids. Within the polyploids, the A genome was somewhat more polymorphic than the B genome, while the D genome
was the most conserved. Among the diploid species with the A genome, the RFLP pattern of T. urartu was closer to that of the A genome of the common wheat cultivar Chinese Spring (CS) than to that of T. monococcum. The pattern in Ae. tauschii was similar to that of the D genome of CS. Only partial resemblance was found between the RFLP patterns of the species with
the S genome and the B genome of CS.
Received: 10 February 2000 / Accepted: 21 February 2000 相似文献
12.
RegA control of bacteriochlorophyll and carotenoid synthesis in Rhodobacter capsulatus 总被引:1,自引:0,他引:1
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We provide in vivo genetic and in vitro biochemical evidence that RegA directly regulates bacteriochlorophyll and carotenoid biosynthesis in Rhodobacter capsulatus. beta-Galactosidase expression assays with a RegA-disrupted strain containing reporter plasmids for Mg-protoporphyrin IX monomethyl ester oxidative cyclase (bchE), Mg-protoporphyrin IX chelatase (bchD), and phytoene dehydrogenase (crtI) demonstrate RegA is responsible for fourfold anaerobic induction of bchE, threefold induction of bchD, and twofold induction of crtI. Promoter mapping studies, coupled with DNase I protection assays, map the region of RegA binding to three sites in the bchE promoter region. Similar studies at the crtA and crtI promoters indicate that RegA binds to a single region equidistant from these divergent promoters. These results demonstrate that RegA is directly responsible for anaerobic induction of bacteriochlorophyll biosynthesis genes bchE, bchD, bchJ, bchI, bchG, and bchP and carotenoid biosynthesis genes crtI, crtB, and crtA. 相似文献
13.
A quantitative study of indole-3-acetic acid (IAA) turnover, and the contribution of tryptophan-dependent and tryptophan-independent
IAA-biosynthesis pathways, was carried out using protoplast preparations and shoot apices obtained from wild-type and transgenic,
IAA-overproducing tobacco (Nicotiana tabacum L.) plants, during a phase of growth when the level of endogenous IAA was stable. Based on the rate of disappearance of [13C6]IAA, the half-life of the IAA pool was calculated to be 1.1 h in wild-type protoplasts and 0.8 h in protoplasts from the
IAA-overproducing line, corresponding to metabolic rates of 59 and 160 pg IAA (μg Chl)−1 h−1, respectively. The rate of conversion of tryptophan to IAA was 15 pg IAA (μg Chl)−1 h−1 in wild-type protoplasts and 101 pg IAA (μg Chl)−1 h−1 in protoplasts from IAA-overproducing plants. In both instances, IAA was metabolised more rapidly than it was synthesised
from tryptophan. As the endogenous IAA pools were in a steady state, these findings indicate that IAA biosynthesis via the
tryptophan-independent pathway was 44 pg IAA (μg Chl)−1 h−1 and 59 pg IAA (μg Chl)−1 h−1, respectively, in the wild-type and transformed protoplast preparations. In a parallel study with apical shoot tissue, the
presumed site of IAA biosynthesis, the rate of tryptophan-dependent IAA biosynthesis exceeded the rate of metabolism of [13C6]IAA despite the steady state of the endogenous IAA pool. The most likely explanation for this anomaly is that, unlike the
protoplast system, injection of substrates into the apical tissues did not result in uniform distribution of label, and that
at least some of the [2H5]tryptophan was metabolised in compartments not normally active in IAA biosynthesis. This demonstrates the importance of using
experimental systems where labelling of the precursor pool can be strictly controlled.
Received: 18 January 2000 / Accepted 24 February 2000 相似文献
14.
15.
Inhibition of plant protoporphyrinogen oxidase by the herbicide acifluorfen-methyl 总被引:1,自引:0,他引:1
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The effect of acifluorfen-methyl on tetrapyrrole synthesis in greening chloroplasts of Cucumis sativus was examined. Formation of Mg-proto-porphyrin IX from δ-aminolevulinate was reduced 98% by 10 micromolar acifluorfen-methyl. Conversion of protoporphyrin IX to Mg-protoporphyrin IX was unaffected, but protoporphyrin IX synthesis from δ-aminolevulinate was blocked, indicating a site of inhibition prior to the Mg-chelatase. The enzymic oxidation of protoporphyrinogen IX to protoporphyrin IX was highly sensitive to acifluorfen-methyl, indicating that the site of action of the herbicide is the protoporphyrinogen oxidase. (© 1989 FMC Corporation. All rights reserved.) 相似文献
16.
Subplastidic preparations from cotyledons of cucumber (Cucumis sativus L.) were tested for their ability to synthesize protoporphyrin IX from the substrate 5-aminolevulinic acid. Envelope or thylakoid
membranes failed to synthesize protoporphyrin IX from the substrate 5-aminolevulinic acid. Stromal preparations synthesized
a very low amount of protoporphyrin IX. In a reconstitution experiment using stroma + envelope membranes, protoporphyrin IX
synthesis from 5-aminolevulinic acid was enhanced by 660% over that of stroma alone. However, when thylakoids were added to
the stroma + envelope mixture, protoporphyrin IX synthesis from 5-aminolevulinic acid was completely inhibited. In the reconstituted
stroma + envelope membrane mixture, the reducing agent dithiothreitol enhanced the protoporphyrin IX-synthesizing ability
and completely abolished the inhibition of protoporphyrin IX synthesis by thylakoids. This suggested that the oxidizing agents
usually associated with the thylakoid membranes inhibited protoporphyrin IX biosynthesis and the inhibition was alleviated
by the reducing power of dithiothreitol. This study exposes the weakness of in vitro reconstitution experiments in mimicking
the in vivo-conditions. Addition of ATP stimulated protoporphyrin IX synthesis by 50% in the supernatant fraction of chloroplast
lysate. This ATP-induced stimulation of protoporphyrin IX synthesis was due to the enhancement of the activities of uroporphyrinogen
decarboxylase and protoporphyrinogen oxidase, involved in tetrapyrrole biosynthesis. The ATP-induced stimulation of porphyrinogen
oxidase activity was an energy-dependent reaction.
Received: 21 March 2000 / Accepted: 9 May 2000 相似文献
17.
In Vitro Synthesis of the Chlorophyll Isocyclic Ring : Transformation of Magnesium-Protoporphyrin IX and Magnesium-Protoporphyrin IX Monomethyl Ester into Magnesium-2,4-Divinyl Pheoporphyrin A(5) 总被引:10,自引:8,他引:2
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Developing chloroplasts of Cucumis sativus L., cv Beit Alpha which were incubated with either Mg-protoporphyrin IX or Mg-protoporphyrin IX monomethyl ester in darkness produced a partially phototransformable protochlorophyllide species that was tentatively identified as Mg-2,4-divinyl pheoporphyrin a5. S-Adenosylmethionine stimulated Mg-2,4-divinyl pheoporphyrin a5 formation irrespective of the starting material used. In the case of Mg-protoporphyrin IX monomethyl ester, this stimulation was attributed to the need to remethylate substrate that had been hydrolyzed by an endogenous methylesterase which converts part of the added Mg-protoporphyrin IX monomethyl ester to Mg-protoporphyrin IX.
NADP and NADPH stimulated the conversion of Mg-protoporphyrin IX to Mg-2,4-divinyl pheoporphyrin a5. The conversion required oxygen and was half saturated at 50 micromolar dissolved O2. The conversion was insensitive to inhibitors of iron-sulfur and heme-containing proteins, to Cu chelators, H2O2, and peroxide scavengers. However, the conversion was extremely sensitive to phenazine methosulfate, methylene blue, and methyl viologen.
A decrease of the plastids' ability to convert Mg-protoporphyrin IX to Mg-2,4-divinyl pheoporphyrin a5 after lysis in 0.1 molar NaCl suggested a requirement for plastid integrity.
相似文献18.
Effects of Iron and Oxygen on Chlorophyll Biosynthesis : I. IN VIVO OBSERVATIONS ON IRON AND OXYGEN-DEFICIENT PLANTS 总被引:7,自引:5,他引:2
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Corn (Zea mays, L.), bean (Phaseolus vulgaris L.), barley (Hordeum vulgare L.), spinach (Spinacia oleracea L.), and sugarbeet (Beta vulgaris L.) grown under iron deficiency, and Potamogeton pectinatus L, and Potamogeton nodosus Poir. grown under oxygen deficiency, contained less chlorophyll than the controls, but accumulated Mg-protoporphyrin IX and/or Mg-protoporphyrin IX monomethyl ester. No significant accumulation of these intermediates was detected in the controls or in the tissue of plants stressed by S, Mg, N deficiency, or by prolonged dark treatment. Treatment of normal plant tissue with δ-aminolevulinic acid in the dark resulted in the accumulation of protochlorophyllide. If this treatment was carried out under conditions of iron or oxygen deficiency, less protochlorophyllide was formed, but a significant amount of Mg-protoporphyrin IX and Mg-protoporphyrin IX monomethyl ester accumulated. 相似文献
19.
Heterologous expression of the bchM gene product from Rhodobacter capsulatus and demonstration that it encodes S-adenosyl-L-methionine:Mg-protoporphyrin IX methyltransferase. 总被引:2,自引:2,他引:2
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The bacteriochlorophyll biosynthesis gene, bchM, from Rhodobacter capsulatus was previously believed to code for a polypeptide involved in formation of the cyclopentone ring of protochlorophyllide from Mg-protoporphyrin IX monomethyl ester. In this study, R. capsulatus bchM was expressed in Escherichia coli and the gene product was subsequently demonstrated by enzymatic analysis to catalyze methylation of Mg-protoporphyrin IX to form Mg-protoporphyrin IX monomethyl ester. Activity required the substrates Mg-protoporphyrin IX and S-adenosyl-L-methionine. 14C-labeled product was formed in incubations containing 14C-methyl-labeled S-adenosyl-L-methionine. On the basis of these and previous results, we also conclude that the bchH gene, which was previously reported to code for Mg-protoporphyrin IX methyltransferase, is most likely involved in the Mg chelation step. 相似文献
20.
Exposure of barley (Hordeum vulgare L.) leaves to strong white light (1500 μmol photons · m−2 · s−1) decreased the quantum yield of photosynthetic oxygen evolution in green light preferentially absorbed by carotenoids (Φ-510)
but not in red light exclusively absorbed by chlorophylls (Φ-650). This phenomenon was observed to be (i) rapidly induced
(within a few minutes), (ii) slowly reversible in darkness (within about 1 h), (iii) insensitive to dithiothreitol and (iv)
maximally induced by photon flux densities higher than about 1000 μmol · m−2 · s−1. Determination of the carotenoid composition of the major light-harvesting complex of PSII (LHCII) and analysis of the thylakoid
membrane lipid fluidity before and after strong illumination of barley leaves in the presence or the absence of dithiothreitol
showed that the light-induced decrease in the Φ-510/Φ-650 ratio did not require the physical detachment of carotenoids from
the pigment antennae. Compared to barley plants grown under moderate light and temperature conditions, plants grown in sustained
high irradiance at elevated temperature exhibited (i) a lower Φ-510/Φ-650 ratio, (ii) a reduced size of the functional PSII
pigment antenna in green light (but not in red light) and (iii) a marked increase in the amount of free carotenoids found
in non-denaturing Deriphat-containing electrophoretic gels of thylakoid membranes. Similarly, the Φ-510/Φ-650 ratio of the
LHCII-deficient chlorina-f2 barley mutant was very low compared to the wild type. Separation and quantification of the cis/trans carotenoid isomers of barley leaves revealed that strong illumination did not induce pronounced cis-trans isomerization of xanthophylls. Taken together, the data suggest that the efficiency of energy transfer from carotenoids to
chlorophylls varies with the light environment both in the short term and in the long term, with excess light energy noticeably
inhibiting the photosynthetic light-harvesting function of carotenoids. The photoprotective significance of this carotenoid
decoupling from the chlorophyll antennae is discussed.
Received: 28 July 1997 / Accepted: 25 October 1997 相似文献