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1.
Seven-day-old dark-grown bean leaves were greened under continuous light. The amount of chlorophyll, the ratio of chlorophyll a to chlorophyll b, the O2 evolving capacity and the primary photochemical activities of Photosystem I and Photosystem II were measured on the leaves after various times of greening. The primary photochemical activities were measured as the photo-oxidation of P700, the photoreduction of C-550, and the photo-oxidation of cytochrome b559 in intact leaves frozen to −196 C. The results indicate that the reaction centers of Photosystem I and Photosystem II begin to appear within the first few minutes and that Photosystem II reaction centers accumulate more rapidly than Photosystem I reaction centers during the first few hours of greening. The very early appearances of the primary photochemical activity of Photosystem II was also confirmed by light-induced fluorescence yield measurements at −196 C.  相似文献   

2.
C.J. Arntzen  C.L. Ditto 《BBA》1976,449(2):259-274
When isolated chloroplasts from mature pea (Pisum sativum) leaves were treated with digitonin under “low salt” conditions, the membranes were extensively solubilized into small subunits (as evidenced by analysis with small pore ultrafilters). From this solubilized preparation, a photochemically inactive chlorophyll · protein complex (chlorophyll ab ratio, 1.3) was isolated. We suggest that the detergent-derived membrane fragment from mature membranes is a structural complex within the membrane which contains the light-harvesting chlorophyll ab protein and which acts as a light-harvesting antenna primarily for Photosystem II.Cations dramatically alter the structural interaction of the light-harvesting complex with the photochemically active system II complex. This interaction has been measured by determining the amount of protein-bound chlorophyll b and Photosystem II activity which can be released into dispersed subunits by digitonin treatment of chloroplast lamellae. When cations are present to cause interaction between the Photosystem II complex and the light-harvesting pigment · protein, the combined complexes pellet as a “heavy” membranous fraction during differential centrifugation of detergent treated lamellae. In the absence of cations, the two complexes dissociate and can be isolated in a “light” submembrane preparation from which the light-harvesting complex can be purified by sucrose gradient centrifugation.Cation effects on excitation energy distribution between Photosystems I and II have been monitored by following Photosystem II fluorescence changes under chloroplast incubation conditions identical to those used for detergent treatment (with the exception of chlorophyll concentration differences and omission of detergents). The cation dependency of the pigment · protein complex and Photosystem II reaction center interactions measured by detergent fractionation, and regulation of excitation energy distribution as measured by fluorescence changes, were identical. We conclude that changes in substructural organization of intact membranes, involving cation induced changes in the interaction of intramembranous subunits, are the primary factors regulating the distribution of excitation energy between Photosystems II and I.  相似文献   

3.
A model is presented describing the relationship between chlorophyll fluorescence quenching and photoinhibition of Photosystem (PS) II-dependent electron transport in chloroplasts. The model is based on the hypothesis that excess light creates a population of inhibited PS II units in the thylakoids. Those units are supposed to posses photochemically inactive reaction centers which convert excitation energy to heat and thereby quench variable fluorescence. If predominant photoinhibition of PS II and cooperativity in energy transfer between inhibited and active units are presumed, a quasi-linear correlation between PS II activity and the ratio of variable to maximum fluorescence, FVFM, is obtained. However, the simulation does not result in an inherent linearity of the relationship between quantum yield of PS II and FVFM ratio. The model is used to fit experimental data on photoinhibited isolated chloroplasts. Results are discussed in view of current hypotheses of photoinhibition.Abbreviations FM maximum total fluorescence - F0 initial fluorescence - FV maximum variable fluorescence - PS Photosystem - QA, QB primary and secondary electron acceptors of Photosystem II  相似文献   

4.
We have examined tobacco transformed with an antisense construct against the Rieske-FeS subunit of the cytochromeb 6 f complex, containing only 15 to 20% of the wild-type level of cytochrome f. The anti-Rieske-FeS leaves had a comparable chlorophyll and Photosystem II reaction center stoichiometry and a comparable carotenoid profile to the wild-type, with differences of less than 10% on a leaf area basis. When exposed to high irradiance, the anti-Rieske-FeS leaves showed a greatly increased closure of Photosystem II and a much reduced capacity to develop non-photochemical quenching compared with wild-type. However, contrary to our expectations, the anti-Rieske-FeS leaves were not more susceptible to photoinhibition than were wild-type leaves. Further, when we regulated the irradiance so that the excitation pressure on photosystem II was equivalent in both the anti-Rieske-FeS and wild-type leaves, the anti-Rieske-FeS leaves experienced much less photoinhibition than wild-type. The evidence from the anti-Rieske-FeS tobacco suggests that rapid photoinactivation of Photosystem II in vivo only occurs when closure of Photosystem II coincides with lumen acidification. These results suggest that the model of photoinhibition in vivo occurring principally because of limitations to electron withdrawal from photosystem II does not explain photoinhibition in these transgenic tobacco leaves, and we need to re-evaluate the twinned concepts of photoinhibition and photoprotection.Abbreviations Chl chlorophyll - DCMU 3-(3,4-dichlophenyl)-1,-dimethylurea - Fo and Fo minimal fluorescence when all PS II reaction centers are open in dark- and light-acclimated leaves, respectively - Fm and Fm maximal fluorescence when all PS II reaction centers are closed in dark- and light-acclimated leaves, respectively - Fv variable fluorescence (Fm-Fo) in dark acclimated leaves - Fv variable fluorescence (Fm-Fo) in lightacclimated leaves - NPQ non-photochemical quenching of fluorescence - PS I and PS II Photosystem I and II - P680 primary electron donor of the reaction center of PS II - PFD photosynthetic flux density - QA primary acceptor quinone of PS II - qp photochemical quenching of fluorescence - V+A+Z violaxanthin+antheraxanthin+zeaxanthin  相似文献   

5.
In Cryptomonas rufescens (Cryptophyceae), phycoerythrin located in the thylakoid lumen is the major accessory pigment. Oxygen action spectra prove phycoerythrin to be efficient in trapping light energy.The fluorescence excitation spectra at ?196°C obtained by the method of Butler and Kitajima (Butler, W.L. and Kitajima, M. (1975) Biochim. Biophys. Acta 396, 72–85) indicate that like in Rhodophycease, chlorophyll a is the exclusive light-harvesting pigment for Photosystem I.For Photosystem II we can observe two types of antennae: (1) a light-harvesting chlorophyll complex connected to Photosystem II reaction centers, which transfers excitation energy to Photosystem I reaction centers when all the Photosystem II traps are closed. (2) A light-harvesting phycoerythrin complex, which transfers excitation energy exclusively to the Photosystem II reaction complexes responsible for fluorescence at 690 nm.We conclude that in Cryptophyceae, phycoerythrin is an efficient light-harvesting pigment, organized as an antenna connected to Photosystem II centers, antenna situated in the lumen of the thylakoid. However, we cannot afford to exclude that a few parts of phycobilin pigments could be connected to inactive chlorophylls fluorescing at 690 nm.  相似文献   

6.
Elisha Tel-Or  Shmuel Malkin 《BBA》1977,459(2):157-174
The photochemical activities and fluorescence properties of cells, spheroplasts and spheroplast particles from the blue-green alga Phormidium luridum were compared. The photochemical activities were measured in a whole range of wavelengths and expressed as quantum yield spectra (quantum yield vs. wavelength). The following reactions were measured: Photosynthesis (O2 evolution) in whole cells; Hill reaction (O2 evolution) with Fe(CN)63? and NADP as electron acceptors (Photosystem II and Photosystem II+Photosystem I reactions); electron transfer from reduced 2,6-dichlorophenolindophenol to diquat (Photosystem I reaction). The fluorescence properties were emission spectra, quantum yield spectra and the induction pattern.On the basis of comparison between the quantum yield spectra and the pigments compositions the relative contribution of each pigment to each photosystem was estimated. In normal cells and spheroplasts it was found that Photosystem I (Photosystem II) contains about 90 % (10 %) of the chlorophyll a, 90 % (10 %) of the carotenoids and 15 % (85 %) of the phycocyanin. In spheroplast particles there is a reorganization of the pigments: they loose a certain fraction (about half) of the phycocyanin but the remaining phycocyanin attaches itself exclusively to Photosystem I (!). This is reflected by the loss of Photosystem II activity, a flat quantum yield vs. wavelength dependence and a loss of the fluorescence induction.The fluorescence quantum yield spectra conform qualitatively to the above conclusion. More quantitative estimation shows that only a fraction (20–40 %) of the chlorophyll of Photosystem II is fluorescent. Total emission spectrum and the ratio of variable to constant fluorescence are in agreement with this conclusion.The fluorescence emission spectrum shows characteristic differences between the constant and variable components. The variable fluorescence comes exclusively from chlorophyll a; the constant fluorescence is contributed, in addition to chlorophyll a, by phycocyanine and an unidentified long wavelength component.The variable fluorescence does not change in the transition from whole cells to spheroplasts. However, the constant fluorescence increases considerably. This indicates the release of a small fraction of pigments from the photosynthetic photochemical apparatus which then become fluorescent.  相似文献   

7.
Addition of ATP to chloroplasts causes a reversible 25–30% decrease in chlorophyll fluorescence. This quenching is light-dependent, uncoupler insensitive but inhibited by DCMU and electron acceptors and has a half-time of 3 minutes. Electron donors to Photosystem I can not overcome the inhibitory effect of DCMU, suggesting that light activation depends on the reduced state of plastoquinone. Fluorescence emission spectra recorded at ?196°C indicate that ATP treatment increases the amount of excitation energy transferred to Photosystem I. Examination of fluorescence induction curves indicate that ATP treatment decreases both the initial (Fo) and variable (Fv) fluorescence such that the ratio of Fv to the maximum (Fm) yield is unchanged. The initial sigmoidal phase of induction is slowed down by ATP treatment and is quenched 3-fold more than the exponential slow phase, the rate of which is unchanged. A plot of Fv against area above the induction curve was identical plus or minus ATP. Thus ATP treatment can alter quantal distribution between Photosystems II and I without altering Photosystem II-Photosystem II interaction. The effect of ATP strongly resembles in its properties the phosphorylation of the light-harvesting complex by a light activated, ATP-dependent protein kinase found in chloroplast membranes and could be the basis of physiological mechanisms which contribute to slow fluorescence quenching in vivo and regulate excitation energy distribution between Photosystem I and II. It is suggested that the sensor for this regulation is the redox state of plastoquinone.  相似文献   

8.
Ta-Yan Leong  Jan M. Anderson 《BBA》1984,766(3):533-541
Light quality was shown to exert well-coordinated regulatory effects on the composition and function of the thylakoid membranes as well as on the photosynthetic rates of intact leaves from Atriplex triangularis grown in continuous blue, white and red lights (50 μE · m?2 · s?1). The higher photosynthetic rates in plants grown in blue light, as compared to those in white and red lights, resulted from marked changes in both light-harvesting complexes and electron carriers. The concentrations of electron carriers such as atrazine binding sites, plastoquinone, cytochromes b and f and P-700 on a chlorophyll basis were markedly increased in Atriplex grown in blue light; and the apparent light-harvesting antenna unit sizes of Photosystems I and II were greatly reduced. Consequently, the electron transport capacities of Photosystems I and II were also increased as was the coupling factor CF1 activity. Atriplex grown in red light had lower photosynthetic rates than those grown in blue or white light by incorporating changes in the composition and function of the thylakoids in a direction opposite to those caused by growth in blue light. When these regulatory effects of light quality were compared with those of light quantity [6,7], it is clear that ChlaChl b ratios, electron transport capacities of Photosystems I and II, concentrations of plastoquinone, atrazine binding sites, coupling factor CF1 activity and the apparent antenna unit size of Photosystem II are more affected by light quantity, whereas light quality has a greater influence on the concentration of P-700, the apparent antenna unit size of Photosystem I and the overall photosynthetic rates of intact leaves.  相似文献   

9.
Effects of photoinhibition on photosynthesis in pea (Pisum sativum L.) leaves were investigated by studying the relationship between the severity of a photoinhibitory treatment (measured as Fv/Fm) and several photoacoustic and chlorophyll a fluorescence parameters. Because of the observed linear relationship between the decline of Fv/Fm and the potential oxygen evolution rate determined by the photoacoustic method, the parameter Fv/Fm was used as an indicator for the severity of photoinhibition. Our analysis revealed that part of the Photosystem II (PS II) reaction centers is inactive in oxygen evolution and is also less sensitive to photoinhibition. Correcting the parameter qP (fraction of open PS II reaction centers) for inactive PS II centers unveiled a strong increase of qP in severely inhibited pea leaves, indicating that the inactivated active centers do no longer contribute to qP and that photoinhibition has an all or none effect on PS II centers. Analysis of qE (energy quenching) demonstrated its initial increase possibly associated with dephosphorylation of LHC II. Analysis of qI (photoinhibition dependent quenching) showed that the half-time of recovery of qI increases steeply below an Fv/Fm of 0.65. This increase of the relaxation half-time corresponds with a decrease of the electron transport rate J and tentatively indicates that the supply of ATP, needed for the recovery, starts to decrease. The data indicate the necessity of correcting for inactive centers in order to make valuable conclusions about effects of photoinhibition on photosynthetic parameters.  相似文献   

10.
The effect of anthocyanic cells of the epidermal layer was investigated on photosynthetic activity of the higher plant Tradescantia pallida. To determine the possible indirect role of anthocyanin in photosynthesis, analysis was done on intact leaves and leaves where anthocyanic epidermal layer was removed. Energy dissipation processes related to Photosystem II (PSII) and Photosystem I (PSI) activity was done using simultaneously Chlorophyll a (Chl a) fluorescence and P700 transmittance signals change. In anthocyanic epidermal-less leaves, PSII photochemical activity was more decreased in dependence to increasing light irradiance exposure. We found that photoinhibition of PSII decreased PSI activity by reducing the electron flow toward PSI, especially under high light intensities. Under those conditions, it resulted in the accumulation of oxidized PSI reaction centers, which was stronger in leaves where the anthocyanic epidermal layer was removed. In conclusion, our results showed that the anthocyanic epidermal layer had a photoprotective effect only on the PSII and not on the PSI of T. pallida leaves, supporting the role of anthocyanin pigments in the regulation of photosynthesis for excess absorbed light irradiance.  相似文献   

11.
Chlorella was used to study the effects of dehydration on photosynthetic activities. The use of unicellular green algae assured that the extent of dehydration was uniform throughout the whole cell population during the course of desiccation. Changes in the activities of the cells were monitored by measurements of fluorescence induction kinetics. It was found that inhibition of most of the photosynthetic activities started at a similar level of cellular water content. They included CO2 fixation, photochemical activity of Photosystem II and electron transport through Photosystem I. The blockage of electron flow through Photosystem I was complete and the whole transition occurred within a relative short time of dehydration. On the other hand, the suppression of Photosystem II activity was incomplete and the transition took a longer time of dehydration. Upon rehydration, the inhibition of Photosystem II activity was fully reversible when samples were in the middle of the transition, but was not thereafter. The electron transport through Photosystem I was also reversible during the transition, but was only partially afterward.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - Fm maximum fluorescence yield - F0 non-variable fluorescence level emitted when all PS II centers are open - Fv variable part of fluorescence - PS photosystem - QA primary quinone acceptor of Photosystem II  相似文献   

12.
Fractions enriched in either Photosystem I or Photosystem II activity have been isolated from the blue-green alga, Synechococcus cedrorum after digitonin treatment. Sedimentation of this homogenate on a 10–30% sucrose gradient yielded three green bands: the upper band was enriched in Photosystem II, the lowest band was enriched in Photosystem I, while the middle band contained both activities. Large quantities of both particles were isolated by zonal centrifugation, and the material was then further purified by chromatography on DEAE-cellulose.The resulting Photosystem II particles carried out light-induced electron transport from semicarbizide to ferricyanide of over 2000 μmol/mg Chlorophyll per h (which was sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea), and was nearly devoid of Photosystem I activity. This particle contains β-carotene, very little phycocyanin, has a chlorophyll absorption maximum at 675 nm, and a liquid N2 fluorescence maximum at 685 nm. The purest Photosystem II particles have a chlorophyll to cytochrome b-559 ratio of 50 : 1. The Photosystem I particle is highly enriched in P-700, with a chlorophyll to P-700 ratio of 40 : 1. The physical structure of the two Photosystem particles has also been studied by gel electrophoresis and electron microscopy. These results indicate that the size and protein composition of the two particles are distinctly different.  相似文献   

13.
The analysis of FDMR spectra, recorded at multiple emission wavelengths, by a global decomposition technique, has allowed us to characterise the triplet populations associated with Photosystem I and Photosystem II of thylakoids in the green alga Chlamydomonas reinhardtii. Three triplet populations are observed at fluorescence emissions characteristic of Photosystem II, and their zero field splitting parameters have been determined. These are similar to the zero field parameters for the three Photosystem II triplets previously reported for spinach thylakoids, suggesting that they have a widespread occurrence in nature. None of these triplets have the zero field splitting parameters characteristic of the Photosystem II recombination triplet observed only under reducing conditions. Because these triplets are generated under non-reducing redox conditions, when the recombination triplet is undetectable, it is suggested that they may be involved in the photoinhibition of Photosystem II. At emission wavelengths characteristic of Photosystem I, three triplet populations are observed, two of which are attributed to the P700 recombination triplet frozen in two different conformations, based on the microwave-induced fluorescence emission spectra and the triplet minus singlet difference spectra. The third triplet population detected at Photosystem I emission wavelengths, which was previously unresolved, is proposed to originate from the antenna chlorophyll of the core or the unusually blue-shifted outer antenna complexes of this organism.  相似文献   

14.
K. Satoh  R. Strasser  W.L. Butler 《BBA》1976,440(2):337-345
Photosystem I activity of Tris-washed chloroplasts was measured at room temperature as the rate of photoreduction of NADP and as the rate of oxygen uptake mediated by methyl viologen in both cases using dichlorophenolindophenol plus ascorbate as the source of electrons for Photosystem I. With both assay systems the rate of electron transport by Photosystem I was stimulated approx. 20 % by the addition of 3-(3,4-dichlorophenyl)-1, 1-dimethylurea which caused the Photosystem II reaction centers to close. Photosystem I activity of chloroplasts was measured at low temperature as the rate of photooxidation of P-700. Chloroplasts suspended in the presence of hydroxylamine and 3-(3,4-dichlorophenyl)-1, 1-dimethylurea were frozen to ?196 °C after adaptation to darkness or after a preillumination at room temperature. The Photosystem II reaction centers of the frozen dark-adapted sample were all open; those of the preilluminated sample were all closed. The rate of photooxidation of P-700 at ?196 °C with the preilluminated sample was approx. 25 % faster than with the dark-adapted sample. We conclude from both the room temperature and the low temperature experiments that there is greater energy transfer from Photosystem II to Photosystem I when the Photosystem II reaction centers are closed and that these results are a direct demonstration of spillover.  相似文献   

15.
Chloroplast thylakoid protein phosphorylation produces changes in light-harvesting properties and in membrane structure as revealed by freeze-fracture electron microscopy. Protein phosphorylation resulted in an increase in the 77 °K fluorescence signal at 735 nm relative to that at 685 nm. In addition, a decrease in connectivity between Photosystem II centers (PS II) and a dynamic quenching of the room temperature variable fluorescence was observed upon phosphorylation. Accompanying these fluorescence changes was a 23% decrease in the amount of stacked membranes. Microscopic analyses indicated that 8.0-nm particles fracturing on the P-face moved from the stacked into the unstacked regions upon phosphorylation. The movement of the 8.0-nm particles was accompanied by the appearance of chlorophyll b and 25 to 29 kD polypeptides in isolated stroma lamellae fractions. We conclude that phosphorylation of a population of the light-harvesting chlorophyll ab protein complexes (LHC) in grana partitions causes the migration of these pigment proteins from the PS II-rich appressed membranes into the Photosystem I (PS I) enriched unstacked regions. This increases the absorptive cross section of PS I. In addition, we suggest that the mobile population of LHC functions to interconnect PS II centers in grana partitions; removal of this population of LHC upon phosphorylation limits PS II → PS II energy transfer and thereby favors spillover of energy from PS II to PS I.  相似文献   

16.
Kenneth Leto  Charles Arntzen 《BBA》1981,637(1):107-117
Despite the total loss of Photosystem II activity, thylakoids isolated from the green nuclear maize mutant hcf1-3 contain normal amounts of the light-harvesting chlorophyll ab pigment-protein complex (LHC). We interpret the spectroscopic and ultrastructural characteristics of these thylakoids to indicate that the LHC present in these membranes is not associated with Photosystem II reaction centers and thus exists in a ‘free’ state within the thylakoid membrane. In contrast, the LHC found in wild-type maize thylakoids shows the usual functional association with Photosystem II reaction centers. Several lines of evidence suggest that the free LHC found in thylakoids isolated from hcf1-3 is able to mediate cation-dependent changes in both thylakoid appression and energy distribution between the photosystems: (1) Thylakoids isolated from hcf1-3 and wild-type seedlings exhibit a similar Mg2+-dependent increase in the short/long wavelength fluorescence emission peak ratio at 77 K. This Mg2+ effect is lost following incubation of thylakoids isolated from either source with low concentrations of trypsin. Such treatment results in the partial proteolysis of the LHC in both membrane types. (2) Thylakoids isolated from both hcf1-3 and wild-type seedlings show a similar Mg2+ dependence for the enhancement of the maximal yield of room temperature fluorescence and light scattering; both Mg2+ effects are abolished by brief incubation of the thylakoids with low concentrations of trypsin (3) Mg2+ acts to reduce the relative quantum efficiency of Photosystem I-dependent electron transport at limiting 650 nm light in thylakoids isolated from hcf1-3. (4) The pattern of digitonin fractionation of thylakoid membranes, which is dependent upon structural membrane interactions and upon LHC in the thylakoids, is similar in thylakoids isolated from both hcf1-3 and wild-type seedlings. We conclude that the surface-exposed segment of the LHC, but not the LHC-Photosystem II core association, is necessary for the cation-dependent changes in both thylakoid appression and energy distribution between the two photosystems, and that the LHC itself is able to transfer excitation energy directly to Photosystem I in a Mg2+-dependent fashion in the absence of Photosystem II reaction centers. The latter phenomenon is equivalent to a cation-induced change in the absorptive cross-section of Photosystem I.  相似文献   

17.
Photoinhibition was studied in osmotically broken chloroplasts isolated from spinach leaves (Spinacia oleracea L.). Both whole chain electron transport (measured as ferricyanide-dependent O2 evolution in the presence of NH4Cl) and photosystem II activity (measured as O2 evolution in the presence of either silicomolybdate plus 3-(3,4-diphenyl)-1,1 dimethylurea or parabenzoquinone) showed similar decreases in activity in response to a photoinhibitory treatment (8 minutes of high light given in the absence of an electron acceptor other than O2). Photosystem I activity was less affected. Photoinhibition of silicomolybdate reduction was largely reversible by an 8 minute dark incubation following the light treatment. Decreasing the O2 concentration during photoinhibition below 2% increased photoinhibition of whole chain electron transport. Addition of superoxide dismutase to the reaction medium did not affect photoinhibition. Photoinhibition of both photosystem I and photosystem II activity increased as the rate of electron transfer during the treatment increased, and was largely prevented when 3-(3,4-diphenyl)-1,1-dimethylurea was present during the photoinhibition period. Noncyclic photophosphorylation was decreased as a consequence of whole chain electron transfer photoinhibition. Since diphenyl carbazide added after light treatment did not relieve photoinhibition of dichlorophenol indophenol reduction, we conclude that the site of inhibition is located within or near the photosystem II reaction center.  相似文献   

18.
G. Dubertret  M. Lefort-Tran 《BBA》1978,503(2):316-332
The relationships between light-harvesting chlorophyll and reaction centers in Photosystem II were analyzed during the chloroplast development of dark-grown, non-dividing Euglena gracilis Z. Comparative measurements included light saturation of photosynthesis, oxygen evolution under flashing-light and fluorescence induction. The results obtained can be summarized as follows: (1) Photosystem II photocenters are formed in parallel with chlorophyll synthesis, but after a longer lag phase. (2) As a consequence, the chlorophyll: reaction center ratio (Emerson's type photosynthetic unit) decreases during greening. (3) This decrease is accompanied by considerable changes in the energy transfer and trapping properties of Photosystem II. Most of the initially synthesized chlorophylls are inactive in the transfer of excitations to active photochemical centers and are shared among newly formed Photosystem II photocenters; as a consequence, the number of chlorophylls functionally connected to each Photosystem II photocenter decreases and cooperativity between these centers appears. Results are discussed in terms of chlorophyll organization in developing photosynthetic membranes with reference to the lake or puddle models of photosynthetic unit organization.  相似文献   

19.
The enzyme lactoperoxidase was used to specifically iodinate the surface-exposed proteins of chloroplast lamellae. This treatment had two effects on Photosystem II activity. The first, occurring at low levels of iodination, resulted in a partial loss of the ability to reduce 2,6-dichlorophenolindophenol (DCIP), even in the presence of an electron donor for Photosystem II. There was a parallel loss of Photosystem II mediated variable yield fluorescence which could not be restored by dithionite treatment under anaerobic conditions. The same pattern of inhibition was observed in either glutaraldehyde-fixed or unfixed membranes. Analysis of the lifetime of fluorescence indicated that iodination changes the rate of deactivation of the excited state chlorophyll. We have concluded that iodination results in the introduction of iodine into the Photosystem II reaction center pigment-protein complex and thereby introduces a new quenching. The data indicate that the reaction center II is surface exposed.At higher levels of iodination, an inhibition of the electron transport reactions on the oxidizing side of Photosystem II was observed. That portion of the total rate of photoreduction of DCIP which was inhibited by this action could be restored by addition of an electron donor to Photosystem II. Loss of activity of the oxidizing side enzymes also resulted in a light-induced bleaching of chlorophyll a680 and carotenoid pigments and a dampening of the sequence of O2 evolution observed during flash irradiation of treated chloroplasts. All effects on electron transport on the oxidizing side of Photosystem II could be eliminated by glutaraldehyde fixation of the chloroplast lamellae prior to lactoperoxidase treatment. It is concluded that the electron carriers on the oxidizing side of Photosystem II are not surface localized; the functioning of these components is impaired by structural disorganization of the membrane occurring at high levels of iodination.Our data are in agreement with previously published schemes which suggest that Photosystem II mediated electron transport traverses the membrane.  相似文献   

20.
Synechocystis sp. PCC 6803 when grown in the presence of sublethal (M) levels of cobalt chloride shows an enhancement of Photosystem II (PS II) catalyzed Hill reaction. This stimulation seems to be induced by cobalt ions as other metal ions inhibit para-benzoquinone catalyzed Hill reaction. At saturating white light intensity, this enhancement is two times over that of the control cells on unit chlorophyll basis. Analysis of the PS II electron transport rate at varying intensities of white, blue or yellow light suggests an increased maximal rates but no change in the quantum yield or effective antenna size of CoCl2-grown cells. There were no structural and functional changes in the phycobilisome as judged by the absence of changes in the phycocyanin/allophycocyanin ratio, fluorescence emission spectra, second derivative absorption spectra at 77 K and SDS-PAGE analysis of isolated phycobilisomes. The 77 K fluorescence emission spectra of the cells showed a decrease in the ratio of Photosystem I emission (F725) to Photosystem II emission (F685) in CoCl2-grown cells compared to the control cells. These observations indicate three possibilities: (1) there is an increase in the number of Photosystem II units; (2) a faster turnover of Photosystem II centers; or (3) an alteration in energy redistribution between PS II and PS I in CoCl2-grown cells which causes stimulation of Photosystem II electron transport rate.Abbreviations APC allophycocyanin - Chl a chlorophyll a - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - EDTA ethylene diamine tetraacetic acid - PBS phycobilisome - PC phycocyanin - PSI Photosystem I - PS II Photosystem II - pBQ p-benzoquinone - PMSF phenyl methyl sulfonyl fluoride  相似文献   

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