PROTEIN COMPOSITION OF SPINACH CHLOROPLASTS AND THEIR PHOTOSYSTEM I AND PHOTOSYSTEM II SUBFRAGMENTS*
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Abstract— The proteins of spinach chloroplasts and their subfragments containing photosystem I and photosystem II, obtained by Triton X‐100 treatment or French‐pressure rupture, were separated by sodium dodecyl sulfate (SDS)‐acrylamide electrophoresis at pH 7·0 in phosphate buffer. The individual protein bands were identified where possible by comparing them with known, isolated and characterized proteins from chloroplasts, and their molecular weights were determined. The protein composition of the chloroplast fragments were correlated to the functional properties of these fragments. Distinct patterns were obtained for photosystem I and photosystem II particles. The major protein of photosystem II is expressed in the 23 kilodalton range and photosystem I proteins seem to be clustered mainly in the 50–70 kilodalton range.Keywords:
P700
Cytochrome b6f complex
Sodium dodecyl sulfate
Kilodalton
P700
Cytochrome b6f complex
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Several chloroplast membrane proteins can become phosphorylated by a membrane protein kinase. The kinase is activated when the electron carriers (plastoquinone) between photosystem II and photosystem I are reduced. Phosphorylation of the light harvesting complex of photosystem II causes a decrease in energy transferred to photosystem II and increase in the rate of excitation of photosystem I. This process, which is mediated by lateral diffusion of the phosphorylated complex serves to regulate the relative rates of electron transfer through the two photosystems. Phosphorylation may also have an important role in balancing the rate of excitation of photosystem II with the capacity for photosynthetic carbon metabolism and in controlling the functional state of photosystem II.
Plastoquinone
Cytochrome b6f complex
P700
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Cytochrome c6, the product of the petJ gene, is a photosynthetic electron carrier in cyanobacteria, which transfers electrons to photosystem I and which is synthesised under conditions of copper deficiency to functionally replace plastocyanin. The photosystem I photochemical activity (energy storage, photoinduced P700 redox changes) was examined in a petJ-null mutant of Synechocystis PCC 6803. Surprisingly, photosystem I activity in the petJ-null mutant grown in the absence of copper was not much affected. However, in a medium with a low inorganic carbon concentration and with NH4+ ion as nitrogen source, the mutant displayed growth inhibition. Analysis showed that, especially in the latter, the isiAB operon, encoding flavodoxin and CP43', an additional chlorophyll a antenna, was strongly expressed in the mutant. These proteins are involved in photosystem I function and organisation and are proposed to assist in prevention of overoxidation of photosystem I at its lumenal side and overreduction at its stromal side.
Plastocyanin
P700
Cytochrome b6f complex
Synechocystis
Flavodoxin
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▪ Abstract Photosystem I is the light-driven plastocyanin-ferredoxin oxidoreductase in the thylakoid membranes of cyanobacteria and chloroplasts. In recent years, sophisticated spectroscopy, molecular genetics, and biochemistry have been used to understand the light conversion and electron transport functions of photosystem I. The light-harvesting complexes and internal antenna of photosystem I absorb photons and transfer the excitation energy to P700, the primary electron donor. The subsequent charge separation and electron transport leads to the reduction of ferredoxin. The photosystem I proteins are responsible for the precise arrangement of cofactors and determine redox properties of the electron transfer centers. With the availability of genomic information and the structure of photosystem I, one can now probe the functions of photosystem I proteins and cofactors. The strong reductant produced by photosystem I has a central role in chloroplast metabolism, and thus photosystem I has a critical role in the metabolic networks and physiological responses in plants.
P700
Plastocyanin
Cytochrome b6f complex
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Illumination of intact Bryopsis corticulans chloroplasts under anaerobic conditions induced a decline of chlorophyll fluorescence and photoinhibition of Photosystems I and II. The time course of the light-induced decline of chlorophyll fluorescence and the decreases of activities of reactions sensitized by Photosystems I and II were compared. Photosystem I activity decreased in parallel with the disappearance of active P700. The time course of the destruction of the reaction center of Photosystem II was similar to that of photoinhibition of 2,6-dichlorophenolindophenol-Hill reaction.It appears that the initial events in photoinhibition are the destruction of the reaction centers of Photosystems I and II and that the reaction centers that are inhibited become quenchers of chlorophyll fluorescence.Effects of inhibitors of electron transfer and of an electron donor to Photosystem I showed that photoinhibition was related to Photosystem I activity.
Photoinhibition
P700
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P700
P680
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P700
DCMU
Plastocyanin
Cytochrome b6f complex
Cytochrome f
Electron donor
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Previous reports from this laboratory described a new concept of three light reactions in plant photosynthesis comprising two short-wavelength (lambda < 700 nm) photoreactions belonging to Photosystem II and one long-wavelength (lambda > 700 nm) photoreaction belonging to Photosystem I. Among the electron carriers assigned to Photosystem II were cytochrome b(559) and plastocyanin and to Photosystem I, cytochrome f.According to a widely held view, the light-induced reduction of NADP by water requires the collaboration of Photosystems I and II and involves specifically cytochrome f and P700 (a portion of chlorophyll a peculiar to Photosystem I). By contrast, the new concept ascribes the light-induced reduction of NADP by water solely to the two photoreactions of Photosystem II, without the participation of Photosystem I and its components, cytochrome f and P700.Further evidence in support of the new concept has now been obtained from chloroplast fragments. Two kinds of chloroplast fragments have been prepared: (a) one with Photosystem II activity, capable-in the presence of plastocyanin-of photoreducing NADP with water but lacking P700 and functional cytochrome f and (b) another having only Photosystem I activity, lacking plastocyanin, and enriched in P700.
P700
Plastocyanin
Cytochrome b6f complex
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P700
Plastoquinone
Cytochrome b6f complex
Heptane
Tetranitromethane
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Modifications of the type and concentration of detergents and buffers used for thylakoid extraction and gel electrophoresis have provided improved resolution of chlorophyll‐protein complexes in mutant barley which lacks chlorophyll b. The use of sodium deoxycholate in the gels and in the solubilization of thylakoids reduced the amount of free chlorophyll significantly. Three chlorophyll‐ a –protein complexes were resolved from mutant barley; two of these (CP1a and CP1) represent the reaction centre complex of photosystem 1, while the third complex, CPa, is the presumed reaction centre of photosystem 2. The fluorescence emission band of mutant barley complex CPa at 684 nm and the low fluorescence yield are consistent with expected characteristics of a reaction centre complex of photosystem 2. More chlorophyll (60 %) is associated with the reaction centre complex of photosystem 1 than with the presumed reaction centre complex of photosystem 2 (30%), and these amounts are double those found in the corresponding complexes of normal barley. It is proposed that the smaller photosynthetic unit of mutant barley thylakoids has twice as much chlorophyll a in photosystem 1 as in photosystem 2, and this chlorophyll is associated only with the two reaction centre complexes which are in direct contact. The sodium deoxycholate method may be useful for the resolution of pigment complexes from other thylakoids with inherently fragile photosystems.
P700
Chlorophyll b
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