Photosynthetic acclimation: structural reorganisation of light harvesting antenna--role of redox-dependent phosphorylation of major and minor chlorophyll a/b binding proteins

In order to carry out photosynthesis, plants and algae rely on the co-operative interaction of two photosystems: photosystem I and photosystem II. For maximum efficiency, each photosystem should absorb the same amount of light. To achieve this, plants and green algae have a mobile pool of chlorophyll a/b-binding proteins that can switch between being light-harvesting antenna for photosystem I or photosystem II, in order to maintain an optimal excitation balance. This switch, termed state transitions, involves the reversible phosphorylation of the mobile chlorophyll a/b-binding proteins, which is regulated by the redox state of the plastoquinone-mediating electron transfer between photosystem I and photosystem II. In this review, we will present the data supporting the function of redox-dependent phosphorylation of the major and minor chlorophyll a/b-binding proteins by the specific thylakoid-bound kinases (Stt7, STN7, TAKs) providing a molecular switch for the structural remodelling of the light-harvesting complexes during state transitions. We will also overview the latest X-ray crystallographic and electron microscopy-derived models for structural re-arrangement of the light-harvesting antenna during State 1-to-State 2 transition, in which the minor chlorophyll a/b-binding protein, CP29, and the mobile light-harvesting complex II trimer detach from the light-harvesting complex II-photosystem II supercomplex and associate with the photosystem I core in the vicinity of the PsaH/L/O/P domain.     

Association of chlorophyll a/b-binding Pcb proteins with photosystems I and II in Prochlorothrix hollandica

Action spectra for photosystem II (PSII)-driven oxygen evolution and of photosystem I (PSI)-mediated H(2) photoproduction and photoinhibition of respiration were used to determine the participation of chlorophyll (Chl) a/b-binding Pcb proteins in the functions of pigment apparatus of Prochlorothrix hollandica. Comparison of the in situ action spectra with absorption spectra of PSII and PSI complexes isolated from the cyanobacterium Synechocystis 6803 revealed a shoulder at 650 nm that indicated presence of Chl b in the both photosystems of P. hollandica. Fitting of two action spectra to absorption spectrum of the cells showed a chlorophyll ratio of 4:1 in favor of PSI. Effective antenna sizes estimated from photochemical cross-sections of the relevant photoreactions were found to be 192+/-28 and 139+/-15 chlorophyll molecules for the competent PSI and PSII reaction centers, respectively. The value for PSI is in a quite good agreement with previous electron microscopy data for isolated Pcb-PSI supercomplexes from P. hollandica that show a trimeric PSI core surrounded by a ring of 18 Pcb subunits. The antenna size of PSII implies that the PSII core dimers are associated with approximately 14 Pcb light-harvesting proteins, and form the largest known Pcb-PSII supercomplexes.     

Disturbed structure and function of chloroplasts during blocked biosynthesis of 5-aminolevulinic acid in the light

Xantha-702 mutant of cotton (Gossypium hirsutum L.) proved to have blocked synthesis of 5-aminolevulinic acid in the light. Accordingly, mutant leaves accumulated 2–5% chlorophyll of baseline. Mutant plants demonstrated disturbed production of pigment-protein complexes of photosystems I (PSI) and II (PSII) and generation of the chloroplast membrane system blocked at the early stages, largely, at the stages of vesicles and single short thylakoid. The functional activity of the PSI and PSII reaction centers was close to zero. Only the chlorophyll a/b light-harvesting complexes of PSI and PSII with the chlorophyll fluorescence peaks at 728 and 681 nm, respectively, were produced in the xantha-702 mutant. We propose that the genetic block of 5-aminolevunilic acid biosynthesis in the light in the xantha-702 mutant disturbs the formation and activity of the complexes of the reaction centers of PS-I and PS-II and inhibits the development of the whole membrane system of chloroplasts.     

Structural-functional organization of chloroplasts in leaves of xantha-702 mutant of Gossypium hirsutum L

For cotton mutant xantha (Gossypium hirsutum L.), it has been established that synthesis of 5-aminolevulinic acid was blocked in the light. In the light this mutant accumulates chlorophyll by 30 times lower as compared to the parent type. In mutant xantha, a very few pigment-protein complexes of PS-I and PS-II are formed in chloroplasts, and formation of membrane system in these is blocked at the early stages, in most cases, at the stage of bubbles and single short thylakoids. Functional activity of reaction centers of PS-I and PS-II is close to zero. Only light-harvesting chlorophyll-a/b protein complexes of the two photosystems are formed in mutant xantha plastid membranes with maximum chlorophyll fluorescence at 728 and 681 nm, respectively. It has been concluded that in mutant xantha genetic block of 5-aminolevulinic acid biosynthesis in the light disturbs the formation and functioning of the complexes of reaction centers of PS-I and PS-II, hindering the development of the whole membrane system in chloroplasts, causing a sharp decrease in productivity.     

Pigment–Protein Complexes and the Number of Reaction Centers of the Photosystems in Pea Chlorophyll Mutants

We studied fluorescent and absorption properties of the chloroplasts and pigment–protein complexes isolated by gel electrophoresis from the leaves of pea, the parent cultivar Torsdag and mutants chlorotica 2004 and 2014. Specific fluorescence peaks of chlorophyll forms in individual complexes have been determined from the absorption and fluorescence spectra of the chloroplast chlorophyll and their second derivatives at 23 and –196°C. The mutant chlorotica 2004 proved to have an increased intensity of a long-wave band of the light-harvesting complex I at both 23°C (745 nm) and –196°C (728 nm). At the same time, this mutant manifested a decreased accumulation of the chlorophyll forms making up the nearest-neighbor antenna of the PS I reaction center (at 690, 697, and 708 nm). No spectral differences have been revealed between chlorotica 2014 mutant and the parent cultivar. Gel electrophoresis revealed the synthesis of all chlorophyll–protein complexes in both mutants. At the same time, analysis of photochemical activity of PS I and PS II reaction centers and calculations of their number and the size of the light-harvesting antenna have shown that the number of reaction centers in the PS I of chlorotica 2004 mutant is reduced by a factor of 1.7 because its chlorophyll a–protein complex is disturbed by the mutation. The primary effect of chlorotica 2014 mutation remains unclear. The proportional changes in the content of photosystem complexes in this mutant suggest that they are secondary and result from a 50% decrease in chlorophyll content.     

Spectral characteristics and the structure of chloroplasts upon blocking the early stages of chlorophyll biosynthesis

The cotton mutant xantha (Gossypium hirsutum L.) with the blocked synthesis of 5-aminolevulinic acid in the light has been shown to accumulate chlorophyll 30 times less than the parent type. In chloroplasts of the mutant xantha, the formation of the membrane system is blocked at the earliest stages, mainly at the stage of bubbles and single short thylakoids. Only light-harvesting chlorophyll-a/b-protein complexes I and II with chlorophyll fluorescence maxima at 728 and 681 nm, respectively, are formed in plastid membranes of the mutant. It has been concluded that the genetic block of chlorophyll biosynthesis in the mutant xantha disturbs the formation and functioning of the complexes in reaction centers of PS-I and PS-II, inhibiting the development of the whole membrane system of chloroplasts at the stage of bubbles and single thylakoids.     

Ultrastructure organization of chloroplasts in chlorotica mutants of Pisum sativum L

A study was made of chlorophyll-protein complexes of photosystems, and of ultrastructural organization of chloroplasts in pea leaves of the primary cultivar Torsdag and of its mutants, chlorotica 2004 and 2014. It has been shown that mutants accumulated 80 and 55% chlorophyll, respectively, and were able to synthesize all four types of photosystem complexes. The value of the light-harvesting antenna in mutant 2014 was close to the control one, and in mutant 2004 it increased significantly (by 30%). These changes were caused by a proportional decrease (40-50%) in any complexes in mutant 2014, whereas the number of PS-I reaction centre complexes, decreased by 50% in mutant 2004 at nearly complete storage of PS-I reaction centre complexes, decreased by 50% in mutant 2004 at nearly complete storage of PS-II complexes. The proportional decrease of PS-I and PS-II complexes in mutant chlorotica 2014 was followed by partial reduction of the entire membrane system in chloroplasts, but with a normal development of both granal and intergranal thylakoids. On the contrary, the loss of PS-I reaction centre complexes in mutant chlorotica 2004 leads to reduction of unstacked sites of thylakoids in chloroplasts. It is concluded that this effect may be associated with localization of PS-I complexes mainly in unstacked sites of thylakoids.     

Energy transfers from Photosystem II to Photosystem I in Cryptomonas rufescens (Cryptophyceae)

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.     

Spectral features and structure of chloroplasts under an early block of chlorophyll synthesis

A xantha mutant of cotton (Gossypium hirsutum L.) with blocked synthesis of 5-aminolevulinic acid in light accumulates 30 times less chlorophyll than the parental strain. Formation of the chloroplast membrane system in the mutant stops at very early stages, mostly vesicles and single short thylakoids. The mutant plastid membranes contain only light-harvesting chlorophyll-a/b-protein complexes I and II with fluorescence maxima at 728 and 681 nm, respectively. Thus, an early block of chlorophyll synthesis impairs the formation and function of photosystem reaction centers and retards the development of the chloroplast membrane system at the stage of proplastids.     

Subunit stoichiometry of the chloroplast photosystem II antenna system and aggregation state of the component chlorophyll a/b binding proteins

Photosystem (PS) II membranes, obtained by the method of Berthold et al. (Berthold, D. A., Babcock, G. T., and Yocum, C. F. (1981) FEBS Lett. 134, 231-234), have been fractionated by a sucrose gradient ultracentrifugation method which allows the quantitative separation of the three major chlorophyll binding complexes in these membranes: the chlorophyll (chl) a binding PSII reaction center core, the major light-harvesting complex II, and the minor chl a/b proteins called CP26, CP29, and CP24. Each fraction has been analyzed for its subunit stoichiometry by quantitative sodium dodecyl sulfate-polyacrylamide gel electrophoresis methods. The results show that 12 mol of light-harvesting complex II and 1.5 mol of each of the minor chl a/b proteins are present per mol of the PSII reaction center complex in PSII membranes. These data suggest a dimeric organization of PSII, in agreement with a recent crystallographic study (Bassi, R., Ghiretti Magaldi, A., Tognon, G., Giacometti, G. M., and Miller, K. (1989) Eur. J. Cell Biol. 50, 84-93) and imply that such a dimeric complex is served by antenna chl a/b proteins whose minimal aggregation state includes three polypeptides. This was confirmed by covalent cross-linking of purified antenna complexes.     

Reduction of the chloroplast membrane system caused by disorders in early stages of chlorophyll biosynthesis

A chlorophyll-deficient xantha mutant of cotton (Gossypium hirsutum L.) was examined with respect to development and structural organization of the chloroplast membrane system as affected by disruption of early stages of chlorophyll biosynthesis in the light. The analysis of early chlorophyll precursors showed that the mutant is unable to synthesize 5-aminolevulinic acid (5-ALA) in the light. The disorders in early stages of chlorophyll biosynthesis arrested the development of chloroplast membrane system at the stage of vesicles and single thylakoids. The accumulation of 2–5% chlorophyll in the mutant was related to the formation of light-harvesting chlorophyll-a/b-protein complexes I and II, whereas pigment-protein complexes composing reaction centers of photosystem I and photosystem II were lacking. It is concluded that the chloroplast membrane system in the mutant with impaired 5-ALA synthesis is incapable of development and is even reduced upon long-term growing under light.     

Spectroscopic characterization of the spinach Lhcb4 protein (CP29), a minor light-harvesting complex of photosystem II.

A spectroscopic characterization is presented of the minor photosystem II chlorophyll a/b-binding protein CP29 (or the Lhcb4 protein) from spinach, prepared by a modified form of a published protocol [Henrysson, T., Schroder, W. P., Spangfort, M. & Akerlund, H.-E. (1989) Biochim. Biophys. Acta 977, 301-308]. The isolation procedure represents a quicker, cheaper means of isolating this minor antenna protein to an equally high level of purity to that published previously. The pigment-binding protein shows similarities to other related light-harvesting complexes (LHCs), including the bulk complex LHCIIb but more particularly another minor antenna protein CP26 (Lhcb5). It is also, in the main, similar to other preparations of CP29, although some significant differences are discussed. In common with CP26, the protein binds about six chlorophyll a and two chlorophyll b molecules. Two chlorophyll b absorption bands are present at 638 and 650 nm and they are somewhat more pronounced than in a recent report [Giuffra, E., Zucchelli, G., Sandonà, D., Croce, R., Cugini, D., Garlaschi, F.M., Bassi, R. & Jennings, R.C. (1997) Biochem. 36, 12984-12993]. The bands give rise to positive and negative linear dichroism, respectively; both show negative CD bands (cf. bands with similar properties at 637 and 650 nm in CP26). Chlorophyll a absorption is dominated by a large contribution at 674 nm which also shows similarities to the major band in LHCIIb and CP26, while (as for CP26) a reduction in absorption around 670 nm is observed relative to the bulk complex. Principal differences from LHCIIb and CP26, and from other CP29 preparations, occur in the carotenoid region.     

Chlorophyll-protein complex composition during chloroplast development: A species comparison

Barley, maize, pea, soybean, and wheat exhibited differences in chlorophyll a/b ratio and chlorophyll-protein (CP) complex composition during the initial stages of chloroplast development. During the first hours of greening, the chlorophyll a/b ratios of barley, pea, and wheat were high (a/b8) and these species contained only the CP complex of photosystem I as measured by mild sodium dodecyl sulfate polyacrylamide gel electrophoresis. A decrease in chlorophyll a/b ratio and the observation of the CP complexes associated with photosystem II and the light-harvesting apparatus occurred at later times in barley, pea, and wheat. In contrast, maize and soybean exhibited low chlorophyll a/b ratios (a/b<8) and contained the CP complexes of both photosytem I and the light-harvesting apparatus at early times during chloroplast development. The species differences were not apparent after 8 h of greening. In all species, the CP complexes were stabilized during the later stages of chloroplast development as indicated by a decrease in the percentage of chlorophyll released from the CP complexes during detergent extraction. The results demonstrate that CP complex synthesis and accumulation during chloroplast development may not be regulated in the same way in all higher plant species.Abbreviations Chl chlorophyll - CP chlorophyll-protein - CPI P700 chlorophyll-a protein complex of photosystem I - CPa electrophoretic band that contains the photosystem II reaction center complexes and a variable amount of the photosystem I light-harvesting complex - LHC the major light-harvesting complex associated with photosystem II - PSI photosystem I - PSII photosystem II - SDS sodium dodecyl sulfate - SDS-PAGE sodium dodecyl sulfate polyacrylamide gel electrophoresis Cooperative investigations of the United States Department of Agriculture, Agricultural Research Service, and the North Carolina Agricultural Research Service, Raleigh, NC 27695-7601. Paper No. 10335 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7601.     

Three-dimensional model and characterization of the iron stress-induced CP43'-photosystem I supercomplex isolated from the cyanobacterium Synechocystis PCC 6803

The cyanobacterium Synechocystis PCC 6803 has been subjected to growth under iron-deficient conditions. As a consequence, the isiA gene is expressed, and its product, the chlorophyll a-binding protein CP43', accumulates in the cell. Recently, we have shown for the first time that 18 copies of this photosystem II (PSII)-like chlorophyll a-binding protein forms a ring around the trimeric photosystem I (PSI) reaction center (Bibby, T. S., Nield, J., and Barber, J. (2001) Nature, 412, 743-745). Here we further characterize the biochemical and structural properties of this novel CP43'-PSI supercomplex confirming that it is a functional unit of approximately 1900 kDa where the antenna size of PSI is increased by 70% or more. Using electron microscopy and single particle analysis, we have constructed a preliminary three-dimensional model of the CP43'-PSI supercomplex and used it as a framework to incorporate higher resolution structures of PSI and CP43 recently derived from x-ray crystallography. Not only does this work emphasize the flexibility of cyanobacterial light-harvesting systems in response to the lowering of phycobilisome and PSI levels under iron-deficient conditions, but it also has implications for understanding the organization of the related chlorophyll a/b-binding Pcb proteins of oxychlorobacteria, formerly known as prochlorophytes.     

Leaf senescence in a non-yellowing mutant of Festuca pratensis: Proteins of photosystem II

The senescence of leaves is characterized by yellowing as chlorophyll pigments are degraded. Proteins of the chloroplasts also decline during this phase of development. There exists a non-yellowing mutant genotype of Festuca pratensis Huds. which does not suffer a loss of chlorophyll during senescence. The fate of chloroplast membrane proteins was studied in mutant and wild-type plants by immune blotting and immuno-electron microscopy. Intrinsic proteins of photosystem II, exemplified by the light-harvesting chlorophyll a/b-binding protein (LHCP-2) and D1, were shown to be unusually stable in the mutant during senescence, whereas the extrinsic 33-kilodalton protein of the oxygen-evolving complex was equally lable in both genotypes. An ultrastructural study revealed that while the intrinsic proteins remained in the internal membranes of the chloroplasts, they ceased to display the heterogenous lateral distribution within the lamellae which was characteristic of nonsenescent chloroplasts. These observations are discussed in the light of possible mechanisms of protein turnover in chloroplasts.Abbreviations kDa kilodalton - LHCP-2 light-harvesting chlorophyll a/b-binding protein - M_r relative molecular mass - PSII photosystem II - SDS sodium dodecyl sulphate     

Self-assembling peptide detergents stabilize isolated photosystem I on a dry surface for an extended time

We used a class of designed peptide detergents to stabilize photosystem I (PS-I) upon extended drying under N2 on a gold-coated-Ni-NTA glass surface. PS-I is a chlorophyll-containing membrane protein complex that is the primary reducer of ferredoxin and the electron acceptor of plastocyanin. We isolated the complex from the thylakoids of spinach chloroplasts using a chemical detergent. The chlorophyll molecules associated with the PS-I complex provide an intrinsic steady-state emission spectrum between 650 and 800 nm at -196.15 degrees C that reflects the organization of the pigment-protein interactions. In the absence of detergents, a large blue shift of the fluorescence maxima from approximately 735 nm to approximately 685 nm indicates a disruption in light-harvesting subunit organization, thus revealing chlorophyll-protein interactions. The commonly used membrane protein-stabilizing detergents, N-dodecyl-beta-D-maltoside and N-octyl-beta-D-glucoside, only partially stabilized the approximately 735-nm complex with approximately 685-nm spectroscopic shift. However, prior to drying, addition of the peptide detergent acetyl-AAAAAAK at increasing concentration significantly stabilized the PS-I complex. Moreover, in the presence of acetyl-AAAAAAK, the PS-I complex is stable in a dried form at room temperature for at least 3 wk. Another peptide detergent, acetyl-VVVVVVD, also stabilized the complex but to a lesser extent. These observations suggest that the peptide detergents may effectively stabilize membrane proteins in the solid-state. These designed peptide detergents may facilitate the study of diverse types of membrane proteins.     

Differential Synthesis of Photosystem Cores and Light-Harvesting Antenna during Proplastid to Chloroplast Development in Spirodela oligorrhiza

Proplastids and etioplasts are common starting points for monitoring chloroplast development in higher plants. Although proplastids are the primary precursor of chloroplasts, most proplastid to chloroplast systems are cumbersome to study temporally. Conversely, the etioplast to chloroplast transition is initiated by light and is readily examined as a function of time. Etioplasts, however, are found mostly in plants germinated in the dark and are not an obligatory step in chloroplast development. We have chosen to study chloroplast ontogeny in Spirodela oligorrhiza (Kurtz) Hegelm (a C₃-monocot) because of its unique ability to grow indefinitely in the dark. Ultrastructural, physiological, and molecular evidence is presented in support of a temporal, light-triggered proplastid to chloroplast transition in Spirodela. The dark-grown plants are devoid of chlorophyll, and upon illumination synchronously green over a 3- to 5-day period. Synthesis of chloroplast proteins involved in photosynthesis is coincident with thylakoid assembly, chlorophyll accumulation, and appearance of CO₂ fixation activity. Interestingly, the developmental sequence in Spirodela was slow enough to reveal that biosynthesis of the D1 photosystem II reaction center protein precedes biosynthesis of the major light-harvesting antenna proteins. This, coupled with the high chlorophyll a/b ratio observed early in development, indicated that reaction center assembly occurred prior to accumulation of the light-harvesting complexes. Thus, with Spirodela one can study proplastid to chloroplast conversions temporally in higher plants and follow the process on a time scale that enables a detailed dissection of plastid maturation processes.     

Chlorophyll-protein complex composition and photochemical activity in developing chloroplasts from greening barley seedlings

The time course for the observation of intact chlorophyll-protein (CP) complexes during barley chloroplast development was measured by mild sodium dodecyl sulfate polyacrylamide gel electrophoresis. The procedure required extraction of thylakoid membranes with sodium bromide to remove extrinsic proteins. During the early stages of greening, the proteins extracted with sodium bromide included polypeptides from the cell nucleus that associate with developing thylakoid membranes during isolation and interfere with the separation of CP complexes by electrophoresis. Photosystem I CP complexes were observed before the photosystem II and light-harvesting CP complexes during the initial stages of barley chloroplast development. Photosystem I activity was observed before the photosystem I CP complex was detected whereas photosystem II activity coincided with the appearance of the CP complex associated with photosystem II. Throughout chloroplast development, the percentage of the total chlorophyll associated with photosystem I remained constant whereas the amount of chlorophyll associated with photosystem II and the light-harvesting complex increased. The CP composition of thylakoid membranes from the early stages of greening was difficult to quantitate because a large amount of chlorophyll was released from the CP complexes during detergent extraction. As chloroplast development proceeded, a decrease was observed in the amount of chlorophyll released from the CP complexes by detergent action. The decrease suggested that the CP complexes were stabilized during the later stages of development.Abbreviations Chl chlorophyll - CP chlorophyll-protein - CPI P700 chlorophyll-a protein complex of photosystem I - CPa electrophoretic band that contains the photosystem II reaction center complexes and a variable amount of the photosystem I light-harvesting complex - CP A/B the major light-harvesting complex associated with photosystem II - DCIP 2,6-dichlorophenolindophenol - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DPC diphenyl carbazide - MV methyl viologen - PAR photosynthetically active radiation - PSI photosystem I - PSII photosystem II - SDS sodium dodecyl sulfate - SDS-PAGE sodium dodecyl sulfate polyacrylamide gel electrophoresis - TEMED N,N,N,N-tetramethylethylenediamine - TMPD N,N,N,N-tetramethyl-p-phenylenediamine Cooperative investigations of the United States Department of Agriculture, Agricultural Research Service, and the North Carolina Agricultural Research Service, Raleigh, NC 27695-7601. Paper No. 9949 of the Journal Series of the North Carolina Agricultural Research Service, Raleight, NC 27695-7601.     

The evolution of light stress proteins in photosynthetic organisms

The Elip (early light-inducible protein) family in pro- and eukaryotic photosynthetic organisms consists of more than 100 different stress proteins. These proteins accumulate in photosynthetic membranes in response to light stress and have photoprotective functions. At the amino acid level, members of the Elip family are closely related to light-harvesting chlorophyll a/b-binding (Cab) antenna proteins of photosystem I and II, present in higher plants and some algae. Based on their predicted secondary structure, members of the Elip family are divided into three groups: (a) one-helix Hlips (high light-induced proteins), also called Scps (small Cab-like proteins) or Ohps (one-helix proteins); (b) two-helix Seps (stress-enhanced proteins); and (c) three-helix Elips and related proteins. Despite having different physiological functions it is believed that eukaryotic three-helix Cab proteins evolved from the prokaryotic Hlips through a series of duplications and fusions. In this review we analyse the occurrence of Elip family members in various photosynthetic prokaryotic and eukaryotic organisms and discuss their evolutionary relationship with Cab proteins.     

Xanthophyll cycle and energy-dependent fluorescence quenching in leaves from pea plants grown under intermittent light

The possible role of zeaxanthin formation and antenna proteins in energy-dependent chlorophyll fluorescence quenching (qE) has been investigated. Intermittent-light-grown pea (Pisum sativum L.) plants that lack most of the chlorophyll a/b antenna proteins exhibited a significantly reduced qE upon illumination with respect to control plants. On the other hand, the violaxanthin content related to the number of reaction centers and to xanthophyll cycle activity, i.e. the conversion of violaxanthin into zeaxanthin, was found to be increased in the antenna-protein-depleted plants. Western blot analyses indicated that, with the exception of CP 26, the content of all chlorophyll a/b-binding proteins in these plants is reduced to less than 10% of control values. The results indicate that chlorophyll a/b-binding antenna proteins are involved in the energy-dependent fluorescence quenching but that only a part of qE can be attributed to quenching by chlorophyll a/b-binding proteins. It seems very unlikely that xanthophylls are exclusively responsible for the qE mechanism.Abbreviations CAB chlorophyll a/b-binding - Chl chlorophyll - F_V variable fluorescence - IML intermittent light - LHC light harvesting complex - PFD photon flux density - qP photochemical quenching of chlorophyll fluoresence - qN non-photochemical quenching - qE energy-dependent quenching - qI photoinhibitory quenching - qT quenching by state transition