Discovery of a Chlorophyll Binding Protein Complex Involved in the Early Steps of Photosystem II Assembly in Synechocystis

Efficient assembly and repair of the oxygen-evolving photosystem II (PSII) complex is vital for maintaining photosynthetic activity in plants, algae, and cyanobacteria. How chlorophyll is delivered to PSII during assembly and how vulnerable assembly complexes are protected from photodamage are unknown. Here, we identify a chlorophyll and β-carotene binding protein complex in the cyanobacterium Synechocystis PCC 6803 important for formation of the D1/D2 reaction center assembly complex. It is composed of putative short-chain dehydrogenase/reductase Ycf39, encoded by the slr0399 gene, and two members of the high-light-inducible protein (Hlip) family, HliC and HliD, which are small membrane proteins related to the light-harvesting chlorophyll binding complexes found in plants. Perturbed chlorophyll recycling in a Ycf39-null mutant and copurification of chlorophyll synthase and unassembled D1 with the Ycf39-Hlip complex indicate a role in the delivery of chlorophyll to newly synthesized D1. Sequence similarities suggest the presence of a related complex in chloroplasts.     

Assembly of D1/D2 complexes of photosystem II: Binding of pigments and a network of auxiliary proteins

Photosystem II (PSII) is the multi-subunit light-driven oxidoreductase that drives photosynthetic electron transport using electrons extracted from water. To investigate the initial steps of PSII assembly, we used strains of the cyanobacterium Synechocystis sp. PCC 6803 arrested at early stages of PSII biogenesis and expressing affinity-tagged PSII subunits to isolate PSII reaction center assembly (RCII) complexes and their precursor D1 and D2 modules (D1_mod and D2_mod). RCII preparations isolated using either a His-tagged D2 or a FLAG-tagged PsbI subunit contained the previously described RCIIa and RCII* complexes that differ with respect to the presence of the Ycf39 assembly factor and high light-inducible proteins (Hlips) and a larger complex consisting of RCIIa bound to monomeric PSI. All RCII complexes contained the PSII subunits D1, D2, PsbI, PsbE, and PsbF and the assembly factors rubredoxin A and Ycf48, but we also detected PsbN, Slr1470, and the Slr0575 proteins, which all have plant homologs. The RCII preparations also contained prohibitins/stomatins (Phbs) of unknown function and FtsH protease subunits. RCII complexes were active in light-induced primary charge separation and bound chlorophylls (Chls), pheophytins, beta-carotenes, and heme. The isolated D1_mod consisted of D1/PsbI/Ycf48 with some Ycf39 and Phb3, while D2_mod contained D2/cytochrome b₅₅₉ with co-purifying PsbY, Phb1, Phb3, FtsH2/FtsH3, CyanoP, and Slr1470. As stably bound, Chl was detected in D1_mod but not D2_mod, formation of RCII appears to be important for stable binding of most of the Chls and both pheophytins. We suggest that Chl can be delivered to RCII from either monomeric Photosystem I or Ycf39/Hlips complexes.

Analysis of isolated assembly complexes provides new insights into the early stages of photosystem II biogenesis.     

Conversion of Chlorophyll b to Chlorophyll a and the Assembly of Chlorophyll with Apoproteins by Isolated Chloroplasts

The photosynthetic apparatus is reorganized during acclimation to various light environments. During adaptation of plants grown under a low-light to high-light environment, the light-harvesting chlorophyll a/b-protein complexes decompose concomitantly with an increase in the core complex of photosystem II. To study the mechanisms for reorganization of photosystems, the assembly of chlorophyll with apoproteins was investigated using isolated chloroplasts. When [14C]chlorophyllide b was incubated with chloroplasts in the presence of phytyl pyrophosphate, it was esterified and some of the [14C]chlorophyll b was converted to [14C]chlorophyll a via 7-hydroxymethyl chlorophyll. [14C]Chlorophyll a and b were incorporated into chlorophyll-protein complexes. Light-harvesting chlorophyll a/b-protein complexes of PSII had a lower [14C]chlorophyll a to [14C]chlorophyll b ratio than P700-chlorophyll a-protein complexes, indicating the specific binding of chlorophyll to apoproteins in our systems. 7-Hydroxymethyl chlorophyll, an intermediate molecule from chlorophyll b to chlorophyll a, did not become assembled with any apoproteins. These results indicate that chlorophyll b is released from light-harvesting chlorophyll a/b-protein complexes of photosystem II and converted to chlorophyll a via 7-hydroxymethyl chlorophyll in the lipid bilayer and is then used for the formation of core complexes of photosystems. These mechanisms provide the fast, fine regulation of the photosynthetic apparatus during construction of photosystems.     

Binding of pigments to the cyanobacterial high-light-inducible protein HliC

Cyanobacteria possess a family of one-helix high-light-inducible proteins (HLIPs) that are widely viewed as ancestors of the light-harvesting antenna of plants and algae. HLIPs are essential for viability under various stress conditions, although their exact role is not fully understood. The unicellular cyanobacterium Synechocystis sp. PCC 6803 contains four HLIPs named HliA–D, and HliD has recently been isolated in a small protein complex and shown to bind chlorophyll and β-carotene. However, no HLIP has been isolated and characterized in a pure form up to now. We have developed a protocol to purify large quantities of His-tagged HliC from an engineered Synechocystis strain. Purified His-HliC is a pigmented homo-oligomer and is associated with chlorophyll and β-carotene with a 2:1 ratio. This differs from the 3:1 ratio reported for HliD. Comparison of these two HLIPs by resonance Raman spectroscopy revealed a similar conformation for their bound β-carotenes, but clear differences in their chlorophylls. We present and discuss a structural model of HliC, in which a dimeric protein binds four chlorophyll molecules and two β-carotenes.     

Participation of Chlorophyll b Reductase in the Initial Step of the Degradation of Light-harvesting Chlorophyll a/b-Protein Complexes in Arabidopsis

The light-harvesting chlorophyll a/b-protein complex of photosystem II (LHCII) is the most abundant membrane protein in green plants, and its degradation is a crucial process for the acclimation to high light conditions and for the recovery of nitrogen (N) and carbon (C) during senescence. However, the molecular mechanism of LHCII degradation is largely unknown. Here, we report that chlorophyll b reductase, which catalyzes the first step of chlorophyll b degradation, plays a central role in LHCII degradation. When the genes for chlorophyll b reductases NOL and NYC1 were disrupted in Arabidopsis thaliana, chlorophyll b and LHCII were not degraded during senescence, whereas other pigment complexes completely disappeared. When purified trimeric LHCII was incubated with recombinant chlorophyll b reductase (NOL), expressed in Escherichia coli, the chlorophyll b in LHCII was converted to 7-hydroxymethyl chlorophyll a. Accompanying this conversion, chlorophylls were released from LHCII apoproteins until all the chlorophyll molecules in LHCII dissociated from the complexes. Chlorophyll-depleted LHCII apoproteins did not dissociate into monomeric forms but remained in the trimeric form. Based on these results, we propose the novel hypothesis that chlorophyll b reductase catalyzes the initial step of LHCII degradation, and that trimeric LHCII is a substrate of LHCII degradation.     

Analysis of an Arabidopsis heat‐sensitive mutant reveals that chlorophyll synthase is involved in reutilization of chlorophyllide during chlorophyll turnover

Chlorophylls, the most abundant pigments in the photosynthetic apparatus, are constantly turned over as a result of the degradation and replacement of the damage‐prone reaction center D1 protein of photosystem II. Results from isotope labeling experiments suggest that chlorophylls are recycled by reutilization of chlorophyllide and phytol, but the underlying mechanism is unclear. In this study, by characterization of a heat‐sensitive Arabidopsis mutant we provide evidence of a salvage pathway for chlorophyllide a. A missense mutation in CHLOROPHYLL SYNTHASE (CHLG) was identified and confirmed to be responsible for a light‐dependent, heat‐induced cotyledon bleaching phenotype. Following heat treatment, mutant (chlg‐1) but not wild‐type seedlings accumulated a substantial level of chlorophyllide a, which resulted in a surge of phototoxic singlet oxygen. Immunoblot analysis suggested that the mutation destabilized the chlorophyll synthase proteins and caused a conditional blockage of esterification of chlorophyllide a after heat stress. Accumulation of chlorophyllide a after heat treatment occurred during recovery in the dark in the light‐grown but not the etiolated seedlings, suggesting that the accumulated chlorophyllides were not derived from de novo biosynthesis but from de‐esterification of the existing chlorophylls. Further analysis of the triple mutant harboring the CHLG mutant allele and null mutations of CHLOROPHYLLASE1 (CLH1) and CLH2 indicated that the known chlorophyllases are not responsible for the accumulation of chlorophyllide a in chlg‐1. Taken together, our results show that chlorophyll synthase acts in a salvage pathway for chlorophyll biosynthesis by re‐esterifying the chlorophyllide a produced during chlorophyll turnover.     

Functional Organization of the Chlorophyll-Containing Complexes of Chlamydomonas reinhardi: A Study of Their Formation and Interconnection with Reaction Centers in the Greening Process of the y-1 Mutant

The stepwise synthesis and assembly of photosynthetic membrane components in the y-1 mutant of Chlamydomonas reinhardi have been previously demonstrated (Ohad 1975 In Membrane Biogenesis, Mitochondria, Chloroplasts and Bacteria, Plenum, pp 279-350). This experimental system was used here in order to investigate the process of formation and interconnection of the energy collecting chlorophylls with the reaction centers of both photosystems I and II. The following measurements were carried out: photosynthetic electron flow at various light intensities, including parts or the entire electron transfer chain; analysis of the kinetics of fluorescence emission at room temperature and fluorescence emission spectra at 77 K, and electrophoretic separation of membrane polypeptides and chlorophyll protein complexes. Based on the data obtained it is concluded that: (a) each photosystem (PSI and PSII) contains, in addition to the reaction center, an interconnecting antenna and a main or light harvesting antenna complex; (b) the formation of the light harvesting complex, interconnecting antenna, and reaction centers for each photosystem can occur independently. (c) the interconnecting antennae link the light harvesting complexes with the respective reaction centers. In their absence, energy transfer between the light harvesting chlorophylls and the reaction centers is inefficient. The formation of the interconnecting antennae and efficient assembly of photosystem components occur simultaneously with the de novo synthesis of chlorophyll and at least three polypeptides, one translated in the cytoplasm and two translated in the chloroplast. The synthesis of these polypeptides was found to be light dependent.     

Removal of both Ycf48 and Psb27 in Synechocystis sp. PCC 6803 disrupts Photosystem II assembly and alters QA oxidation in the mature complex

The Photosystem II (PS II) assembly factors Psb27 and Ycf48 are transiently associated with PS II during its biogenesis and repair pathways. We investigated the function of these proteins by constructing knockout mutants in Synechocystis sp. PCC 6803. In ΔYcf48 cells, PS II electron transfer and stable oxygen evolution were perturbed. Additionally, Psb27 was required for photoautotrophic growth of cells lacking Ycf48 and assembly beyond the RC47 assembly complex in ΔYcf48:ΔPsb27 cells was impeded. Our results suggest the RC47 complex formed in ΔYcf48 cells is defective and that this deficiency is exacerbated if CP43 binds in the absence of Psb27.     

Regulation of chlorophyll apoprotein expression and accumulation. Requirements for carotenoids and chlorophyll.

Chlorophyll apoprotein accumulation and expression were examined in mutants of Chlamydomonas reinhardtii blocked at specific steps of carotenoid or chlorophyll synthesis. In the absence of carotenoids: 1) apoproteins of the core and light-harvesting complexes of photosystem I (CCI and LHCI, respectively) and photosystem II (CCII and LHCII, respectively) do not accumulate; 2) mRNAs for the CCI, CCII, and LHCII apoproteins accumulate to normal levels; and 3) synthesis of the chlorophyll apoproteins is differentially affected, or in some cases, not affected. In the absence of chlorophylls: 1) the apoproteins fail to accumulate; 2) mRNA levels for CCI and CCII apoproteins are relatively unchanged; 3) levels of LHCII apoprotein mRNA, but not rates of LHCII mRNA synthesis, are reduced in a light-dependent chlorophyll-synthesis mutant (ya12); and 4) synthesis of chlorophyll apoproteins is differentially affected or not affected in the case of several chloroplast-encoded apoproteins. These results demonstrate a direct role for carotenoids as well as chlorophylls in the stabilization of certain chlorophyll apoproteins and, for others, possibly in their translation. The data also indicate a role for chlorophyll synthesis in the stability of LHCII mRNA.     

Y3IP1, a Nucleus-Encoded Thylakoid Protein, Cooperates with the Plastid-Encoded Ycf3 Protein in Photosystem I Assembly of Tobacco and Arabidopsis

The intricate assembly of photosystem I (PSI), a large multiprotein complex in the thylakoid membrane, depends on auxiliary protein factors. One of the essential assembly factors for PSI is encoded by ycf3 (hypothetical chloroplast reading frame number 3) in the chloroplast genome of algae and higher plants. To identify novel factors involved in PSI assembly, we constructed an epitope-tagged version of ycf3 from tobacco (Nicotiana tabacum) and introduced it into the tobacco chloroplast genome by genetic transformation. Immunoaffinity purification of Ycf3 complexes from the transplastomic plants identified a novel nucleus-encoded thylakoid protein, Y3IP1 (for Ycf3-interacting protein 1), that specifically interacts with the Ycf3 protein. Subsequent reverse genetics analysis of Y3IP1 function in tobacco and Arabidopsis thaliana revealed that knockdown of Y3IP1 leads to a specific deficiency in PSI but does not result in loss of Ycf3. Our data indicate that Y3IP1 represents a novel factor for PSI biogenesis that cooperates with the plastid genome-encoded Ycf3 in the assembly of stable PSI units in the thylakoid membrane.     

Analysis of photosynthetic complexes from a cyanobacterial ycf37 mutant

The Ycf37 protein has been suggested to be involved in the biogenesis and/or stability of the cyanobacterial photosystem I (PSI) [A. Wilde, K. Lünser, F. Ossenbühl, J. Nickelsen, T. Börner, Characterization of the cyanobacterial ycf37: mutation decreases the photosystem I content, Biochem. J. 357 (2001) 211-216]. With Ycf37 specific antibodies, we analyzed the localization of Ycf37 within the thylakoid membranes of the cyanobacterium Synechocystis sp. PCC 6803. Inspection of a sucrose gradient profile indicated that small amounts of Ycf37 co-fractionated with monomeric photosynthetic complexes, but not with trimeric PSI. Isolating 3xFLAG epitope-tagged Ycf37 by affinity-tag purification rendered several PSI subunits that specifically co-precipitated with this protein. Blue-native PAGE newly revealed two monomeric PSI complexes (PSI and PSI*) in wild-type thylakoids. The lower amount of PsaK present in PSI* may explain its higher electrophoretic mobility. PSI* was more prominent in high-light grown cells and interestingly proved absent in the Δycf37 mutant. PSI* appeared again when the mutant was complemented in trans with the wild-type ycf37 gene. In the Δycf37 mutant the amount of trimeric PSI complexes was reduced to about 70% of the wild-type level with no significant changes in photochemical activity and subunit composition of the remaining photosystems. Our results indicate that Ycf37 plays a specific role in the preservation of PSI* and the biogenesis of PSI trimers.     

Chlorophyll-protein complexes of a marine green alga,Caulerpa cactoides

Three main chlorophyll-protein complexes have been resolved by gel electrophoresis from a marine green alga, Caulerpa cactoides, which has a low chlorophyll a/chorophyll b ratio of 1.62. Of the 6 chlorophyll-protein complexes resolved, two are chlorophyll a-proteins related to the reaction centre complex of photosystem 1, one is the chlorophyll a-protein of the presumed reaction centre complex of photosystem 2, and three are chlorophyll a/b-proteins of the light-harvesting complex. Some 61% of the total chlorophyll was associated with the light-harvesting complex and 23% and 6% with the reaction centre complexes of photosystems 1 and 2, respectively. In contrast to the light-harvesting complexes of higher plants which have equimolar amounts of chlorophylls a and b, the light-harvesting complex of Caulerpa has 1.45 times as much chlorophyll b as chlorophyll a. Variations in the pigment contents of the photosynthetic units of chlorophyll b-containing plants are reflected not only in varying amounts of total chlorophyll associated with each of the three main chlorophyll-protein complexes, but also in the stoichiometric amounts of chlorophyll a and chlorophyll b present in the light-harvesting complexes.     

Pigment ligation to proteins of the photosynthetic apparatus in higher plants

Ligation of pigments to proteins of the thylakoid membrane is a central step in the assembly of the photosynthetic apparatus in higher plants. Because of the potentially damaging photooxidative activity of chlorophylls, it is likely that between their biosynthesis and final assembly, chlorophylls will always be bound to protein complexes in which photooxidation is prevented by quenchers such as carotenoids. Such complexes may include chlorophyll carriers and/or membrane receptors involved in protein insertion into the membrane. Many if not all pigment-protein complexes of the thylakoid are stabilised towards protease attack by bound pigments. The major light-harvesting chlorophyll a/b protein (Lhebl,2) folds into its native structure in vitro only when it binds pigments. Pigment-induced folding may also be a general feature of chlorophyll-carotenoid proteins of the photosynthetic apparatus.     

The light-harvesting system of Euglena gracilis during the cell cycle

The apoproteins of the light-harvesting chlorophyll-protein complexes LHCI and CP29 (apparent molecular weights of 27 kDa and 29 kDa, respectively) of Euglena gracilis were identified immunologically. Both complexes are present in the thylakoids of autotrophically cultured Euglena cells during the whole cell cycle. The relative amount of each apoprotein tends to increase towards the end of the cell cycle. The light-harvesting chlorophyll-protein complex of photosystem II, LHCII, of E. gracilis contains chlorophyll a, chlorophyll b, neoxanthin, diadinoxanthin and beta-carotene. Its chlorophyll a/b ratio is about 1.7 during the whole cell cycle. About 9 h after cell division the ratio of diadinoxanthin to chlorophyll a is doubled for a time of 3–4 h. The relevance of this increase during one developmental stage is discussed in relation to the insertion and-or assembly of newly synthesized LHCII.Abbreviations LHCP light-harvesting chlorophyll-protein complex - PS photosystem This research was partly supported by the Deutsche Forschungsge meinschaft.     

Chlorophyll Turnover in Skeletonema costatum, a Marine Plankton Diatom

[³H]- and δ-[¹⁴C]Aminolevulinic acids were incorporated into the chlorophylls of Skeletonema costatum, a marine plankton diatom. In the stationary phase of growth, the tetrapyrrole-based pigments reached steady-state labeling after 10 hours. Under conditions of exponential cell division and chlorophyll accumulation, ³H was rapidly lost from the labeled chlorophylls and was replaced with ¹⁴C derived from δ-[4−¹⁴C]aminolevulinic acid. The kinetics of isotope dilution suggests recycling of tetrapyrrole precursors and/or two pigment pools, containing both chlorophyll a and chlorophyllide c, one which turns over rapidly (10 hours) and another which turns over more slowly (100 hours). Calculation of turnover times varied from 3 to 10 hours for chlorophyll a and from 7 to 26 hours for chlorophyllide c. The data suggest the dynamics of chlorophyll metabolism in S. costatum and explain the diatom's ability to undergo light-shade adaptation within a generation time.     

Chlorophyll analysis by high-performance liquid chromatography

The separation and determination of chlorophylls by high-performance liquid chromatography (HPLC) is described. Chlorophylls and their derivatives were separated by reversed-phase HPLC based on hydrophobic interaction between solute and support, using an octadecyl silica column and elution with 100% methanol. Separated pigments were detected fluorometrically with a sensitivity in the picomole range: the fluorescence response was linear over a wide pigment concentration range. Resolution of five chlorophylls a and four protochlorophyll species esterified with different alcohols was achieved within 22 min in a single experiment. This method can be used for the determination of chlorophyll b, bacteriochlorophyll a esters and products synthesized from chlorophyll, but not for nonesterified pigments, i.e., chlorophyllide, protochlorophyllide and chlorophyll c. The chromatographic mobility of chlorophyll a esterified with different alcohols increases with increasing number of carbon atoms in the esterifying alcohols. The plots obtained from the logarithm of the capacity factor (k′) of these pigments versus the numbers of carbon atoms of the alcohol molecule gave a straight line, thus permitting the estimation of the chain length of unknown pigment esterifying alcohols. This HPLC separation technique did not cause the formation of artifacts. The deviation of the individual retention time for each pigment is less than ±0.5%, thus making this method suitable for the rapid identification and quantification of unknown pigments.     

Overexpression of chlorophyllide a oxygenase (CAO) enlarges the antenna size of photosystem II in Arabidopsis thaliana

The light-harvesting efficiency of a photosystem is thought to be largely dependent on its photosynthetic antenna size. It has been suggested that antenna size is controlled by the biosynthesis of chlorophyll b. To verify this hypothesis, we overexpressed the enzyme for chlorophyll b biosynthesis, chlorophyllide a oxygenase (CAO), in Arabidopsis thaliana by transforming the plant with cDNA for CAO under the control of the 35S cauliflower mosaic virus promoter. In the early de-etiolation phase, when the intrinsic CAO expression is very low, the chlorophyll a: b ratio was drastically decreased from 28 to 7.3, indicating that enhancement of chlorophyll b biosynthesis had been successfully achieved. We made the following observations in full-green rosette leaves of transgenic plants. (1) The chlorophyll a : b ratio was reduced from 2.85 to 2.65. (2) The ratio of the peripheral light-harvesting complexes (LHCII) to the core antenna complex (CPa) resolved with the green-gel system increased by 20%. (3) The ratio of the light-harvesting complex II apoproteins (LHCP) to 47-kDa chlorophyll a protein (CP47), which was estimated by the results of immunoblotting, increased by 40%. These results indicated that the antenna size increased by at least 10-20% in transgenic plants, suggesting that chlorophyll b biosynthesis controls antenna size. To the best of our knowledge, this is the first report on enlargement of the antenna size by genetic manipulations.     

Chlorophyll Synthesis in Dark-Grown Pine Primary Needles

The pigment content of dark-grown primary needles of Pinus jeffreyi L. and Pinus sylvestris L. was determined by high-performance liquid chromatography. The state of protochlorophyllide a and of chlorophylls during dark growth were analyzed by in situ 77 K fluorescence spectroscopy. Both measurements unambiguously demonstrated that pine primary needles are able to synthesize chlorophyll in the dark. Norflurazon strongly inhibited both carotenoid and chlorophyll synthesis. Needles of plants treated with this inhibitor had low chlorophyll content, contained only traces of xanthophylls, and accumulated carotenoid precursors. The first form of chlorophyll detected in young pine needles grown in darkness had an emission maximum at 678 nm. Chlorophyll-protein complexes with in situ spectroscopic properties similar to those of fully green needles (685, 695, and 735 nm) later accumulated in untreated plants, whereas in norflurazon-treated plants the photosystem I emission at 735 nm was completely lacking. To better characterize the light-dependent chlorophyll biosynthetic pathway in pine needles, the 77 K fluorescence properties of in situ protochlorophyllide a spectral forms were studied. Photoactive and nonphotoactive protochlorophyllide a forms with emission properties similar to those reported for dark-grown angiosperms were found, but excitation spectra were substantially red shifted. Because of their lower chlorophyll content, norflurazon-treated plants were used to study the protochlorophyllide a photoreduction process triggered by one light flash. The first stable chlorophyllide photoproduct was a chlorophyllide a form emitting at 688 nm as in angiosperms. Further chlorophyllide a shifts usually observed in angiosperms were not detected. The rapid regeneration of photoactive protochlorophyllide a from nonphotoactive protochlorophyllide after one flash was demonstrated.     

Effects of chlorophyll a, chlorophyll b, and xanthophylls on the in vitro assembly kinetics of the major light-harvesting chlorophyll a/b complex, LHCIIb

The major light-harvesting chlorophyll a/b complex (LHCIIb) of photosystem II in higher plants can be reconstituted with pigments in lipid-detergent micelles. The pigment-protein complexes formed are functional in that they perform efficient internal energy transfer from chlorophyll b to chlorophyll a. LHCIIb formation in vitro, can be monitored by the appearance of energy transfer from chlorophyll b to chlorophyll a in time-resolved fluorescence measurements. LHCIIb is found to form in two apparent kinetic steps with time constants of about 30 and 200 seconds. Here we report on the dependence of the LHCIIb formation kinetics on the composition of the pigment mixture used in the reconstitution. Both kinetic steps slow down when the concentration of either chlorophylls or carotenoids is reduced. This suggests that the slower 200 seconds formation of functional LHCIIb still includes binding of both chlorophylls and carotenoids. LHCIIb formation is accelerated when the chlorophylls in the reconstitution mixture consist predominantly of chlorophyll a although the complexes formed are thermally less stable than those reconstituted with a chlorophyll a:b ratio < or = 1. This indicates that although chlorophyll a binding is more dominant in the observed rate of LHCIIb formation, the occupation of (some) chlorophyll binding sites with chlorophyll b is essential for complex stability. The accelerating effect of various carotenoids (lutein, zeaxanthin, violaxanthin, neoxanthin) on LHCIIb formation correlates with their affinity to two lutein-specific binding sites. We conclude that the occupation of these two carotenoid binding sites but not of the third (neoxanthin-specific) binding site is an essential step in the assembly of LHCIIb in vitro.     

Preferential accumulation of apoproteins of the light-harvesting chlorophyll a/b-protein complex in greening barley leaves treated with 5-aminolevulinic acid

In order to study the coordinate accumulation of chlorophyll (Chl) and apoproteins of Chl-protein complexes (CPs) during chloroplast development, we examined changes in the accumulation of the apoproteins in barley (Hordeum vulgare L.) leaves when the rate of Chl synthesis was altered by feeding 5-aminolevulinic acid (ALA), a precursor of Chl biosynthesis. Pretreatment with ALA increased the accumulation of Chl a and Chl b 1.5- and 2.3-fold, respectively, after 12 cycles of intermittent light (2 min light followed by 28 min darkness). Apoproteins of the light-harvesting Chl a/b-protein complex of photosystem II (LHCII) were increased 2.4-fold with ALA treatment. However, apoproteins of the P700-Chl a-protein complex (CP1) and the 43-kDa apoprotein of a Chl a-protein complex of photosystem II (CPa) were not increased by ALA application. With respect to CPs themselves, LHCII was increased when Chl synthesis was raised by ALA feeding, whereas CP1 exhibited no remarkable increase. These results indicate that LHCII serves a role in maintaining the stoichiometry of Chl to apoproteins by acting as a temporary pool for Chl molecules.Abbreviations ALA 5-aminolevulinic acid - Chl chlorophyll - CP chlorophyll-protein complex - CPa chlorophyll a-protein complex of PSII - CP1 P700-chlorophyll a-protein complex - LDS lithium dodecyl sulfate - LHCII light-harvesting chlorophyll a/b-protein complex of PSII This work was supported by the Grants-in-Aid for Scientific Research (04304004) from the Ministry of Education, Science and Culture, Japan.