Biosynthesis of the light-harvesting chlorophyll a/b protein. Polypeptide turnover in darkness

At the origin of the selectivity of the chlorophyll-binding sites in light harvesting complex II (LHCII)

The photosynthetic light-harvesting complexes (LHCs) are responsible for light absorption due to their pigment-binding properties. These pigments are primarily Chlorophyll (Chl) molecules of type a and b, which ensure an excellent coverage of the visible light spectrum. To date, it is unclear which factors drive the selective binding of different Chl types in the LHC binding pockets. To gain insights into this, we employed molecular dynamics simulations on LHCII binding different Chl types. From the resulting trajectories, we have calculated the binding affinities per each Chl-binding pocket using the Molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) model. To further examine the importance of the nature of the axial ligand in tuning the Chl selectivity of the binding sites, we used Density Functional Theory (DFT) calculations. The results indicate that some binding pockets have a clear Chl selectivity, and the factors governing these selectivities are identified. Other binding pockets are promiscuous, which is consistent with previous in vitro reconstitution studies. DFT calculations show that the nature of the axial ligand is not a major factor in determining the Chl binding pocket selectivity, which is instead probably controlled by the folding process.

Development and characterization of chlorophyll-amended montmorillonite clays for the adsorption and detoxification of benzene

After disasters, such as forest fires and oil spills, high levels of benzene (> 1 ppm) can be detected in the water, soil, and air surrounding the disaster site, which poses a significant health risk to human, animal, and plant populations in the area. While remediation methods with activated carbons have been employed, these strategies are limited in their effectiveness due to benzene's inherent stability and limited retention to most surfaces. To address this problem, calcium and sodium montmorillonite clays were amended with a mixture of chlorophyll (a) and (b); their binding profile and ability to detoxify benzene were characterized using in vitro, in silico, and well-established ecotoxicological (ecotox) bioassay methods. The results of in vitro isothermal analyses indicated that chlorophyll-amended clays showed an improved binding profile in terms of an increased binding affinity (Kf = 668 vs 67), increased binding percentage (52% vs 11%), and decreased rates of desorption (28% vs 100%), compared to the parent clay. In silico simulation studies elucidated the adsorption mechanism and validated that the addition of the chlorophyll to the clays increased the adsorption of benzene through Van der Waals forces (i.e., aromatic π-π stacking and alkyl-π interactions). The sorbents were also assessed for their safety and ability to protect sensitive ecotox organisms (Lemna minor and Caenorhabditis elegans) from the toxicity of benzene. The inclusion of chlorophyll-amended clays in the culture medium significantly reduced benzene toxicity to both organisms, protecting C. elegans by 98-100% from benzene-induced mortality and enhancing the growth rates of L. minor. Isothermal analyses, in silico modeling, and independent bioassays all validated our proof of concept that benzene can be sequestered, tightly bound, and stabilized by chlorophyll-amended montmorillonite clays. These novel sorbents can be utilized during disasters and emergencies to decrease unintentional exposures from contaminated water, soil, and air.

The terminal enzymes of (bacterio)chlorophyll biosynthesis

(Bacterio)chlorophylls are modified tetrapyrroles that are used by phototrophic organisms to harvest solar energy, powering the metabolic processes that sustain most of the life on Earth. Biosynthesis of these pigments involves enzymatic modification of the side chains and oxidation state of a porphyrin precursor, modifications that differ by species and alter the absorption properties of the pigments. (Bacterio)chlorophylls are coordinated by proteins that form macromolecular assemblies to absorb light and transfer excitation energy to a special pair of redox-active (bacterio)chlorophyll molecules in the photosynthetic reaction centre. Assembly of these pigment-protein complexes is aided by an isoprenoid moiety esterified to the (bacterio)chlorin macrocycle, which anchors and stabilizes the pigments within their protein scaffolds. The reduction of the isoprenoid 'tail' and its addition to the macrocycle are the final stages in (bacterio)chlorophyll biosynthesis and are catalysed by two enzymes, geranylgeranyl reductase and (bacterio)chlorophyll synthase. These enzymes work in conjunction with photosynthetic complex assembly factors and the membrane biogenesis machinery to synchronize delivery of the pigments to the proteins that coordinate them. In this review, we summarize current understanding of the catalytic mechanism, substrate recognition and regulation of these crucial enzymes and their involvement in thylakoid biogenesis and photosystem repair in oxygenic phototrophs.

Fine Mapping and Characterization of a Major Gene Responsible for Chlorophyll Biosynthesis in Brassica napus L

Rapeseed (Brassica napus L.) is mainly used for oil production and industrial purposes. A high photosynthetic efficiency is the premise of a high yield capable of meeting people’s various demands. Chlorophyll-deficient mutants are ideal materials for studying chlorophyll biosynthesis and photosynthesis. In a previous study, we obtained the mutant yl1 for leaf yellowing throughout the growth period by ethyl methanesulfonate mutagenesis of B. napus. A genetic analysis showed that the yl1 chlorophyll-deficient phenotype was controlled by one incompletely dominant gene, which was mapped on chromosome A03 by a quantitative trait loci sequencing analysis and designated as BnA03.Chd in this study. We constructed an F2 population containing 5256 individuals to clone BnA03.Chd. Finally, BnA03.Chd was fine-mapped to a 304.7 kb interval of the B. napus ‘ZS11’ genome containing 58 annotated genes. Functional annotation, transcriptome, and sequence variation analyses confirmed that BnaA03g0054400ZS, a homolog of AT5G13630, was the most likely candidate gene. BnaA03g0054400ZS encodes the H subunit of Mg-chelatase. A sequence analysis revealed a single-nucleotide polymorphism (SNP), causing an amino-acid substitution from glutamic acid to lysine (Glu1349Lys). In addition, the molecular marker BnaYL1 was developed based on the SNP of BnA03.Chd, which perfectly cosegregated with the chlorophyll-deficient phenotype in two different F2 populations. Our results provide insight into the molecular mechanism underlying chlorophyll synthesis in B. napus.

Linking mode of action of the model respiratory and photosynthesis uncoupler 3,5-dichlorophenol to adverse outcomes in Lemna minor

Standard chemical toxicity testing guidelines using aquatic plant Lemna minor have been developed by several international standardisation organisations. Although being highly useful for regulatory purposes by focusing on traditional adverse endpoints, these tests provide limited information about the toxic mechanisms and modes of action (MoA). The present study aimed to use selected functional assays in L. minor after exposure to 3,5-dichlorophenol (3,5-DCP) as a model to characterise the toxic mechanisms causing growth inhibition and lethality in primary producers. The results demonstrated that 3,5-DCP caused concentration-dependent effects in chloroplasts and mitochondria. Uncoupling of oxidative phosphorylation (OXPHOS), reduction in chlorophyll (Chlorophyll a and b) content, reproduction rate and frond size were the most sensitive endpoints, followed by formation of reactive oxygen species (ROS), lipid peroxidation (LPO), reduction of carotenoid content and impairment of photosynthesis efficiency. Suppression of photosystem II (PSII) efficiency, electron transport rate (ETR), chlorophyll (a and b) contents and oxidative phosphorylation (OXPHOS) were closely correlated while ROS production and LPO were negative correlated with ETR, carotenoid content and growth parameters. A network of conceptual Adverse Outcome Pathways (AOPs) was developed to decipher the causal relationships between molecular, cellular, and apical adverse effects occurring in L. minor to form a basis for future studies with similar compounds.

Biogenesis of light harvesting proteins

The LHC family includes nuclear-encoded, integral thylakoid membrane proteins, most of which coordinate chlorophyll and xanthophyll chromophores. By assembling with the core complexes of both photosystems, LHCs form a flexible peripheral moiety for enhancing light-harvesting cross-section, regulating its efficiency and providing protection against photo-oxidative stress. Upon its first appearance, LHC proteins underwent evolutionary diversification into a large protein family with a complex genetic redundancy. Such differentiation appears as a crucial event in the adaptation of photosynthetic organisms to changing environmental conditions and land colonization. The structure of photosystems, including nuclear- and chloroplast-encoded subunits, presented the cell with a number of challenges for the control of the light harvesting function. Indeed, LHC-encoding messages are translated in the cytosol, and pre-proteins imported into the chloroplast, processed to their mature size and targeted to the thylakoids where are assembled with chromophores. Thus, a tight coordination between nuclear and plastid gene expression, in response to environmental stimuli, is required to adjust LHC composition during photoacclimation. In recent years, remarkable progress has been achieved in elucidating structure, function and regulatory pathways involving LHCs; however, a number of molecular details still await elucidation. In this review, we will provide an overview on the current knowledge on LHC biogenesis, ranging from organization of pigment-protein complexes to the modulation of gene expression, import and targeting to the photosynthetic membranes, and regulation of LHC assembly and turnover. Genes controlling these events are potential candidate for biotechnological applications aimed at optimizing light use efficiency of photosynthetic organisms. This article is part of a Special Issue entitled: Chloroplast biogenesis.

Overexpression of protochlorophyllide oxidoreductase C regulates oxidative stress in Arabidopsis

Light absorbed by colored intermediates of chlorophyll biosynthesis is not utilized in photosynthesis; instead, it is transferred to molecular oxygen, generating singlet oxygen (¹O₂). As there is no enzymatic detoxification mechanism available in plants to destroy ¹O₂, its generation should be minimized. We manipulated the concentration of a major chlorophyll biosynthetic intermediate i.e., protochlorophyllide in Arabidopsis by overexpressing the light-inducible protochlorophyllide oxidoreductase C (PORC) that effectively phototransforms endogenous protochlorophyllide to chlorophyllide leading to minimal accumulation of the photosensitizer protochlorophyllide in light-grown plants. In PORC overexpressing (PORCx) plants exposed to high-light, the ¹O₂ generation and consequent malonedialdehyde production was minimal and the maximum quantum efficiency of photosystem II remained unaffected demonstrating that their photosynthetic apparatus and cellular organization were intact. Further, PORCx plants treated with 5-aminolevulinicacid when exposed to light, photo-converted over-accumulated protochlorophyllide to chlorophyllide, reduced the generation of ¹O₂ and malonedialdehyde production and reduced plasma membrane damage. So PORCx plants survived and bolted whereas, the 5-aminolevulinicacid-treated wild-type plants perished. Thus, overexpression of PORC could be biotechnologically exploited in crop plants for tolerance to ¹O₂-induced oxidative stress, paving the use of 5-aminolevulinicacid as a selective commercial light-activated biodegradable herbicide. Reduced protochlorophyllide content in PORCx plants released the protochlorophyllide-mediated feed-back inhibition of 5-aminolevulinicacid biosynthesis that resulted in higher 5-aminolevulinicacid production. Increase of 5-aminolevulinicacid synthesis upregulated the gene and protein expression of several downstream chlorophyll biosynthetic enzymes elucidating a regulatory net work of expression of genes involved in 5-aminolevulinicacid and tetrapyrrole biosynthesis.

The roles of chloroplast proteases in the biogenesis and maintenance of photosystem II

Photosystem II (PSII) catalyzes one of the key reactions of photosynthesis, the light-driven conversion of water into oxygen. Although the structure and function of PSII have been well documented, our understanding of the biogenesis and maintenance of PSII protein complexes is still limited. A considerable number of auxiliary and regulatory proteins have been identified to be involved in the regulation of this process. The carboxy-terminal processing protease CtpA, the serine-type protease DegP and the ATP-dependent thylakoid-bound metalloprotease FtsH are critical for the biogenesis and maintenance of PSII. Here, we summarize and discuss the structural and functional aspects of these chloroplast proteases in these processes. This article is part of a Special Issue entitled: SI: Photosystem II.

Regulation of protein metabolism: Coupling of photosynthetic electron transport to in vivo degradation of the rapidly metabolized 32-kilodalton protein of the chloroplast membranes

In Spirodela oligorrhiza, mature chloroplasts copiously synthesize and degrade a 32-kilodalton membrane protein. The rates of synthesis and degradation are controlled by light intensity, the protein being unstable in the light and stable in the dark. Light-driven synthesis, but not degradation, is dependent on ATP. Degradation is blocked by herbicides inhibiting photosystem II electron transport, such as diuron and atrazine. Thus, both anabolism and catabolism of the 32-kilodalton protein are photoregulated, with degradation coupled to electron transport rather than phosphorylation.

Chlorophyll biosynthesis and assembly into chlorophyll-protein complexes in isolated developing chloroplasts

Isolated developing plastids from greening cucumber cotyledons or from photoperiodically grown pea seedlings incorporated (14)C-labeled 5-aminolevulinic acid (ALA) into chlorophyll (Chl). Incorporation was light dependent, enhanced by S-adenosylmethionine, and linear for 1 hr. The in vitro rate of Chl synthesis from ALA was comparable to the in vivo rate of Chl accumulation. Levulinic acid and dioxoheptanoic acid strongly inhibited Chl synthesis but not plastid protein synthesis. Neither chloramphenicol nor spectinomycin affected Chl synthesis, although protein synthesis was strongly inhibited. Components of thylakoid membranes from plastids incubated with [(14)C]ALA were resolved by electrophoresis and then subjected to autoradiography. This work showed that (i) newly synthesized Chl was assembled into Chl-protein complexes and (ii) the inhibition of protein synthesis during the incubation did not alter the labeling pattern. Thus, there was no observable short-term coregulation between Chl synthesis (from ALA) and the synthesis of membrane proteins in isolated plastids.

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.

Consecutive Binding of Chlorophylls a and b During the Assembly in Vitro of Light-harvesting Chlorophyll-a/b Protein (LHCIIb)

The apoprotein of the major light-harvesting chlorophyll a/b complex (LHCIIb) is post-translationally imported into the chloroplast, where membrane insertion, protein folding, and pigment binding take place. The sequence and molecular mechanism of the latter steps is largely unknown. The complex spontaneously self-organises in vitro to form structurally authentic LHCIIb upon reconstituting the unfolded recombinant protein with the pigments chlorophyll a, b, and carotenoids in detergent micelles. Former measurements of LHCIIb assembly had revealed two apparent kinetic phases, a faster one (tau1) in the range of 10 s to 1 min, and a slower one (tau2) in the range of several min. To unravel the sequence of events we analysed the binding of chlorophylls into the complex by using time-resolved fluorescence measurements of resonance energy transfer from chlorophylls to an acceptor dye attached to the apoprotein. Chlorophyll a, offered in the absence of chlorophyll b, bound with the faster kinetics (tau1) exclusively whereas chlorophyll b, in the absence of chlorophyll a, bound predominantly with the slower kinetics (tau2). In double-jump experiments, LHCIIb assembly could be dissected into a faster chlorophyll a and a subsequent, predominantly slower chlorophyll b-binding step. The assignment of the faster and the slower kinetic phase to predominantly chlorophyll a and exclusively chlorophyll b binding, respectively, was verified by analysing the assembly kinetics with a circular dichroism signal in the visible domain presumably reflecting the establishment of pigment-pigment interactions. We propose that slow chlorophyll binding is confined to the exclusively chlorophyll b binding sites whereas faster binding occurs to the chlorophyll a binding sites. The latter sites can bind both chlorophylls a and b but in a reversible fashion as long as the complex is not stabilised by proper occupation of the chlorophyll b sites. The resulting two-step model of LHCIIb assembly is able to reconcile the highly specific binding sites containing either chlorophyll a or b, as seen in the recent crystal structures of LHCIIb, with the observation of promiscuous binding sites able to bind both chlorophyll a and b in numerous reconstitution analyses of LHCIIb assembly.

Evidence in support of a role for plant-mediated proteolysis in the rumens of grazing animals

The present work aimed to differentiate between proteolytic activities of plants and micro-organisms during the incubation of grass in cattle rumens. Freshly cut ryegrass was placed in bags of varying permeability and incubated for 16 h in the rumens of dairy cows that had previously grazed a ryegrass sward, supplemented with 4 kg dairy concentrate daily. Woven polyester bags (50 microm pore size) permitted direct access of the micro-organisms and rumen fluid enzymes to the plant material. The polythene was impermeable even to small molecules such as NH(3). Dialysis tubing excluded micro-organisms and rumen enzymes/metabolites larger than 10 kDa. DM loss was 46.3 % in polyester, 36.2 % in polythene and 38.1 % in dialysis treatments. It is possible that the DM loss within polythene bags occurred due to a solubilisation of plant constituents (e.g. water-soluble carbohydrates) rather than microbial attachment/degradation processes. The final protein content of the herbage residues was not significantly different between treatments. Regardless of bag permeability, over 97 % of the initial protein content was lost during incubations in situ. Electrophoretic separation showed that Rubisco was extensively degraded in herbage residues whereas the membrane-associated, light-harvesting protein remained relatively undegraded. Protease activity was detected in herbage residues and bathing liquids after all incubation in situ treatments. Although rumen fluid contains proteases (possibly of plant and microbial origin), our results suggest that, owing to cell compartmentation, their activity against the proteins of intact plant cells is limited, supporting the view that plant proteases are involved in the degradation of proteins in freshly ingested herbage.

Folding in vitro of light-harvesting chlorophyll a/b protein is coupled with pigment binding

The major light-harvesting chlorophyll a/b protein (LHCIIb) of the plant photosynthetic apparatus is able to self-organise in vitro. When the recombinant apoprotein, Lhcb1, is solubilised in the denaturing detergent sodium (or lithium) dodecylsulfate (SDS or LDS) and then mixed with chlorophylls and carotenoids under renaturing conditions, structurally authentic LHCIIb forms. Assembly of functional LHCIIb, as indicated by the establishment of energy transfer between complex-bound chlorophyll molecules, occurs in two apparent kinetic steps with time constants of 10 to 30 seconds and 50 to 300 seconds, depending on the reaction conditions. Here, we use circular dichroism (CD) in the far-UV range to monitor the folding of the LHCIIb apoprotein as it is complexed with pigments. The alpha-helix content in the protein's secondary structure increases in two apparent kinetic steps with time constants similar to those observed for the establishment of chlorophyll energy transfer. When the carotenoid concentration in the reaction mixture is reduced, the time constants of alpha-helix formation increase, as do those for the appearance of chlorophyll energy transfer. This indicates that both processes, pigment assembly and secondary structure formation, are tightly coupled. A substantial amount of alpha-helix is present in dodecylsulfate-solubilised LHCIIb apoprotein and appears to be distributed among various protein domains.

Inhibition of the light-independent synthesis of chlorophyll in pine cotyledons at low temperature

Cotyledons of Japanese black pine (Pinus thunbergii) were yellow when they developed in darkness at 8 degrees C since the light-independent synthesis of chlorophyll was almost completely inhibited in these cotyledons. The level of chlorophyll in dark-grown cotyledons was less than one-twentieth of that in light-grown cotyledons at the same temperature. In the yellow cotyledons, levels of transcripts of cab, rbcS, rbcL and psbA genes were quite high. The large and small subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase were also detected at relatively high levels in yellow cotyledons. However, the accumulation of the two apoproteins of the light-harvesting chlorophyll a/b-binding protein of PSII was limited because of the limited supply of chlorophyll.

Cloning and characterisation of chlorophyll synthase from Avena sativa

The chlorophyll synthase gene from oat (Avena sativa) was cloned and expressed in Escherichia coli. The deduced amino acid sequence consists of 378 amino acids including a presequence of 46 amino acids. Deletion mutants show that a core protein comprising amino acid residues 88 to 377 is enzymatically active. The sequence of the mature protein shows 85% identity with the chlorophyll synthase of Arabidopsis thaliana and 62% identity with the chlorophyll synthase of Synechocystis PCC 6803. The gene is constitutively expressed as the same transcript level is found in dark-grown and in light-grown seedlings. The enzyme requires magnesium ions for activity; manganese ions can reconstitute only part of the activity. Diacetyl and N-phenylmaleimide (NPM) inhibit the enzyme activity. Site-directed mutagenesis reveals that, out of the 4 Arg residues present in the active core protein, Arg-91 and Arg-161 are essential for the activity. Five cysteine residues are present in the core protein, of which only Cys-109 is essential for the enzyme activity. Since the wild-type and all other Cys-mutants with the exception of the mutant C304A are inhibited by N-phenylmaleimide, we conclude that the inhibitor binds to a non-essential Cys residue to abolish activity. The role of the various Arg and Cys residues is discussed.

The N-terminal domain of the light-harvesting chlorophyll a/b-binding protein complex (LHCII) is essential for its acclimative proteolysis

Variations in the amount of the light-harvesting chlorophyll a/b-binding protein complex (LHCII) is essential for regulation of the uptake of light into photosystem II. An endogenous proteolytic system was found to be involved in the degradation of LHCII in response to elevated light intensities and the proteolysis was shown to be under tight regulation [Yang, D.-H. et al. (1998) Plant Physiol. 118, 827-834]. In this study, the substrate specificity and recognition site towards the protease were examined using reconstituted wild-type and mutant recombinant LHCII. The results show that the LHCII apoprotein and the monomeric form of the holoprotein are targeted for proteolysis while the trimeric form is not. The N-terminal domain of LHCII was found to be essential for recognition by the regulatory protease and the involvement of the N-end rule pathway is discussed.

Light-harvesting chlorophyll a/b-binding protein stably inserts into etioplast membranes supplemented with Zn-pheophytin a/b

Light-harvesting chlorophyll a/b-binding protein, LHCP, or its precursor, pLHCP, cannot be stably inserted into barley etioplast membranes in vitro. However, when these etioplast membranes are supplemented with the chlorophyll analogs Zn-pheophytin a/b, synthesized in situ from Zn-pheophorbide a/b and digeranyl pyrophosphate, pLHCP is inserted into a protease-resistant state. This proves that chlorophyll is the only component lacking in etioplast membranes that is necessary for stable LHCP insertion. Synthesis of Zn-pheophytin b alone promotes insertion of LHCP in vitro into a protease-resistant state, whereas synthesis of Zn-pheophytin a alone does not. Insertion of pLHCP into etioplast membranes can also be stimulated by adding chlorophyll a and chlorophyll b to the membranes, albeit at a significantly lower efficiency as compared with Zn-pheophytin a/b synthesized in situ. When pLHCP is inserted into chlorophyll- or Zn-pheophytin-supplemented etioplast membranes and then assayed with protease, only the protease digestion product indicative of the monomeric major light-harvesting chlorophyll a/b complex (LHCII) is found but not the one indicating trimeric complexes. In this respect, chlorophyll- or Zn-pheophytin-supplemented etioplast membranes resemble thylakoid membranes at an early greening stage: pLHCP inserted into plastid membranes from greening barley is assembled into trimeric LHCII only after more than 1 h of greening.

Photoregulation of chloroplast developm ent: transcriptional, translational and post-translational controls?

Chloroplast development involves the nucleus, the cytoplasm and the chloroplast of plant cells. This may be illustrated by reference to the two most abundant proteins of the chloroplast: (i) the soluble CO 2 -fixing enzyme ribulose 1,5-bisphosphate carboxylase—oxygenase, whose large subunit (LSU) is encoded in chloroplast DNA and synthesized on chloroplast ribosomes and whose small subunit (SSU) is encoded in nuclear DNA, synthesized on cytoplasmic ribosomes in precursor form and transported into chloroplasts, and (ii) the thylakoid-bound light-harvesting chlorophyll a/b complex, whose pigment components are synthesized in the chloroplast and whose apoproteins resemble the SSU in site of coding and site of synthesis. We have examined the extent to which biosynthetic events in the nucleocytoplasmic compartments are coordinated with those inside the chloroplast during the de-etiolation of pea seedlings. We have examined the levels of LSU, SSU and the light-harvesting chlorophyll a/b protein (LHCP) by using a highly specific radioimmune assay. The steady-state levels of the corresponding mRNAs have been determined using specific cloned DNA probes. With the SSU, the mRNA and protein levels are near the limit of detection in dark-grown plants but increase markedly under continuous white light, with a lag of about 24 h. The protein appears to be under simple phytochrome control at the level of the steady-state concentration of its mRNA. The LSU also appears to be regulated through the steady-state concentration of its mRNA but in this case the mRNA is not under simple phytochrome control. The LHCP mRNA is readily detectable in dark-grown plants and accumulates further under illumination in a phytochrome-mediated manner. However, the LHCP itself (like chlorophyll) is not detectable in dark-grown plants and accumulates to high levels only under continuous illumination, with a lag of about 6 h. Post-translational control is particularly important in the accumulation of the LHCP: continuous chlorophyll synthesis is required for the stabilization of the protein within the thylakoid membrane, at least during the early stages of chloroplast development.

Unassembled subunits of the photosynthetic oxygen-evolving complex present in the thylakoid lumen are long-lived and assembly-competent

Physiologically healthy pea chloroplasts contain unassembled, soluble subunits of the oxygen-evolving complex in the thylakoid lumen. We report that the lifetimes of two of these subunits, both on and off the membrane, exceed 8 h in vitro. We also demonstrate that each of the subunits present in the thylakoid lumen is competent for assembly into the membrane-bound complex. These data are consistent with the postulate that the soluble lumen-resident subunits play a role in photosystem II homeostasis. We also demonstrate that the reconstitution of the 33 kDa subunit is inhibited by extremely low concentrations of Triton X-100, suggesting that hydrophobic interactions are involved in the binding of this subunit to the photosystem II reaction center.

Light is required for efficient translation elongation and subsequent integration of the D1-protein into photosystem II

The light dependence of translation and successive assembly of the D1 reaction center protein into Photosystem II subcomplexes was followed in fully developed chloroplasts isolated from the dark phase of diurnally grown spinach. The incorporation of synthesized D1 protein into Photosystem II (PSII) was analyzed by fractionation of radiolabeled unassembled protein and PSII (sub)complexes on sucrose density gradients. The ribosomes with attached nascent chains were recovered as pellets in the same gradients, and nascent chains of the D1 protein were immunoprecipitated. The analysis showed that absence of light during translation leads to an increased accumulation of polysome-bound D1 translation intermediates, indicating that light is required for efficient elongation of the D1 protein. The accumulation of the D1 protein and CP43 decreased three-fold in darkness, whereas accumulation of the D2 reaction center protein was not affected by light. In addition, light was also required for efficient incorporation of the D1 protein into the PSII core complex. In darkness, the newly synthesized D1 protein accumulated predominantly as unassembled protein or in PSII subcomplexes smaller than 100 kDa.

The regulation of enzymes involved in chlorophyll biosynthesis

All living organisms contain tetrapyrroles. In plants, chlorophyll (chlorophyll a plus chlorophyll b) is the most abundant and probably most important tetrapyrrole. It is involved in light absorption and energy transduction during photosynthesis. Chlorophyll is synthesized from the intact carbon skeleton of glutamate via the C5 pathway. This pathway takes place in the chloroplast. It is the aim of this review to summarize the current knowledge on the biochemistry and molecular biology of the C5-pathway enzymes, their regulated expression in response to light, and the impact of chlorophyll biosynthesis on chloroplast development. Particular emphasis will be placed on the key regulatory steps of chlorophyll biosynthesis in higher plants, such as 5-aminolevulinic acid formation, the production of Mg(2+)-protoporphyrin IX, and light-dependent protochlorophyllide reduction.

Immunological detection of proteins similar to bacterial proteases in higher plant chloroplasts

Despite numerous demonstrations of protein degradation in chloroplasts of higher plants, little is known about the identity of the proteases involved in these reactions. To identify chloroplast proteases by immunological means, we investigated two proteins: ClpP, a protein similar to the proteolytic subunit of the bacterial ATP-dependent Clp protease, for which a gene is found in the chloroplast genome [Maurizi, M.R., Clark, W.P., Kim, S. H. & Gottesman, S. (1990) J. Biol. Chem. 265, 12546-12552] and PrcA, a cyanobacterial Ca2+-stimulated protease [Maldener, I., Lockau, W., Cai, Y. & Wolk, P. (1991) Mol. & Gen. Genet. 225, 113-120]. We expressed the clpP gene from rice in Escherichia coli, purified its product, and generated antibodies against the product. Western blot analysis revealed the ClpP protein in different leaf extracts. Analysis of fractionated barley chloroplasts revealed that the protein was associated with the stromal fraction. The expression of ClpP is light independent and tissue specific, as it was found in green and etiolated barley leaves, but not in roots. A second protein, similar to the cyanobacterial protease PrcA, was also detected in chloroplasts. Antibody against this protease recognized proteins in various leaf extracts. When pea chloroplasts were fractionated, the antibody only recognized a stromal protein. The expression of this protein is regulated by light, as it was found in green leaves, but not in etiolated leaves. The tissue specificity of PrcA was similar to that of ClpP in that it could not be detected in root extracts.

Degradation of mistargeted OEE33 in the chloroplast stroma

OEE33, a component of the oxygen-evolving enzyme in chloroplasts, normally resides in the thylakoid lumen. In an attempt to study the fate of mistargeted proteins in chloroplasts, we substituted the bipartite transit peptide of OEE33 with that of CAB7, an integral thylakoid-membrane protein. As a result, when imported into isolated chloroplasts, the chimeric protein protein was targeted to the stroma instead of the thylakoid lumen. Whereas the wild-type OEE33 was totally stable for at least 2 h, the chimeric protein was rapidly degraded, with a half-life of 60 min. Degradation of the chimeric protein was stimulated by ATP supplementation. Degradation could also be observed in lysed chloroplasts, in an ATP-stimulated manner. When lysates were fractionated, the proteolytic activity was found to be associated mainly with the stromal fraction. This activity was very effectively inhibited by all tested inhibitors of serine proteases. Western blot analysis demonstrated that the stromal fraction active in degrading the chimeric OEE33 contains ClpC and ClpP, homologues of the regulatory and proteolytic subunits, respectively, of the bacterial, ATP-dependent, serine-type Clp protease.

The nuclear-encoded chlorophyll-binding photosystem II-S protein is stable in the absence of pigments

The 22-kDa chlorophyll a/b-binding protein (CAB) (psbS gene product) is associated with photosystem II and related to the CAB gene family. Here we report that the PSII-S protein unlike other chlorophyll-binding proteins is stable in the absence of pigments. It is present in etiolated spinach plants and accumulates in the dark progressively with the cellular development of the seedlings. Furthermore, it is present in several pigment-deficient mutants. Analysis of the pigment composition of the PSII-S protein isolated from etiolated plants suggests that neither carotenoids nor chlorophyll precursors are involved in its stabilization in the dark. Exposure of etiolated spinach to light leads to further accumulation of the PSII-S protein, which appears more early than other chlorophyll-binding proteins. Accumulation of the PSII-S protein in green plants is developmentally regulated and restricted to photosynthetic tissues. It is suggested that the function of the PSII-S protein may not be light-harvesting but it could act as a ligand chaperone required for transient binding of pigments during biogenesis or turnover of chlorophyll-binding proteins. Such function would be essential for coordination between pigment biosynthesis and ligation as well as avoiding toxic effects of non-bound chlorophyll molecules.

Light-dependent chlorophyll a biosynthesis upon chlL deletion in wild-type and photosystem I-less strains of the cyanobacterium Synechocystis sp. PCC 6803

Part of the chlL gene encoding a component involved in light-independent protochlorophyllide reduction was deleted in wild type and in a photosystem I-less strain of Synechocystis sp. PCC 6803. In resulting mutants, chlorophyll biosynthesis was fully light-dependent. When these mutants were propagated under light-activated heterotrophic growth conditions (in darkness except for 15 min of weak light a day) for several weeks, essentially no chlorophyll was detectable but protochlorophyllide accumulated. Upon return of the chlL- mutant cultures to continuous light, within the first 6 h chlorophyll was synthesized at the expense of protochlorophyllide at a rate independent of the presence of photosystem I. Chlorophyll biosynthesized during this time gave rise to a 685 nm fluorescence emission peak at 77 K in intact cells. This peak most likely originates from a component different from those known to be directly associated with photosystems II and I. Development of 695 and 725 nm peaks (indicative of intact photosystem II and photosystem I, respectively) required longer exposures to light. After 6 h of greening, the rate of chlorophyll synthesis slowed as protochlorophyllide was depleted. In the chlL- strain, greening occurred at the same rate at two different light intensities (5 and 50 microE m-2 s-1), indicating that also at low light intensity the amount of light is not rate-limiting for protochlorophyllide reduction. Thus, in this system the rate of chlorophyll biosynthesis is limited neither by biosynthesis of photosystems nor by the light-dependent protochlorophyllide reduction. We suggest the presence of a chlorophyll-binding 'chelator' protein (with 77 K fluorescence emission at 685 nm) that binds newly synthesized chlorophyll and that provides chlorophyll for newly synthesized photosynthetic reaction centers and antennae.

Degradation of plastocyanin in copper-deficient Chlamydomonas reinhardtii. Evidence for a protease-susceptible conformation of the apoprotein and regulated proteolysis

In the green alga Chlamydomonas reinhardtii, the copper-dependent accumulation of plastocyanin is effected via the altered stability of the protein in copper-deficient versus copper-sufficient medium (t1/2) < 20 min versus several hours). To understand the mechanism of plastocyanin degradation in vivo, the purified apoprotein was characterized relative to the holoprotein with respect to conformation and protease susceptibility. Circular dichroism spectroscopy revealed that the apoprotein in solution did not display the characteristic secondary structure displayed by the native or reconstituted holoprotein. The apoprotein was also susceptible to digestion in vitro by chymotrypsin whereas the holoprotein was resistant. High ionic conditions, which stabilize the folded structure of apoplastocyanin, also inhibit its degradation by chymotrypsin. These results suggest that one explanation for plastocyanin degradation in copper-deficient cells in vivo might be the increased susceptibility of the apo form to a lumenal protease. Since apoplastocyanin is a normal biosynthetic intermediate for the formation of holoplastocyanin, the increased susceptibility of apoplastocyanin to proteolysis implies that degradative and biosynthetic activities would compete for the same substrate. However, characterization of an apoplastocyanin-accumulating mutant suggests that a plastocyanin-degrading protease is active only in copper-deficient cells. Thus, apoplastocyanin is rapidly degraded in copper-deficient cells, whereas its major fate in copper-supplemented cells is holoplastocyanin formation.

Circadian expression of the light-harvesting protein of Photosystem II in etiolated bean leaves following a single red light pulse: Coordination with the capacity of the plant to form chlorophyll and the thylakoid-bound protease

The appearance of the light harvesting II (LHC II) protein in etiolated bean leaves, as monitored by immunodetection in LDS-solubilized leaf protein extracts, is under phytochrome control. A single red light pulse induces accumulation of the protein, in leaves kept in the dark thereafter, which follows circadian oscillations similar to those earlier found for Lhcb mRNA (Tavladoraki et al. (1989) Plant Physiol 90: 665-672). These oscillations are closely followed by oscillations in the capacity of the leaf to form Chlorophyll (Chl) in the light, suggesting that the synthesis of the LHC II protein and its chromophore are in close coordination. Experiments with levulinic acid showed that PChl(ide) resynthesis does not affect the LHC II level nor its oscillations, but new Chl a synthesis affects LHC II stabilization in thylakoids, implicating a proteolytic mechanism. A proteolytic activity against exogenously added LHC II was detected in thylakoids of etiolated bean leaves, which was enhanced by the light pulse. The activity, also under phytochrome control, was found to follow circadian oscillations in verse to those in the stabilization of LHC II protein in thylakoids. Such a proteolytic mechanism therefore, may account for the circadian changes observed in LHC II protein level, being implicated in pigment-protein complex assembly/stabilization during thylakoid biogenesis.

Biosynthesis of cytochrome f in Chlamydomonas reinhardtii: analysis of the pathway in gabaculine-treated cells and in the heme attachment mutant B6

Chlamydomonas reinhardtii uses two c-type cytochromes for photosynthetic electron transfer: the thylakoid membrane-bound cytochrome f of the cytochrome b6f complex and the soluble cytochrome c6. Previously, a class of photosynthesis-minus, acetate-requiring mutants was identified which were deficient in both c-type cytochromes, and biochemical analyses of cytochrome c6 biosynthesis in these strains indicated that they were each blocked at the step of heme attachment to apocytochrome c6. In order to demonstrate that the deficiency in cytochrome f results from the same biochemical and genetic defect, cytochrome f biosynthesis was examined in the B6 mutant (a representative of this phenotypic class) and in spontaneous suppressor strains derived from B6. Pulse-radiolabeling experiments show that B6 synthesizes a form of cytochrome f that is rapidly degraded in vivo. This polypeptide is membrane associated and migrates with an electrophoretic mobility identical to that of standard apocytochrome f produced in vitro but slightly greater than that of standard holocytochrome f produced in vivo by wild-type cells. These findings suggest that the B6 strain is unable to convert apocytochrome f to holocytochrome f and that apocytochrome f is unstable in vivo. In the suppressed strains, accumulation of both holocytochrome f and holocytochrome c6 is restored. One suppressor mutation (strain B6R) displays uniparental inheritance whereas another (B6T3) displays Mendelian inheritance. In both cases, the three phenotypes, photosynthesis-plus, b6f+ and cyt c6+ co-segregate in genetic crosses. This study therefore confirms that the dual cyt b6f-/cytc6- deficiency in B6 results from a single mutation that affects a step in holocytochrome formation that is common to the biosynthetic pathways of both plastidic c-type cytochromes. The study also confirms that pre-apocytochrome f synthesis, processing and association with the membrane is not dependent on heme attachment. Synthesis of cytochrome f does, however, appear to be dependent on heme availability. In cells depleted of tetrapyrrole pathway intermediates by gabaculine treatment, cytochrome f synthesis was significantly reduced. Since gabaculine treatment did not affect the stability of cytochrome f nor the accumulation of cytochrome f-encoding transcripts, the reduction is attributed to post-transcriptional regulation of preapocytochrome f synthesis via a pathway that is sensitive to the availability of heme or a tetrapyrrole pathway intermediate.

Post-transcriptional and post-translational regulatory steps are crucial in controlling the appearance and stability of thylakoid polypeptides during the transition of etiolated tobacco seedlings to white light

We have investigated the expression of nuclear-encoded chloroplast proteins that are not associated with chlorophyll (the lumenal 33-kDa and 23-kDa polypeptides of the oxygen-evolving system of photosystem II, plastocyanin and the Rieske Fe/S protein) by comparing mRNA-accumulation rates with those of the corresponding proteins during illumination of etiolated tobacco seedlings. Using subcellular fractionation, pulse/chase, Northern and Western techniques, we found that the biogenesis and stability of these proteins are regulated both translationally, as well as post-translationally, including the efficiency of mRNA uptake into polysomes, processes that operate between translation and assembly or monitor the status (soluble and membrane-attached) of a terminally processed polypeptide. Polypeptide synthesis is generally not limited by mRNA amounts. For instance, steady-state transcript levels may increase 10-fold during illumination, while those associated with polysomes increase only 2-3-fold without measurable influence on the rate of protein synthesis. The 23-kDa and Rieske polypeptides are predominantly membrane associated, but plastocyanin and the 33-kDa polypeptide are distributed among both soluble and membrane-associated protein fractions. Plastocyanin appears to be comparably stable in both forms. However, for the 33-kDa polypeptide, only the membrane-attached form is stable (> 8 h) and only this pool increases upon illumination. Its soluble form is rapidly degraded with a half-life of approximately 1 h under the chosen conditions. Our findings probably reflect part of a more general regulatory principle operating in the differentiation and maintenance of subcellular structure.

Mutation of histidine residues in CP47 leads to destabilization of the photosystem II complex and to impairment of light energy transfer

Site-directed mutagenesis has been used to change conserved histidine residues in hydrophobic regions of the photosystem II chlorophyll-binding protein CP47 in the cyanobacterium Synechocystis sp. PCC 6803. Nine mutants with one, four mutants with two, and four mutants with three His mutations in CP47 have been generated and characterized. Mutation of any one of seven different His residues to Tyr leads to slower photoautotrophic growth and apparent destabilization of the PS II complex. Mutations introduced into multiple His residues in one mutant exhibited a cumulative effect. Replacing His by Asn leads to a much smaller effect than observed upon mutation to Tyr. This is consistent with the hypothesis that the mutated His residues are chlorophyll ligands: Asn can substitute as chlorophyll ligand, whereas Tyr cannot. Further evidence supporting a role of the mutated His residues in chlorophyll binding comes from measurements of the light intensity needed to half-saturate oxygen evolution. All His mutants with impaired PS II function needed higher light intensities for half-saturation than wild type. A possible explanation for this decrease in antenna efficiency in the mutants is a loss of the Mg in the chlorophyll due to a loss of the fifth ligand, and thus the formation of a pheophytin molecule in the antenna. We conclude that conserved His residues in hydrophobic regions of CP47 indeed are chlorophyll ligands and that these ligands are important for PS II stability as well as efficient antenna function.

Synthesis of chlorophyll a regulates translation of chlorophyll a apoproteins P700, CP47, CP43 and D2 in barley etioplasts

Accumulation of plastid-encoded chlorophyll apoproteins and chlorophyll synthesis are controlled by light in angiosperms. An in vitro system utilizing isolated and lysed barley (Hordeum vulgare L.) etioplasts revealed the specific accumulation of P700, CP47, CP43 and D2 triggered by de novo synthesis of chlorophyll. Accumulation rates of radiolabelled chlorophyll apoproteins were linear for about 30 min. Pulse/chase translation assays showed that synthesis of chlorophyll does not result in increased chlorophyll apoprotein stability. Instead turnover rates of chlorophyll apoproteins were higher in the presence than in the absence of chlorophyll. Chlorophyll-dependent accumulation of chlorophyll apoproteins must therefore be regulated on the level of translation. Translation of chlorophyll apoproteins was blocked to about 50% by addition of 30-50 microM aurintricarboxylic acid or 20 microM kasugamycin. The kinetics of chlorophyll-dependent translation indicated that the in vitro translation system is capable of translation initiation. The capability of translation initiation was lost in lysed etioplasts after preincubation for at least 5 min without chlorophyll synthesis. The results suggest that initiation is involved in chlorophyll-dependent regulation of translation.

Characterization of a prolyl endopeptidase from spinach thylakoids

A prolyl endopeptidase (PEPase, EC 3.4.21.26) that specifically cleaves the 18-kDa protein of photosystem II was extracted from photosystem II membranes with 1 M NaCl. Proteolytic activity measured with artificial substrates was less than a quarter of that with the protein. Studies on inhibition of the proteolysis by an artificial substrate suggested that the protease recognizes the scissile prolyl bond. The protease was inhibited by CuCl2, but not by diisopropyl fluorophosphate or p-chloromercuriphenylsulfonic acid. These findings suggest that the protease represents a new class of PEPase. The specificity of the enzyme is discussed in relation to the structure of the 18-kDa protein.