Intracellular coding sites of polypeptides associated with photosynthetic oxygen evolution of photosystem II

Assembly and Repair of Photosystem II in Chlamydomonas reinhardtii

Oxygenic photosynthetic organisms use Photosystem II (PSII) to oxidize water and reduce plastoquinone. Here, we review the mechanisms by which PSII is assembled and turned over in the model green alga Chlamydomonas reinhardtii. This species has been used to make key discoveries in PSII research due to its metabolic flexibility and amenability to genetic approaches. PSII subunits originate from both nuclear and chloroplastic gene products in Chlamydomonas. Nuclear-encoded PSII subunits are transported into the chloroplast and chloroplast-encoded PSII subunits are translated by a coordinated mechanism. Active PSII dimers are built from discrete reaction center complexes in a process facilitated by assembly factors. The phosphorylation of core subunits affects supercomplex formation and localization within the thylakoid network. Proteolysis primarily targets the D1 subunit, which when replaced, allows PSII to be reactivated and completes a repair cycle. While PSII has been extensively studied using Chlamydomonas as a model species, important questions remain about its assembly and repair which are presented here.

The single-copy genepsbScodes for a phylogenetically intriguing 22 kDa polypeptide of photosystem II

Recombinant phages that encode the complete precursor polypeptide for the 22 kDa polypeptide associated with photosystem II have been serologically selected from two lambda gt11 expression libraries made from polyadenylated RNA of spinach seedlings. The cDNAs hybridize to a 1.3 kb RNA species. The precursor protein is comprised of 274 amino acid residues and carries an N-terminal transit peptide of probably 69 amino acid residues. The mature protein exhibits four predicted transmembrane segments and is shown to be an integral component of photosystem II originating in a single-copy gene. The unique characteristics of this protein are: (i) it is the result of a gene-internal duplication of an ancestor with two membrane spans, (ii) a striking resemblance to LHC I/II, CP24/CP29 apoproteins, and ELIPs, although it does not bind chlorophyll and is present in cyanobacteria, and, as these proteins, (iii) it integrates into the membrane with uncleaved routing signals that display remarkable resemblance to patterns found in bipartite transit peptides.

Isolation of high-chlorophyll-fluorescence mutants of Arabidopsis thaliana and their characterisation by spectroscopy, immunoblotting and northern hybridisation

Thirty-four recessive photosynthetic mutants of the high-chlorophyll-fluorescence (hcf) phenotype have been isolated by screening 7700 M2 progenies of ethyl methane sulfonate-treated seeds of Arabidopsis thaliana. Most of the mutants isolated were found to be seedling-lethal, but could be grown on sucrose-supplemented media. Chlorophyll (Chl) fluorescence induction, absorption changes in the reaction-centre chlorophyll of PS I (P700) at 830 nm and Chl a/Chl b ratios were recorded in order to probe the photosynthetic functions and to define the mutational lesion. These studies were complemented by immunoblot and Northern analyses which finally led to the classification of the mutants into six different groups. Four classes of mutants were affected in PS I, PS II (two different classes) or the intersystem electron-transport chain, respectively. A fifth mutant class was of pleiotropic nature and the sixth class comprised a Chl b-deficient mutant. Several of the mutants showed severe deficiencies in the levels of subunits of PS I, PS II or the cytochrome b6/f complex. Thus the mutational lesions could be located precisely. Only one mutant was defective in the transcript patterns of some plastid-encoded photosynthesis genes. Hence most of the mutants isolated appear to be affected in translational and post-translational regulatory processes of thylakoid membrane biogenesis or in structural genes encoding constituent subunits of the thylakoid protein complexes.

The assembly of chloroplast membranes

During the last five or six years there has been a shift in focus in the field of chloroplast protein transport, with greater emphasis being placed on understanding the sorting of proteins to the thylakoids and the envelope membranes. As a result, we have a much-improved understanding of the variety of important pathways that function during chloroplast biogenesis. It is now clear that a considerable number of distinct intraorganellar sorting mechanisms operate to direct imported proteins to their correct destinations. Some of the underlying mechanisms are also beginning to emerge, although it is accurate to say that we are still a long way from understanding in genuine detail how proteins are translocated across chloroplast membranes. However, the availability of such a range of efficient in vitro import assays should ensure that rapid progress is made in coming years. The major gaps in this field now concern the identities and roles of the elements of the important apparatus: Although at least two distinct translocation systems operate during chloroplast biogenesis, none of these components has been identified, purified, or cloned. This is primarily because these proteins are often difficult to assay individually, and they are usually of very low abundance. Nevertheless, it is essential that progress is made in this area soon in order to maintain the present momentum.

Sorting of precursor proteins between isolated spinach leaf mitochondria and chloroplasts

The precursors of the F1-ATPase beta-subunits from Nicotiana plumbaginifolia and Neurospora crassa were imported into isolated spinach (Spinacia oleracea L.) leaf mitochondria. Both F1 beta precursors were imported and processed to mature size products. No import of the mitochondrial precursor proteins into isolated intact spinach chloroplasts was seen. Moreover, the precursor of the 33 kDa protein of photosynthetic water-splitting enzyme was not imported into the leaf mitochondria. This study provides the first experimental report of in vitro import of precursor proteins into plant mitochondria isolated from photosynthetic tissue and enables studies of protein sorting between mitochondria and chloroplasts in a system which is homologous with respect to organelles. The results suggest a high organellar specificity in the plant cell for the cytoplasmically synthesized precursor proteins.

Differential biogenesis of photosystem-II in mesophyll and bundle-sheath cells of 'malic' enzyme NADP(+)-type C4 plants. A comparative protein and RNA analysis

We have investigated the photosystem-II organization in differentiating-bundle-sheath cells of the three malate dehydrogenase (oxaloacetate decarboxylating) (NADP+)-type C4 species maize, Sorghum and Pennisetum. Using a set of nine different antisera raised against individual subunits of photosystem-II, we demonstrate that photosystem-II components constitute a substantial part of the thylakoid membranes of young bundle-sheath chloroplasts. The abundance of subunits of the photosystem-II core, i.e. the 47-and 43-kDa chlorophyll-a-binding proteins, polypeptides D1 and D2, cytochrome b559, and the 34-kDa polypeptide, varies with the developmental state of the plant. However, the levels of the 23-kDa, 16-kDa and 10-kDa extrinsic polypeptides of the water-oxidation complex are drastically reduced in bundle-sheath chloroplasts of all three species analyzed, regardless of their state of differentiation. The reduction in protein abundance is also reflected at the transcript level: only traces of the nuclear-encoded mRNAs are found in differentiating bundle-sheath cells of Sorghum, suggesting that the transcription of these genes has been switched off. Our data are compatible with the idea that the water-oxidation complex is a prime site for initiating or maintaining the process leading to photosystem-II depletion during differentiation of bundle-sheath cells.

Polymorphism and Mendelian inheritance of photosystem II 23-kilodalton polypeptide

Soluble proteins from leaves of Nicotiana glauca Grah., N. langsdorffii Weinm., their reciprocal hybrids and amphiploid hybrid (N. glaucaxN. langsdorffii) were resolved by two-dimensional gel electrophoresis. Among a group of well-resolved polypeptides, in the isoelectric-point range of 5-5.5 and relative-molecular-mass (Mr) range of 18-23 kilodaltons (kDa), species-specific variation was observed. Polypeptides designated "L" and "l" are specific to N. langsdorffii, and "G" and "g" to N. glauca, while "C" is common to both species. Polypeptides "L", "G" and "C" are localized in the chloroplasts and associated with thylakoid membranes. Polypeptide "L" is more acidic than polypeptide "G", and both polypeptides have an Mr of 23 kDa. They were isolated from two-dimensional gels and their first 13 N-terminal amino-acid sequences were determined. These were found to be identical to the 13N-terminal amino acids of the photosystem II (PSII) 23-kDa polypeptide from spinach (T. Jansen et al. (1987) FEBS Lett. 216, 234-240) and, except for one change, to those from pea (R. Wales et al. (1989) Plant Molec. Biol., in press). Polypeptides "G" and "L" cross-react with antiserum against the PSII 23-kDa polypeptide from pea. Therefore, polypeptides "G" and "L" are extrinsic PSII 23-kDa polypeptides. They appear jointly and in equal amounts in the reciprocal hybrids. Since chloroplasts in Nicotiana are maternally inherited, these results demonstrate that polypeptides "G" and "L" are encoded by nuclear genes, are polymorphic variants of the PSII 23-kDa polypeptide, and are inherited in a Mendelian manner.

Primary structure of spinach-chloroplast thioredoxin f. Protein sequencing and analysis of complete cDNA clones for spinach-chloroplast thioredoxin f

The primary structure of thioredoxin f from spinach chloroplasts was determined by standard amino acid sequencing and furthermore by sequencing the corresponding nuclear genome region. The protein, with a calculated molecular mass of 12,564 Da and a molar absorption coefficient at 280 nm of 17,700 M-1 cm-1, consists of 113 residues and exhibits 24% residue identities with spinach chloroplast thioredoxin mb or Escherichia coli thioredoxin. A monospecific antibody elicited against thioredoxin f has been used to select recombinant phage from spinach cDNA libraries in lambda gt11. The inserts of positive clones were sequenced. They code for a polypeptide of 190 amino acids, composed of the thioredoxin f sequence (113 residues) and an upstream element (77 residues) which most probably constitutes the N-terminal transit peptide that directs the polypeptide into chloroplasts. In vitro transcription and translation of this construct generates a polypeptide of approximately 21 kDa, which is imported by isolated spinach chloroplasts and processed to the mature 12.5-kDa protein.

Analysis of the genes of the OEE1 and OEE3 proteins of the photosystem II complex from Chlamydomonas reinhardtii

The sequences of the nuclear genes of the 33 kDa (OEE1) and the 16 kDa (OEE3) polypeptides of the oxygen evolving complex of Chlamydomonas reinhardtii have been established. Comparison between the OEE1 protein sequences of C. reinhardtii and higher plants and cyanobacteria reveals 67 and 47% homology. In contrast, C. reinhardtii and higher plants have only 28% overall homology for OEE3 which is mostly limited to the central portion of the protein. The transit peptides of the C. reinhardtii proteins consist of 52 (OEE1) and, most likely, 51 (OEE1) amino acids. They have a basic amino terminal region and, at least in the case of OEE1, a hydrophobic segment at their carboxy terminal end typical of thylakoid lumen proteins. Comparison of the genomic and cDNA clones indicates that the OEE1 and OEE3 genes contain five and four introns, respectively, some of which are located within the coding sequences of the transit peptides.

The extrinsic 33 kDa polypeptide of the oxygen-evolving complex of photosystem II is a putative calcium-binding protein and is encoded by a multi-gene family in pea

The extrinsic 33 kDa polypeptide of the water-oxidizing complex has been extracted from pea photosystem II particles by washing with alkaline-Tris and purified by ion-exchange chromatography. The N-terminal amino acid sequence has been determined, and specific antisera have been raised in rabbits and used to screen a pea leaf cDNA library in λgt11. Determination of the nucleotide sequence of positive clones revealed an essentially full-length cDNA for the 33 kDa polypeptide, the deduced amino acid sequence showing it to code for a mature protein of 248 amino acids with an N-terminal transit peptide of 81 amino acids. The protein showed a high degree of conservation with previously reported sequences for the 33 kDa protein from other species and the sequence contained a putative Ca(2+)-binding site with homology to mammalian intestinal calcium-binding proteins. Northern analysis of total pea RNA indicated a message of approximately 1.4 kb, in good agreement with the size of the cDNA obtained at 1.3 kbp. Southern blots of genomic DNA probed with the labelled cDNA give rise to several bands suggesting that the 33 kDa polypeptide is coded by a multi-gene family.

A 10 kDa polypeptide associated with the oxygen-evolving complex of photosystem II has a putative C-terminal non-cleavable thylakoid transfer domain

The N-terminal amino acid sequence of the 10 kDa polypeptide associated with the oxygen-evolving complex of wheat photosystem II has been determined and shown to be homologous to the amino acid sequence of the product of the ST-LS1 gene from potato. The N-terminal sequence of the mature protein indicates that the polypeptide is synthesized with a 39 amino acid N-terminal presequence which is similar to chloroplast import sequences but which lacks a hydrophobic domain for transfer of the protein across the thylakoid membrane. The mature polypeptide has a C-terminal hydrophobic region which shows homology to the hydrophobic thylakoid transfer domain of other lumenal proteins and this hydrophobic region of the 10 kDa polypeptide is suggested to facilitate transfer of the protein across the thylakoid membrane.

Cloning, nucleotide sequence and mutational analysis of the gene encoding the Photosystem II manganese-stabilizing polypeptide of Synechocystis 6803

Affinity purified, polyclonal antibodies raised against the Photosystem II 33 kDa manganese-stabilizing polypeptide of the spinach oxygen-evolving complex were used to isolate the gene encoding the homologous protein from Synechocystis 6803. Comparison of the amino acid sequence deduced from the Synechocystis psb1 nucleotide sequence with recently published sequences of spinach and pea confirms the homology indicated by antigenic cross-reactivity and shows that the cyanobacterial and higher plant sequences are 43% identical and 63% conserved. Regions of identity, varying in length from 1 to 10 consecutive residues, are distributed throughout the protein. The 28 residues at the amino terminus of the psb1 gene product, characteristic of prokaryotic signal peptides, show homology with the carboxyl-terminal third of the transit sequences of pea and spinach and are most likely needed for the transport of the manganese-stabilizing protein across the thylakoid membrane to its destination of the lumen. Synechocystis mutants which contain a kanamycin resistance gene cassette inserted into the coding region for the 32 kDa polypeptide were constructed. These mutants contain no detectable 32 kDa polypeptide, do not evolve oxygen, and are incapable of photoautotrophic growth.

Light-dependent accumulation and localization of photosystem II proteins in maize

We have raised antibodies against several major components of photosystem II. These antisera, which are directed against the apoproteins of two chlorophyll-binding proteins (CPa-1 and CPa-2), the apoprotein of light-harvesting complex II and the 33-kDa extrinsic protein of the oxygen-evolving complex, were used to examine the light regulation of photosystem II assembly in maize. The principal findings of this study are as follows. The 33-kDa protein is present in dark-grown maize and the content increases 5-10-fold upon illumination. The level of the protein is mediated at least in part by phytochrome and is independent of the accumulation of chlorophyll. In contrast, none of the three chlorophyll-binding proteins examined was detectable in leaves of maize grown in darkness or under other light regimes where chlorophyll does not accumulate. Even in the absence of photosystem II assembly, the 33-kDa protein is properly transported across the thylakoid into the lumen. However, the protein does not attach in the normal way to the inner surface of the membrane under these conditions.

Differential expression of oxygen-evolving polypeptide genes in maize leaf cell types

Three polypeptides of 33 kD, 23 kD and 16 kD are released from maize photosystem II particles by alkaline Tris solution treatment and shown to cross-react with antisera to purified spinach oxygen-evolving (OE) polypeptides of 34 kD, 23 kD and 17 kD, respectively. They are not located exclusively in mesophyll cells but each is about 4 times more abundant in the thylakoid membranes of mesophyll than bundle sheath cells of etiolated, greening and green leaves. Three maize cDNA clones (OE33, OE23, OE16) have been identified by hybrid-selection, in vitro translation and immunoprecipitation with antisera against spinach OEs. Transcripts of all three genes are already detectable in both mesophyll and bundle sheath cells of etiolated leaves; however, they accumulate transiently and coordinately in mesophyll cells but remain at a constant low level in bundle sheath cells upon illumination of dark-grown maize seedlings. Moreover, the level of each protein increases in mesophyll cells following the accumulation of transcripts during greening and remains high in late greening and green leaves, despite the decline in each corresponding mRNA. The accumulation of all three OE proteins is also stimulated by light in bundle sheath cells without increases in their corresponding mRNAs. The preferential localization of these three proteins in mesophyll cells is due to both transcriptional and translational regulation.

Energy dependence of protein translocation into chloroplasts

The translocation of in vitro synthesized precursor proteins into intact spinach chloroplasts was investigated with respect to its energy requirement. It was demonstrated that MgATP itself, and not a transmembrane electrochemical gradient across the envelope membrane, promotes protein import. By manipulating the external and the stromal level of MgATP, we provided evidence that MgATP energized the protein import not within the chloroplast but at the outside of the envelope membrane. It is postulated that an MgATP-dependent phosphorylation/dephosphorylation cycle at the outer membrane face was involved in the course of protein translocation into the chloroplast.

In vitro synthesis and assembly of photosystem II proteins of spinach chloroplasts

The synthesis and assembly of photosystem II (PS II) proteins of spinach chloroplasts were investigated in three different in vitro systems, i.e., protein synthesis in isolated chloroplasts (in organello translation), read-out translation of thylakoid-bound ribosomes, and transport of translation products from spinach leaf polyadenylated RNA into isolated chloroplasts. Polyacrylamide gel electrophoresis of labeled thylakoid polypeptides in the presence of sodium dodecyl sulfate revealed that the first two systems were capable of synthesizing the reaction center proteins of PS II (47 and 43 kDa), the herbicide-binding protein, and cytochrome b559. The reaction center proteins synthesized in organello were shown to bind chlorophyll and to assemble properly into the PS II core complex. One of the reaction center proteins translated by the thylakoid-bound ribosomes (47 kDa) was also found to be integrated in situ into the complex but was lacking bound chlorophyll. Incorporation of radioactivity into the three extrinsic proteins of the oxygen-evolution system (33, 24, and 18 kDa) was detected only when intact chloroplasts were incubated with the translation products from polyadenylated RNA, showing that these proteins are coded for by nuclear DNA. The occurrence of a precursor polypeptide 6 kDa larger than the 33-kDa protein was immunochemically detected in the translation products.

Transport of proteins into chloroplasts. The effect of incorporation of amino acid analogues on the import and processing of chloroplast polypeptides

We have synthesized abnormal precursors of imported chloroplast proteins by incorporating amino acid analogues during translation in a cell-free wheat germ system. Incorporation of analogues of either proline, arginine or leucine markedly inhibits both the import of Pisum sativum ribulosebisphosphate carboxylase small subunit precursor by isolated chloroplasts and processing to the mature size by the purified processing enzyme. One effect of the arginine analogue is to remove a positive charge(s) in the precursor essential for efficient recognition by the processing enzyme. Incorporation of a lysine analogue results in moderate inhibition of the import of small subunit precursor but complete inhibition of import of the chlorophyll a/b-binding polypeptide precursor. The effect of carboxymethylation on the import of chloroplast proteins is also analyzed. The results indicate that residues essential for transport of the imported proteins by the chloroplast vary among different protein precursors.

Localization of the gene for apocytochromeb-559 on the plastid chromosome of spinach

The gene for cytochromeb-559, associated with the photosystem II reaction center, has been located on the spinach plastid chromosome by cell-free coupled transcription-translation and RNA-programmed hybrid selection translation using appropriate recombinant DNAs, RNA fractions, and monospecific antisera. The gene is located in the large single-copy segment of the plastid chromosome between the genes for cytochomef and the P680 chlorophylla apoprotein of photosystem II and transcribed in the opposite direction relative to these genes. The 10 kd protein is decoded from a bicistronic 1.0 kb mRNA and is apparently not made as a precursor in cell-free rabbit reticulocyte andE. coli lysates.