Identification of a new gene in the chloroplast genome encoding a low-molecular-mass polypeptide of photosystem II complex

The PsbK subunit is required for the stable assembly and stability of other small subunits in the PSII complex in the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1

PsbK is a small membrane protein of the PSII core complex and is highly conserved from cyanobacteria to plants. Here, we studied its role in the thermophilic cyanobacterium, Thermosynechococcus elongatus BP-1, by focusing on a psbK disruptant with hexahistidine-tagged CP47. The psbK disruptant showed photoautotrophic growth comparable with that of the wild type under a wide range of light conditions. The mutant PSII complex retained the oxygen-evolving activity with a unique modification of the acceptor Q(B) site. N-terminal sequencing revealed that Ycf12 and PsbZ proteins were lost in the PSII complex prepared from the mutant. Immunoblotting detected reduced accumulation of PsbZ in the mutant thylakoid. These results suggest that PsbK is required not only for association of PsbZ and Ycf12 with the isolated PSII complex but also for the stabilization of PsbZ in the thylakoid membrane.

Insights into the function of PsbR protein in Arabidopsis thaliana

The functional state of the Photosystem (PS) II complex in Arabidopsis psbR T-DNA insertion mutant was studied. The DeltaPsbR thylakoids showed about 34% less oxygen evolution than WT, which correlates with the amounts of PSII estimated from Y(D)(ox) radical EPR signal. The increased time constant of the slow phase of flash fluorescence (FF)-relaxation and upshift in the peak position of the main TL-bands, both in the presence and in the absence of DCMU, confirmed that the S(2)Q(A)(-) and S(2)Q(B)(-) charge recombinations were stabilized in DeltaPsbR thylakoids. Furthermore, the higher amount of dark oxidized Cyt-b559 and the increased proportion of fluorescence, which did not decay during the 100s time span of the measurement thus indicating higher amount of Y(D)(+)Q(A)(-) recombination, pointed to the donor side modifications in DeltaPsbR. EPR measurements revealed that S(1)-to-S(2)-transition and S(2)-state multiline signal were not affected by mutation. The fast phase of the FF-relaxation in the absence of DCMU was significantly slowed down with concomitant decrease in the relative amplitude of this phase, indicating a modification in Q(A) to Q(B) electron transfer in DeltaPsbR thylakoids. It is concluded that the lack of the PsbR protein modifies both the donor and the acceptor side of the PSII complex.

PsbI affects the stability, function, and phosphorylation patterns of photosystem II assemblies in tobacco

Photosystem II (PSII) core complexes consist of CP47, CP43, D1, D2 proteins and of several low molecular weight integral membrane polypeptides, such as the chloroplast-encoded PsbE, PsbF, and PsbI proteins. To elucidate the function of PsbI in the photosynthetic process as well as in the biogenesis of PSII in higher plants, we generated homoplastomic knock-out plants by replacing most of the tobacco psbI gene with a spectinomycin resistance cartridge. Mutant plants are photoautotrophically viable under green house conditions but sensitive to high light irradiation. Antenna proteins of PSII accumulate to normal amounts, but levels of the PSII core complex are reduced by 50%. Bioenergetic and fluorescence studies uncovered that PsbI is required for the stability but not for the assembly of dimeric PSII and supercomplexes consisting of PSII and the outer antenna (PSII-LHCII). Thermoluminescence emission bands indicate that the presence of PsbI is required for assembly of a fully functional Q(A) binding site. We show that phosphorylation of the reaction center proteins D1 and D2 is light and redox-regulated in the wild type, but phosphorylation is abolished in the mutant, presumably due to structural alterations of PSII when PsbI is deficient. Unlike wild type, phosphorylation of LHCII is strongly increased in the dark due to accumulation of reduced plastoquinone, whereas even upon state II light phosphorylation is decreased in delta psbI. These data attest that phosphorylation of D1/D2, CP43, and LHCII is regulated differently.

A suppressor mutation in the alpha-phycocyanin gene in the light/glucose-sensitive phenotype of the psbK-disruptant of the cyanobacterium Synechocystis sp. PCC 6803

psbK encodes a small transmembrane component of PSII. Here we report that the psbK-disruptant of Synechocystis sp. PCC 6803 cannot survive under photomixotrophic conditions of light and glucose after transient growth, while the wild type is able to grow. A spontaneous yellow-green mutant that recovered the sustained growth under the same conditions was isolated from the psbK-disruptant. Instead of recovery, the mutant largely lost photoautotrophic growth. By phenotype complementation, the mutation was identified in cpcA as a sequence replacement with a close downstream segment, generating an inverted repeat of 23 bp. The mutant phenotype was characterized by (i) the complete loss of alpha- and beta-phycocyanin; (ii) increased accumulation of PSII; and (iii) greatly reduced transcripts harboring cpcA in abundance and in size. The inverted repeat generated in cpcA probably led to the early termination of transcription. A possible mechanism for such a mutation is discussed.

The low molecular mass subunits of the photosynthetic supracomplex, photosystem II

The photosystem II (PSII) complex is located in the thylakoid membrane of higher plants, algae and cyanobacteria and drives the water oxidation process of photosynthesis, which splits water into reducing equivalents and molecular oxygen by solar energy. Electron and X-ray crystallography analyses have revealed that the PSII core complex contains between 34 and 36 transmembrane alpha-helices, depending on the organism. Of these helices at least 12-14 are attributed to low molecular mass proteins. However, to date, at least 18 low molecular mass (<10 kDa) subunits are putatively associated with the PSII complex. Most of them contain a single transmembrane span and their protein sequences are conserved among photosynthetic organisms. In addition, these proteins do not have any similarity to any known functional proteins in any type of organism, and only two of them bind a cofactor. These findings raise intriguing questions about why there are so many small protein subunits with single-transmembrane spans in the PSII complex, and their possible functions. This article reviews our current knowledge of this group of proteins. Deletion mutations of the low molecular mass subunits from both prokaryotic and eukaryotic model systems are compared in an attempt to understand the function of these proteins. From these comparisons it seems that the majority of them are involved in stabilization, assembly or dimerization of the PSII complex. The small proteins may facilitate fast dynamic conformational changes that the PSII complex needs to perform an optimal photosynthetic activity.

Structure, function and assembly of Photosystem II and its light-harvesting proteins

Photosystem II (PSII) is a multisubunit chlorophyll-protein complex that drives electron transfer from water to plastoquinone using energy derived from light. In green plants, the native form of PSII is surrounded by the light-harvesting complex (LHCII complex) and thus it is called the PSII-LHCII supercomplex. Over the past several years, understanding of the structure, function, and assembly of PSII and LHCII complexes has increased considerably. The unicellular green alga Chlamydomonas reinhardtii has been an excellent model organism to study PSII and LHCII complexes, because this organism grows heterotrophically and photoautotrophically and it is amenable to biochemical, genetic, molecular biological and recombinant DNA methodology. Here, the genes encoding and regulating components of the C. reinhardtii PSII-LHCII supercomplex have been thoroughly catalogued: they include 15 chloroplast and 20 nuclear structural genes as well as 13 nuclear genes coding for regulatory factors. This review discusses these molecular genetic data and presents an overview of the structure, function and assembly of PSII and LHCII complexes.

Evidence that the PsbK polypeptide is associated with the photosystem II core antenna complex CP43

PsbK is encoded by the chloroplast psbK gene and is one of the small polypeptides of photosystem II (PSII). This polypeptide is required for accumulation of the PSII complex. In the present study, we generated an antibody against recombinant mature PsbK of Chlamydomonas and used it in Western blots to localize PsbK in the PSII core complex. PsbK was found in the thylakoid membranes, and purification of the PSII core complex from detergent-solubilized thylakoid membranes showed that PsbK is tightly associated with the PSII core complex. We used potassium thiocyanate to separate PSII into subcore complexes, including the D1/D2/cytochrome b559 reaction center complex, CP47, and CP43, and we found that PsbK co-purifies with one of the core antenna complexes, CP43, during ion exchange chromatography. Subsequent gel filtration chromatography of the purified CP43 confirmed that PsbK is tightly associated with CP43. Steady-state levels of PsbK were also determined in Chlamydomonas mutants expressing various levels of PSII. Quantitative Western blotting revealed that the levels of PsbK in these mutants are approximately equal to those of CP43, suggesting that PsbK is stable only when associated with CP43 in the chloroplast. Together, our results indicate that PsbK is an integral part of the PSII complex and may participate in the assembly and stability of the PSII complex.

Isolation and characterization of monomeric and dimeric CP47-reaction center photosystem II complexes

Using the detergents n-dodecyl beta-D-maltoside and heptyl thioglycopyranoside, a subcore complex of photosystem II (PSII) has been isolated that contains the chlorophyll-binding protein, CP47, and the reaction center components, D1, D2, and cytochrome b559. We have found, by using sucrose density centrifugation, that the resulting preparation consisted of a mixture of dimeric and monomeric forms of the CP47 reaction center (RC) complex, having molecular masses of 410 +/- 30 and 200 +/- 28 kDa, respectively, as estimated by size exclusion chromatography. The level of the dimer in the preparation is significantly higher than the monomeric form. Both the monomer and dimer contain the proteins CP47, D1, and D2 and the alpha- and beta-subunits of cytochrome b559. Analyses by mass spectrometry and N-terminal sequencing showed that both forms of the CP47-RC complex contain the products of the psbI, psbTc (chloroplast gene), and psbW with molecular masses of 4195.5, 3849.6, and 5927.4 Da, respectively. In contrast to the monomeric form, the CP47-RC dimer contained two extra proteins with low molecular weights, identified as the products of the psbL and psbK genes having molecular masses of 4365.5 and 4292.1, respectively. It was also found that the dimer contained slightly more molecules of chlorophyll a (21 +/- 2.5) than the monomer (18 +/- 1.5), a characteristic also observed in the room temperature absorption spectrum by comparing the ratio of absorption at 416 and 435 nm. Of particular note was the finding that the dimer, but not the monomer, contained plastoquinone-9 (estimated to be 1.5 +/- 0.3 molecules per RC). The results indicate that the CP47-RC monomer is derived from the dimeric form of the complex, and therefore the latter is likely to represent an in vivo conformation. The PsbTc as well as the PsbI and PsbW proteins are identified as being intimately associated with the D1 and D2 proteins, and in the case of the dimer, importance is placed on the PsbL and PsbK proteins in sustaining plastoquinone binding and maintenance of the dimeric organization. Assuming only one copy of the alpha- and beta-subunits of cytochrome b559, the monomeric and dimeric forms of the complex would be expected to contain 21 and 23 x 2 transmembrane helices, respectively.

Calcium binding to the subunit c of E. coli ATP-synthase and possible functional implications in energy coupling

The 8-kDa subunit c of the E. coli F0 ATP-synthase proton channel was tested for Ca++ binding activity using a 45Ca++ ligand blot assay after transferring the protein from SDS-PAGE gels onto polyvinyl difluoride membranes. The purified subunit c binds 45Ca++ strongly with Ca++ binding properties very similar to those of the 8-kDa CF0 subunit III of choloroplast thylakoid membranes. The N-terminal f-Met carbonyl group seems necessary for Ca++ binding capacity, shown by loss of Ca++ binding following removal of the formyl group by mild acid treatment. The dicyclohexylcarbodiimide-reactive Asp-61 is not involved in the Ca++ binding, shown by Ca++ binding being retained in two E. coli mutants, Asp61-->Asn and Asp61-->Gly. The Ca++ binding is pH dependent in both the E. coli and thylakoid 8-kDa proteins, being absent at pH 5.0 and rising to a maximum near pH 9.0. A treatment predicted to increase the Ca++ binding affinity to its F0 binding site (chlorpromazine photoaffinity attachment) caused an inhibition of ATP formation driven by a base-to-acid pH jump in whole cells. Inhibition was not observed when the Ca++ chelator EGTA was present with the cells during the chlorpromazine photoaffinity treatment. An apparent Ca++ binding constant on the site responsible for the UV plus chlorpromazine effect of near 80-100 nM was obtained using an EGTA-Ca++ buffer system to control free Ca++ concentration during the UV plus chlorpromazine treatment. The data are consistent with the notion that Ca++ bound to the periplasimic side of the E. coli F0 proton channel can block H+ entry into the channel. A similar effect occurs in thylakoid membranes, but the Ca++ binding site is on the lumen side of the thylakoid, where Ca+2 binding can modulate acid-base jump ATP formation. The Ca+2 binding to the F0 and CF0 complexes is consistent with a pH-dependent gating mechanism for control of H+ ion flux across the opening of the H+ channel.

A 64-kDa protein is a substrate for phosphorylation by a distinct thylakoid protein kinase

Solubilization of spinach thylakoids with the nonionic detergent n-octyl-β-D-glucopyranoside (OG) releases active protein kinase from the membrane. Further purification was reported to demonstrate that a 64-kDa protein is the origin of this kinase activity (Coughlan S J and Hind G (1986) J Biol Chem 261: 11378-11385). The N-terminal sequence of this protein was subsequently determined (Gal A, Herrmann R, Lottspiech F and Ohad I (1992) FEBS Lett 298: 33-35). Liquid phase isoelectric focusing of the OG extract and an hydroxylapatite-purified fraction, derived from the OG preparation, reveals that the 64-kDa protein with this documented N-terminal sequence can be separated from the protein kinase activity. Experimental conditions were optimised by manipulation of ampholyte and detergent concentrations to maximise protein solubility and enzyme activity. The kinase-containing fraction was able to catalyze the phosphorylation of several proteins including the 64-kDa which was identified using antibodies raised against a synthetic peptide corresponding to the N-terminal sequence. The results described indicate that this 64-kDa protein is not the protein kinase responsible for the phosphorylation of the light-harvesting complex associated with Photosystem II.

Three-dimensional structure of the higher-plant photosystem II reaction centre and evidence for its dimeric organization in vivo

The three-dimensional structure of photosystem II (PSII) has been determined by conventional transmission electron microscopy and computerized three-dimensional reconstruction. Both the complete system and that lacking the oxygen-evolving complex have been analyzed. The PSII complex has a four-lobed structure with twofold symmetry. An estimate of the molecular mass and the results of Deriphat/PAGE analysis suggest that a reaction centre is present in each half of the structure resolved by electron microscopy. Stepwise removal of components of the complex showed that the removal of CP47 (a 47-kDa chlorophyll-protein complex) induces monomerization of PSII, which indicates the importance of this subunit for the dimeric structure.

Directed disruption of the Chlamydomonas chloroplast psbK gene destabilizes the photosystem II reaction center complex

Using particle gun-mediated chloroplast transformation we have disrupted the psbK gene of Chlamydomonas reinhardtii with an aadA expression cassette that confers resistance to spectinomycin. The transformants are unable to grow photoautotrophically, but they grow normally in acetate-containing medium. They are deficient in photosystem II activity as measured by fluorescence transients and O2 evolution and they accumulate less than 10% of wild-type levels of photosystem II as measured by immunochemical means. Pulse-labeling experiments indicate that the photosystem II complex is synthesized normally in the transformants. These results differ from those obtained previously with similar cyanobacterial psbK mutants that were still capable of photoautotrophic growth (Ikeuchi et al., J. Biol. Chem. 266 (1991) 1111-1115). In C. reinhardtii the psbK product is required for the stable assembly and/or stability of the photosystem II complex and essential for photoautotrophic growth. The data also suggest that the stability requirements of the photosynthetic complexes differ considerably between C. reinhardtii and cyanobacteria.

Subunit III of the chloroplast ATP-synthase can form a Ca(2+)-binding site on the lumenal side of the thylakoid membrane

Subunit III, the 8 kDa component of the chloroplast CFo H+ channel, was isolated and purified from pea thylakoids for the purpose of studying its Ca(2+)-binding properties. After n-butanol extraction and ether precipitation, HPLC purification was accomplished using a poly(styrene-divinylbenzene) column which removes lipid and protein contaminations. The main components of protein contamination were two hydrophobic proteins of near 4 kDa molecular mass, the psaI and psbK gene products associated with PSI and PSII reaction centers, respectively. Purified subunit III as well as the unfractionated organic-solvent soluble preparation were used in a 45Ca(2+)-ligand blot assay known to detect high affinity Ca(2+)-binding sites in proteins. Polypeptides were separated with SDS-PAGE and were transferred onto PVDF membranes. Treatment of the membrane with 45CaCl2 in the presence of 10-fold excess of MgCl2 and 200-fold excess KCl led to the labeling of only the 8 kDa polypeptide. The Ca2+ binding was inhibited after derivatizing aqueously exposed carboxyl groups with a water soluble carbodiimide plus a nucleophile, after de-formylation of the N-terminal methionine, or with a subsequent treatment with La3+. Ca2+ binding was maximum at pH 7.5-8.5 and was greatly decreased at acidic pH. Dicyclohexylcarbodiimide treatment (no nucleophile was added) of thylakoid membranes, which derivatizes the hydrophobically located Glu-61, decreased the electrophoretical mobility of isolated subunit III but did not inhibit the Ca2+ binding. The data indicate that the carbonyl group of the formylated N-terminal Met-1 and probably the carboxyl group of the subunit III C-terminal Val-81 provide some of seven essential oxygen ligands normally required for defining a Ca(2+)-binding site in proteins. It is probable, but not yet established that an oligomeric form of subunit III polypeptides is essential for forming the Ca(2+)-binding site. Based on the accepted models for the hairpin conformation of the subunit III, it does seem clear that the Ca(2+)-binding site can form on the lumenal side of the membrane in the functional CFo structure.

Characterization of the psbK locus of Synechocystis sp. PCC 6803 in terms of Photosystem II function

The psbK gene encodes a small protein of Photosystem II. The gene has previously been cloned and sequenced in Synechocystis sp. PCC 6803. Our new results, presented here, confirm the conclusions of Ikeuchi et al. Based on Northern hybridization and primer extension analyses, we show that psbK is transcribed as a monocistronic message in this cyanobacterium. Analysis of DNA sequence immediately upstream of the transcription start site revealed an E. coli-like-10 consensus sequence. A deletion mutant was constructed where the psbK gene was replaced by a kanamycin resistant cartridge. In situ complementation experiments, as well as Southern and Northern hybridization analyses, confirmed that the mutant strain contains a lesion in psbK. The psbK-less mutant could grow photoautotrophically as well as photoheterotrophically both in liquid culture and on agar plates. The rate of growth was slightly less compared with the wild-type as clearly observed by in situ complementation experiments. Although the mutant showed correspondingly lower rates of electron transport, thermoluminescence, oxygen flash yield and chlorophyll a fluorescence measurements did not detect any significant modification of the reactions of PS II. Moreover, the mutant was no more susceptible to excess light than the wild-type. It is, therefore, concluded that the product of the psbK gene is not crucial for PS II activity and possibly plays some other role in the metabolism of Synechocystis.

Sequence and transcriptional analysis of the gene cluster trnQ-zfpA-psaI-ORF231-petA in pea chloroplasts

A 5.1 kb segment of pea chloroplast DNA containing the upstream region of petA was sequenced. RNAs produced from this DNA were characterized. This region encodes putative genes for psbK, trnQ, zfpA, psaI, ORF231, and petA. These genes are all on the same reading strand except for psbK. The gene organization is somewhat different from that of tobacco, rice, and liverwort, which lack the psbK-trnQ genes in this region and contain ORF184/185. Northern blot and primer extension analysis show that the pea transcript covers the zfpA-psaI-ORF231-petA gene cluster and trnQ. These results indicated that the psbK-trnQ genes have been rearranged and a new transcription unit was formed.

Two promoters within the psbK-psbI-trnG gene cluster in tobacco chloroplast DNA

Transcription of the 2.6 kbp psbK-psbI-trnG cluster in tobacco chloroplasts has been studied. This cluster contains, in linear sequence, the genes encoding two low-molecular-mass polypeptides, K and I, of photosystem II (psbK and psbI, respectively), and tRNA(Gly) (UCC) (trnG). Northern blot hybridization revealed that the largest transcript (2.6 kb) hybridizes to psbK, psbI and trnG, but not to the following trnR-UCU. Ten other transcripts ranging from 0.1 to 1.3 kb were also detected. Three of these transcripts overlap the divergent transcript arising from trnS-GCU located on the opposite DNA strand. S1 mapping and primer extension experiments showed that these multiple transcripts comprise eight distinct 5' ends. By in vitro capping assays two of them were determined to be transcriptional initiation sites; one is located 163 bp upstream of psbK and the other is 6 bp upstream of trnG. The 3' ends of transcripts were determined by S1 mapping; one lies between psbI and trnG and the other is at the end of trnG. The presence of dual promoters of trnG is discussed.

Pea chloroplast genes encoding a 4 kDa polypeptide of photosystem I and a putative enzyme of C1 metabolism

The nucleotide sequence of 3.2 kbp of pea chloroplast DNA located upstream from the petA gene for cytochrome f, and previously reported to contain the gene for a photosystem I polypeptide, has been determined. Three open reading frames of 587, 40 and 157 codons have been identified. Orf40 encodes a highly conserved, hydrophobic, membrane-spanning polypeptide, and is identified as the gene psaI for the 4 kDa subunit of photosystem I. Orf587 is an extended version of the gene zfpA previously identified as encoding a conserved putative zinc-finger protein. The product of orf587 shows extensive homology to an unidentified open reading frame cotranscribed with a gene for folate metabolism in Escherichia coli and local homology to a region of the beta subunit of rat mitochondrial propionyl-CoA carboxylase. It is suggested that the product of orf587 is an enzyme of C1 metabolism and is unlikely to be a regulatory DNA-binding protein. Orf157 potentially encodes an unidentified basic protein, but the protein sequence is not conserved in other plants.

Cleavage of the precursor of pea chloroplast cytochrome f by leader peptidase from Escherichia coli

Leader peptidase from Escherichia coli was able to process the precursor of pea cytochrome f synthesised in vitro. N-Terminal sequencing established that cleavage by leader peptidase generated the same mature sequence as in pea chloroplasts. Processing by leader peptidase was much more efficient co-translationally rather than post-translationally, and the extent of post-translational processing declined with time suggesting that the cytochrome f precursor folded to an uncleavable conformation. Detergent extracts of pea thylakoid membranes were unable to process the cytochrome f precursor co- or post-translationally.

Differential expression of the psbB and psbH genes encoding the 47 kDa chlorophyll a-protein and the 10 kDa phosphoprotein of photosystem II during chloroplast development in wheat

The nucleotide sequence of a region of wheat chloroplast DNA containing the psbB gene for the 47 kDa chlorophyll a-binding protein of photosystem II has been determined. The gene encodes a polypeptide of 508 amino acid residues which is predicted to contain six hydrophobic membrane-spanning regions. The psbB gene is located 562 bp upstream of the psbH gene for the 10 kDa phosphoprotein of photosystem II. A small open reading frame of 38 codons is located between psbB and psbH, and on the opposite strand the psbN gene, encoding a photosystem II polypeptide of 43 amino acid residues, is located between orf38 and psbH. S1 nuclease mapping indicated that the 5' ends of transcripts were located 371 and 183 bp upstream of the psbB translation initiation codon. Predominant transcripts of 2.1 kb and 1.8 kb for psbB and 0.4 kb for psbH were present in RNA isolated from etiolated and greening wheat seedlings. Immunodecoration of Western blots indicated that the 47 kDa polypeptide was absent, or present in very low amounts, in dark-grown tissue and accumulated on greening, whereas the 10 kDa polypeptide was present in similar amounts in both dark-grown and greening seedlings. The 10 kDa polypeptide was phosphorylated in vitro by incubating wheat etioplast membranes with [gamma 32P] ATP.

The chloroplast psbK operon from mustard (Sinapis alba L.): multiple transcripts during seedling development and evidence for divergent overlapping transcription

The mustard chloroplasts genes psbK and psbI are co-transcribed, giving rise to precursor transcripts of several size classes, which are processed to the monocistronic mature RNAs. The psbK and psbI coding regions are flanked by the two tRNA genes trnS-GCU and trnQ-UUG on the opposite DNA strand. Transcript mapping indicates that the (primary) psbK-psbI transcript overlaps the complete trnS-GCU and trnQ-UUG transcripts. The transcription start site of the psbK operon appears to overlap that of the rps16 gene. During seedling development, the psbK and psbI precursors and mature transcripts all become detectable between 30 and 48 h after sowing and then remain at constant levels without much difference either in light or in darkness.

An open reading frame encoding a putative haem-binding polypeptide is cotranscribed with the pea chloroplast gene for apocytochrome f

The nucleotide sequence of a 1 kbp region of pea chloroplast DNA upstream from the gene petA encoding apocytochrome f has been determined. An open reading frame of 231 codons (ORF231) encoding a putative membrane-spanning polypeptide is separated by 205 bp from the coding region of petA. The open reading frame is homologous to open reading frames located in a similar position with respect to petA in chloroplast DNA from Marchantia polymorpha, tobacco, rice, wheat and Vicia faba. The sequence around a conserved histidine residue in a putative membrane-spanning region of the polypeptide resembles sequences present in cytochrome b from chromaffin granules and neutrophil membranes, suggesting that the open reading frame may encode a haem-binding polypeptide, possibly a b-type cytochrome. Northern hybridisation analysis indicates the presence in pea chloroplasts of a complex pattern of transcripts containing ORF231. Large transcripts of 5.5 kb, 4.3 kb, 3.4 kb and 2.7 kb encode both ORF231 and apocytochrome f, indicating that ORF231 and petA are co-transcribed.

Temporal and spatial pattern of plastid gene expression during crucifer seedling development and embryogenesis

Several genes which are located close together on mustard (Sinapis alba L.) chloroplast DNA have been found to differ in their temporal mode of expression throughout seedling development. One predominant expression program, exemplified by thepsbA gene, is characterized by an early (light-independent) rise in transcript levels, followed by subsequent further accumulation to levels which are much higher in the light than in darkness (development of 'competence' for photocontrol). Other genes located next to thepsb A gene show transient or constitutive modes of expression, with no light-dark difference in transcript levels throughout seedling development. The characteristics of light-responsive expression were shown for the nuclearrbcS gene family inBrassica napus L. andSinapis alba L. cotyledons. The spatial distribution ofrbcS andpsbA transcripts across sections of crucifer cotyledons appeared to be relatively uniform, but restricted to photosynthetically active cells. Finally, assessment of these transcripts in immature seeds and embryos ofCapsella bursa-pastoris has provided in-situ evidence for tissuespecific gene expression during early development.

Sequence and transcriptional analysis of the barley ctDNA region upstream of psbD-psbC encoding trnK(UUU), rps16, trnQ(UUG), psbK, psbI, and trnS(GCU)

A 6.25 kbp barley plastid DNA region located between psbA and psbD-psbC were sequenced and RNAs produced from this DNA were analyzed. TrnK(UUU), rps16 and trnQ(UUG) were located upstream of psbA. These genes were transcribed from the same DNA strand as psbA and multiple RNAs hybridized to them. TrnK and rsp16 contained introns; a 504 amino acid open reading frame (ORF504) was located within the trnK intron. Between trnQ and psbD-psbC was a 2.24 kbp region encoding psbK, psbI and trnS(GCU). PsbK and psbI are encoded on the same DNA strand as psbD-psbC whereas trnS(GCU) is transcribed from the opposite strand. Two large RNAs accumulate in barley etioplasts which contain psbK, psbI, anti-sense trnS(GCU) and psbD-psbC sequences. Other RNAs encode psbK and psbI only, or psbK only. The divergent trnS(GCU) located upstream of psbD-psbC and a second divergent trnS(UGA) located downstream of psbD-psbC were both expressed. Furthermore, RNA complementary to psbK and psbI mRNA was detected, suggesting that transcription from divergent overlapping transcription units may modulate expression from this DNA region.

Nucleotide sequence and expression of the two genes encoding D2 protein and the single gene encoding the CP43 protein of Photosystem II in the cyanobacterium synechococcus sp. PCC 7002

The unicellular photoheterotrophic cyanobacterium Synechococcus sp. PCC 7002 was shown to encode two genes for the Photosystem II reaction center core protein D2 and one gene for the reaction center chlorophyhll-binding protein CP43. These three genes were cloned and their DNA sequences determined along with their flanking DNA sequences. Northern hybridization experiments show that both genes which encode D2, psbD1 and psbD2, are expressed at roughly equivalent levels. For each of the two psbD genes, there are 18 nucleotide differences among the 1059 nucleotides which are translated. The DNA sequences surrounding the coding sequences are nearly 70% divergent. Despite the DNA sequence differences in the genes, the proteins encoded by the two genes are predicted to be identical. The proteins encoded by psbD1 and psbD2 are ∼92% homologous to other sequenced cyanobacterial psbD genes and ∼86% homologous to sequenced chloroplast-encoded psbD genes.The single gene for CP43, psbC, overlaps the 3' end of psbD1 and is co-transcribed with it. Results from previous sequencing of psbC genes encoded by chloroplasts suggest that the 5' end of the psbC gene overlaps the 3' end of the coding sequence of psbD by ∼50 nucleotides. In Synechococcus sp. PCC 7002, the methionine codon previously proposed to be the start codon for psbC is replaced by an ACG (threonine) codon. We propose an alternative start for the psbC gene at a GTG codon 36 nucleotides downstream from the threonine codon. This GTG codon is preceded by a consensus E. coli-like ribosome binding sequence. Both the GTG start codon and its preceding ribosome binding sequence are conserved in all psbC genes sequenced from cyanobacteria and chloroplasts. This suggests that all psbC genes start at this alternative GTG codon. Based on this alternative start codon, the gene product is ∼85% identical to other cyanobacterial psbC gene products and ∼77% identical to eucaryotic chloroplast-encoded psbC gene products.

Current perceptions of Photosystem II

In the last few years our knowledge of the structure and function of Photosystem II in oxygen-evolving organisms has increased significantly. The biochemical isolation and characterization of essential protein components and the comparative analysis from purple photosynthetic bacteria (Deisenhofer, Epp, Miki, Huber and Michel (1984) J Mol Biol 180: 385-398) have led to a more concise picture of Photosystem II organization. Thus, it is now generally accepted that the so-called D1 and D2 intrinsic proteins bind the primary reactants and the reducing-side components. Simultaneously, the nature and reaction kinetics of the major electron transfer components have been further clarified. For example, the radicals giving rise to the different forms of EPR Signal II have recently been assigned to oxidized tyrosine residues on the D1 and D2 proteins, while the so-called Q400 component has been assigned to the ferric form of the acceptor-side iron. The primary charge-separation has been meaured to take place in about 3 ps. However, despite all recent major efforts, the location of the manganese ions and the water-oxidation mechanism still remain largely unknown. Other topics which lately have received much attention include the organization of Photosystem II in the thylakoid membrane and the role of lipids and ionic cofactors like bicarbonate, calcium and chloride. This article attempts to give an overall update in this rapidly expanding field.

Evidence that the amino-terminus of the 33 kDa extrinsic protein is required for binding to the Photosystem II complex

Chymotrypsin and Staphylococcus aureus (strain V8) proteinase eliminated sixteen and eighteen amino acid residues, respectively, from the amino-terminal side of the extrinsic 33 kDa protein of the oxygen-evolving Photosystem II (PS II) complex of spinach. The carboxy-terminus of the resultant large fragments was found to be identical with that of the intact protein. Neither fragment could rebind to PS II membranes depleted of all the extrinsic proteins. Circular dichroism spectroscopy did not reveal any major conformational change within the two fragments. These results suggest that the amino-terminal region of the 33 kDa protein contains a domain essential for binding to the PS II complex.

Mutation at the Chlamydomonas nuclear NAC2 locus specifically affects stability of the chloroplast psbD transcript encoding polypeptide D2 of PS II

The nuclear NAC2 locus of C. reinhardtii encodes a trans-acting factor that confers stability on the chloroplast message for the D2 protein of photosystem II (PS II). A mutant that carries a lesion in the NAC2 gene is unable to accumulate the psbD transcript encoding D2, but synthesizes all other PS II proteins normally. However, the PS II complex is completely unstable in the nac2-26 mutant, and all major PS II polypeptides, including the three oxygen-evolving enhancing proteins, are absent or greatly reduced in this strain because of posttranslational degradation. In C. reinhardtii the second exon of the trans-spliced psaA mRNA is cotranscribed with psbD (Choquet et al., 1988), but the nac2-26 mutation normally has no effect on psaA message maturation or stability. However, in double mutants carrying the nac2-26 mutation, as well as a mutation that prevents psaA splicing, splicing intermediates carrying psbD sequences are degraded. These results suggest that the NAC2 gene product acts in a very specific manner to control the half-life of psbD transcripts.

N-terminal sequencing of low-molecular-mass components in cyanobacterial photosystem II core complex. Two components correspond to unidentified open reading frames of plant chloroplast DNA

We recently reported the presence of several low-molecular-mass protein components in the PS II O2-evolving core complex from the thermophilic cyanobacterium, Synechococcus vulcanus [(1989) FEBS Lett. 244, 391-396]. Here we have characterized the three components (4.1, 4.7, 5 kDa) of the same cyanobacterial core complex by N-terminal sequencing. There were two components in the 4.7 kDa region, both having a blocked N-terminus. One has a sequence highly homologous to open reading frame 34 of plant chloroplast DNA (tentatively designated psbM), while the other has a sequence partially homologous to open reading frame 43 of chloroplast DNA (designated psbN), although neither of the two gene products has yet been confirmed in chloroplasts. The cyanobacterial 4.1 kDa protein partially corresponds to the 4.1 kDa nuclear-encoded core component of higher plant PS II. The cyanobacterial 5 kDa component, however, shows a sequence that is unrelated to any other known proteins.

The complete sequence of the rice (Oryza sativa) chloroplast genome: intermolecular recombination between distinct tRNA genes accounts for a major plastid DNA inversion during the evolution of the cereals

The entire chloroplast genome of the monocot rice (Oryza sativa) has been sequenced and comprises 134525 bp. Predicted genes have been identified along with open reading frames (ORFs) conserved between rice and the previously sequenced chloroplast genomes, a dicot, tobacco (Nicotiana tabacum), and a liverwort (Marchantia polymorpha). The same complement of 30 tRNA and 4 rRNA genes has been conserved between rice and tobacco. Most ORFs extensively conserved between N. tabacum and M. polymorpha are also conserved intact in rice. However, several such ORFs are entirely absent in rice, or present only in severely truncated form. Structural changes are also apparent in the genome relative to tobacco. The inverted repeats, characteristic of chloroplast genome structure, have expanded outward to include several genes present only once per genome in tobacco and liverwort and the large single copy region has undergone a series of inversions which predate the divergence of the cereals. A chimeric tRNA pseudogene overlaps an apparent endpoint of the largest inversion, and a model invoking illegitimate recombination between tRNA genes is proposed which accounts simultaneously for the origin of this pseudogene, the large inversion and the creation of repeated sequences near the inversion endpoints.

Low-molecular-mass proteins in cyanobacterial photosystem II: identification of psbH and psbK gene products by N-terminal sequencing

The O2-evolving photosystem II core complex was isolated from a thermophilic cyanobacterium, Synechococcus vulcanus Copeland. Analysis by SDS-polyacrylamide gel electrophoresis revealed that the complex contained at least seven low-molecular-mass proteins in addition to the well characterized CP47 apoprotein, CP43 apoprotein, 33 kDa extrinsic protein, D1 protein, D2 protein and large subunit of cytochrome b-559. The separation of these low-molecular-mass proteins were very similar between cyanobacterial and higher plant PS II. N-terminal sequences of the 6.5 kDa and 3.9 kDa proteins of cyanobacterial core complex were determined after blotting to a polyvinylidene difluoride membrane. The sequence of the 6.5 kDa protein showed high homology with an internal sequence of plant psbH gene product, so-called 10 kDa phosphoprotein, but did not conserve the Thr residue which is specifically phosphorylated in plants. The sequence of the 3.9 kDa protein corresponded to the K protein of higher plants (mature form of psbK gene product). These results indicate that the products of both psbH and psbK genes are present in cyanobacterial PS II as well as being associated with the O2-evolving core complex.

A photosystem II polypeptide is encoded by an open reading frame co-transcribed with genes for cytochrome b-559 in wheat chloroplast DNA

The N-terminal amino acid sequence of a 3.2 kDa photosystem II polypeptide is shown to be identical to that of a polypeptide encoded by an open reading frame of 38 codons (orf38) in wheat chloroplast DNA. Orf38 is located just downstream of the psbE and psbF genes for the polypeptides of cytochrome b-559. Analysis of the transcription of this region of chloroplast DNA shows that psbE, psbF and orf38 are co-transcribed to give a 1.1 kb polycistronic transcript which also contains another open reading frame of 40 codons. The orf38 and orf40 products are hydrophobic polypeptides which are both predicted to span the thylakoid membrane once. Orf38 and orf40 are highly conserved, and map to similar locations adjacent to psbE and psbF, in all organisms from which this region of DNA has been sequenced. We propose that orf38 is named psbL.

N-terminal sequencing of photosystem II low-molecular-mass proteins. 5 and 4.1 kDa components of the O2-evolving core complex from higher plants

High resolution gel electrophoresis in the low-molecular-mass region combined with electroblotting using polyvinylidene difluoride membranes enabled us to sequence the low-molecular-mass proteins of photosystem II membrane fragments from spinach and wheat. The determined N-terminal sequences, all showing considerable homology between the two plants, involved two newly determined sequences for the 4.1 kDa protein and one for the 5 kDa proteins. The sequence of the 4.1 kDa protein did not match any part of the chloroplast DNA sequence from tobacco or liverwort, suggesting that it is encoded by the nuclear genome. In contrast, the sequence of the 5 kDa protein matched ORF38, which is located just downstream of psbE and psbF in the chloroplast DNA and is assumed to be co-transcribed with them. These two components were associated with the O2-evolving core complex. Sequences of other low-molecular-mass proteins confirmed the previous identification as photosystem II components.

A new photosystem II reaction center component (4.8 kDa protein) encoded by chloroplast genome

The photosystem II reaction center complex, so-called D1-D2-cytochrome b-559 complex, isolated from higher plants contains a new component of about 4.8 kDa [(1988) Plant Cell Physiol. 29, 1233-1239]. The partial amino acid sequence of this component from spinach was determined after release of N-terminal blockage. The determined sequence matched an open reading frame (ORF36) of the chloroplast genome from tobacco and liverwort, which is located downstream from the psbK gene and forms an operon with psbK. The predicted product consists of 36 amino acid residues and has a single membrane-spanning segment. High homology between the tobacco and liverwort genes, and its presence in the reaction center complex suggest an important role for this component in the photosystem II complex. Since this gene corresponds to a part of the formerly designated psbI gene, we propose to revise the definition of psbI as the gene encoding the 4.8 kDa reaction center component.