Isolation and characterisation of a Dl/D2/cytochrome b-559 complex from Synechocystis 6803

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

Photosystem II (PSII) is the multi-subunit light-driven oxidoreductase that drives photosynthetic electron transport using electrons extracted from water. To investigate the initial steps of PSII assembly, we used strains of the cyanobacterium Synechocystis sp. PCC 6803 arrested at early stages of PSII biogenesis and expressing affinity-tagged PSII subunits to isolate PSII reaction center assembly (RCII) complexes and their precursor D1 and D2 modules (D1mod and D2mod). RCII preparations isolated using either a His-tagged D2 or a FLAG-tagged PsbI subunit contained the previously described RCIIa and RCII* complexes that differ with respect to the presence of the Ycf39 assembly factor and high light-inducible proteins (Hlips) and a larger complex consisting of RCIIa bound to monomeric PSI. All RCII complexes contained the PSII subunits D1, D2, PsbI, PsbE, and PsbF and the assembly factors rubredoxin A and Ycf48, but we also detected PsbN, Slr1470, and the Slr0575 proteins, which all have plant homologs. The RCII preparations also contained prohibitins/stomatins (Phbs) of unknown function and FtsH protease subunits. RCII complexes were active in light-induced primary charge separation and bound chlorophylls (Chls), pheophytins, beta-carotenes, and heme. The isolated D1mod consisted of D1/PsbI/Ycf48 with some Ycf39 and Phb3, while D2mod contained D2/cytochrome b559 with co-purifying PsbY, Phb1, Phb3, FtsH2/FtsH3, CyanoP, and Slr1470. As stably bound, Chl was detected in D1mod but not D2mod, formation of RCII appears to be important for stable binding of most of the Chls and both pheophytins. We suggest that Chl can be delivered to RCII from either monomeric Photosystem I or Ycf39/Hlips complexes.

Photosynthetic pigment localization and thylakoid membrane morphology are altered in Synechocystis 6803 phycobilisome mutants

Cyanobacteria are oxygenic photosynthetic prokaryotes that are the progenitors of the chloroplasts of algae and plants. These organisms harvest light using large membrane-extrinsic phycobilisome antenna in addition to membrane-bound chlorophyll-containing proteins. Similar to eukaryotic photosynthetic organisms, cyanobacteria possess thylakoid membranes that house photosystem (PS) I and PSII, which drive the oxidation of water and the reduction of NADP+, respectively. While thylakoid morphology has been studied in some strains of cyanobacteria, the global distribution of PSI and PSII within the thylakoid membrane and the corresponding location of the light-harvesting phycobilisomes are not known in detail, and such information is required to understand the functioning of cyanobacterial photosynthesis on a larger scale. Here, we have addressed this question using a combination of electron microscopy and hyperspectral confocal fluorescence microscopy in wild-type Synechocystis species PCC 6803 and a series of mutants in which phycobilisomes are progressively truncated. We show that as the phycobilisome antenna is diminished, large-scale changes in thylakoid morphology are observed, accompanied by increased physical segregation of the two photosystems. Finally, we quantified the emission intensities originating from the two photosystems in vivo on a per cell basis to show that the PSI:PSII ratio is progressively decreased in the mutants. This results from both an increase in the amount of photosystem II and a decrease in the photosystem I concentration. We propose that these changes are an adaptive strategy that allows cells to balance the light absorption capabilities of photosystems I and II under light-limiting conditions.

Isolation and spectral characterization of Photosystem II reaction center from Synechocystis sp. PCC 6803

We isolated highly-purified photochemically active photosystem (PS) II reaction center (RC) complexes from the cyanobacterium Synechocystis sp. PCC 6803 using a histidine-tag introduced to the 47 kDa chlorophyll protein, and characterized their spectroscopic properties. Purification was carried out in a one-step procedure after isolation of PS II core complex. The RC complexes consist of five polypeptides, the same as in spinach. The pigment contents per two molecules of pheophytin a were 5.8 +/- 0.3 chlorophyll (Chl) a and 1.8 +/- 0.1 beta-carotene; one cytochrome b(559) was found per 6.0 Chl a molecules. Overall absorption and fluorescence properties were very similar to those of spinach PS II RCs; our preparation retains the best properties so far isolated from cyanobacteria. However, a clear band-shift of pheophytin a and beta-carotene was observed. Reasons for these differences, and RC composition, are discussed on the basis of the three-dimensional structure of complexes.

Herbicide binding and thermal stability of photosystem II isolated from Thermosynechococcus elongatus

Binding of herbicides to photosystem II inhibits the electron transfer from Q(A) to Q(B) due to competition of herbicides with plastoquinone bound at the Q(B) site. We investigated herbicide binding to monomeric and dimeric photosystem II core complexes (PSIIcc) isolated from Thermosynechococcus elongatus by a combination of different methods (isothermal titration and differential scanning calorimetry, CD spectroscopy and measurements of the oxygen evolution) yielding binding constants, enthalpies and stoichiometries for various herbicides as well as information regarding stabilization/destabilization of the complex. Herbicide binding to detergent-solubilized PSIIcc can be described by a model of single independent binding sites present on this important membrane protein. Interestingly, binding stoichiometries herbicide:PSIIcc are lower than 1:1 and vary depending on the herbicide under study. Strong binding herbicides such as terbutryn stabilize PSIIcc in thermal unfolding experiments and endothermically binding herbicides like ioxynil probably cause large structural changes accompanied with the binding process as shown by differential scanning calorimetry experiments of the unfolding reaction of PSIIcc monomer in the presence of ioxynil. In addition we studied the occupancy of the Q(B) sites with plastoquinone (PQ9) by measuring flash induced fluorescence relaxation yielding a possible explanation for the deviations of herbicide binding from a 1:1 herbicide/binding site model.

Isolation and characterization of photosystem II subcomplexes from cyanobacteria lacking photosystem I

A photosystem II (PSII) core complex lacking the internal antenna CP43 protein was isolated from the photosystem II of Synechocystis PCC6803, which lacks photosystem I (PSI). CP47-RC and reaction centre (RCII) complexes were also obtained in a single procedure by direct solubilization of whole thylakoid membranes. The CP47-RC subcore complex was characterized by SDS/PAGE, immunoblotting, MALDI MS, visible and fluorescence spectroscopy, and absorption detected magnetic resonance. The purity and functionality of RCII was also assayed. These preparations may be useful for mutational analysis of PSII RC and CP47-RC in studying primary reactions of oxygenic photosynthesis.

Investigation of the structure of spinach photosystem II reaction center complex

The photosystem II (PSII) reaction center (RC) complex was isolated from spinach and characterized by gel electrophoresis, gel filtration and analytical ultracentrifugation. The purified complex contained the PsbA, PsbD, PsbE, PsbF and PsbI subunits. Gel filtration and analytical ultracentrifugation indicated the presence of a homogeneous complex. The mass of the RC complexes was found to be 107 kDa by analytical ultracentrifugation and 132 kDa by scanning transmission electron microscopy (STEM). The mass obtained showed the isolated complex to exist as a monomer and only one cytochrome b559 (cyt b559) to be associated with the RC complex. Digital images of negatively stained RC complexes were recorded by STEM and analyzed by single-particle averaging. The complex was 9 nm long and 5 nm wide, and exhibited a pronounced quasi-twofold symmetry. This supports the symmetric organization of the PSII complex, with the PsbA and the PsbD proteins in the center and symmetrically arranged PsbB and PsbC proteins at the periphery of the monomeric complex.

Comparison of primary charge separation in the photosystem II reaction center complex isolated from wild-type and D1-130 mutants of the cyanobacterium Synechocystis PCC 6803

We compare primary charge separation in a photosystem II reaction center preparation isolated from a wild-type (WT) control strain of the cyanobacterium Synechocystis sp. PCC 6803 and from two site-directed mutants of Synechocystis in which residue 130 of the D1 polypeptide has been changed from a glutamine to either a glutamate (mutant D1-Gln130Glu), as in higher plant sequences, or a leucine residue (mutant D1-Gln130Leu). The D1-130 residue is thought to be close to the pheophytin electron acceptor. We show that, when P680 is photoselectively excited, the primary radical pair state P680+Ph- is formed with a time constant of 20-30 ps in the WT and both mutants; this time constant is very similar to that observed in Pisum sativum (a higher plant). We also show that a change in the residue at position D1-130 causes a shift in the peak of the pheophytin Qx-band. Nanosecond and picosecond transient absorption measurements indicate that the quantum yield of radical pair formation (phi RP), associated with the 20-30-ps component, is affected by the identify of the D1-130 residue. We find that, for the isolated photosystem II reaction center particle, phi RP higher plant > phi RP D1-Gln130Glu mutant > phi RP WT > phi RP D1-Gln130Leu mutant. Furthermore, the spectroscopic and quantum yield differences we observe between the WT Synechocystis and higher plant photosystem II, seem to be reversed by mutating the D1-130 ligand so that it is the same as in higher plants. This result is consistent with the previously observed natural regulation of quantum yield in Synechococcus PS II by particular changes in the D1 polypeptide amino acid sequence (Clark, A.K., Hurry, V. M., Gustafsson, P. and Oquist, G. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 11985-11989).

Pigment-protein complexes from the photosynthetic membrane of the cyanobacterium Synechocystis sp. PCC 6803

Photosystem I and II core complexes were resolved in a single step from the thylakoid membrane of Synechocystis sp. PCC 6803 by using a mild solubilization procedure in dodecyl beta-D-maltoside and Deriphat/PAGE. For each photosystem, two green bands were obtained containing oligomeric and monomeric forms of the core complexes of either photosystem. The oligomers are likely to be trimers in the case of photosystem I and dimers for photosystem II. The absorption spectra, polypeptide and pigment composition of green bands corresponding to either photosystem I or photosystem II were identical for monomeric and oligomeric forms. The cytochrome b-559 content of photosystem II was evaluated to be one cytochrome b-559/reaction centre both in the monomeric and dimeric forms. Two new 15-kDa and 22-kDa carotenoid-binding protein were isolated and their polypeptides purified to homogeneity.

Bacterial luciferase as a reporter of circadian gene expression in cyanobacteria

To allow continuous monitoring of the circadian clock in cyanobacteria, we previously created a reporter strain (AMC149) of Synechococcus sp. strain PCC 7942 in which the promoter of the psbAI gene was fused to Vibrio harveyi luciferase structural genes (luxAB) and integrated into the chromosome. Northern (RNA) hybridization and immunoblot analyses were performed to examine changes in abundance of the luxAB mRNA, the native psbAI mRNA, and the luciferase protein to determine whether bioluminescence is an accurate reporter of psbAI promoter activity in AMC149. Under constant light conditions, the mRNA abundances of both luxAB and psbAI oscillated with a period of approximately 24 h for at least 2 days. The expression of these two genes following the same pattern: both mRNAs peaked in the subjective morning, and their troughs occurred near the end of the subjective night. The amount of luciferase protein also oscillated with a period of approximately 24 h, and the protein rhythm is in phase with the bioluminescence rhythm. The rhythm of the luciferase mRNA phase-leads the rhythms of luciferase protein and in vivo bioluminescence by several hours. Comparable results were obtained with a short-period mutant of AMC149. Together, these results indicate that the bioluminescence rhythm in AMC149 is due primarily to circadian oscillation of psbAI promoter activity in this cyanobacterium.

Purification of a Photosystem II reaction center from a thermophilic cyanobacterium using immobilized metal affinity chromatography

Oxygen-evolving PS II particles from the thermophilic cyanobacterium Synechococcus elongatus are partially purified by centrifugation on a sucrose gradient and are bound to a Chelating Sepharose column loaded with Cu(2+) ions. Bound particles are then transformed into PS II RC complexes by two washing steps. First, washing with a phosphate buffer (pH=6.5) containing 0.02% of SB 12 removes the rest of phycobilins and leaves pure PS II core particles on the column. Second, washing with a phosphate buffer (pH=6.2) containing 0.2 M LiClO4 and 0.05% of DM removes CP 47 and CP 43 and leaves bare PS II RC complexes on the column. These are then eluted with a phosphate buffer containing 1% of dodecylmaltoside (DM). The molar ratio of pigments in the eluate changes with the progress of elution but around the middle of the elution period a nearly stable ratio is maintained of Chl a: Pheo a: Car: Cyt b 559 equal to 2.9: 1: 0.9: 0.8. In these fractions the photochemical separation of charges could be demonstrated by accumulation of reduced pheophytin (ΔA of 430-440 nm) and by the flash induced formation of P680(+) (ΔA at 820 nm). The relatively slow relaxation kinetics of the latter signal (t1/2 ≈ 1 ms) may suggest that in a substantial fraction of the RCs QA remains bound to the complex.

Comparison of cytochrome b-559 content in photosystem II complexes from spinach and Synechocystis species PCC 6803

Cytochrome b-559 is an integral component of photosystem II complexes from both plants and cyanobacteria. However, the number of cytochrome b-559 associated with the photosystem II reaction center has been the subject of controversy. Some studies have concluded that there is one heme equivalent of cytochrome b-559 per reaction center, some studies have found two, and some studies have reported intermediate values. Most of the previous experiments have used only one method to quantitate the antenna size of the preparation. In this study, we compare the cytochrome b-559 content in a cyanobacterial and a plant photosystem II preparation. The plant preparation is derived from spinach, and previous work has shown that it has an antenna size of approximately 100 chlorophylls [MacDonald, G. M., & Barry, B. A. (1992) Biochemistry 31, 9848-9856]. The cyanobacterial preparation is from Synechocystis sp. PCC 6803, and previous work has shown that it has an antenna size of approximately 60 chlorophylls [Noren, G. H., Boerner, R. J., & Barry, B. A. (1991) Biochemistry 30, 3943-3950]. Both preparations are isolated through the use of ion-exchange chromatography, and both preparations are monodisperse in the same nonionic detergent. In our comparative study, we quantitate antenna size by three different methods. Our work shows that, depending on the method used to estimate antenna size, the oxygen-evolving spinach photosystem II preparation contains 0.82-1.0 cytochrome b-559 per reaction center, while the oxygen-evolving cyanobacterial preparation contains 1.5-2.1 cytochrome b-559 per reaction center.

Adaptation to high light intensity in Synechococcus sp. strain PCC 7942: regulation of three psbA genes and two forms of the D1 protein

The three psbA genes in the cyanobacterium Synechococcus sp. strain PCC 7942 encode two distinct forms of the D1 protein of photosystem II. The psbAI message, which encodes form I, dominates the psbA transcript pool at low to moderate light intensities; however, exposure to high light triggers a response in which the psbAI message is actively degraded while psbAII and psbAIII, which encode form II, are transcriptionally induced. We addressed whether these changes result from a generalized stress response and examined the consequence of light-responsive psbA regulation on the composition of D1 in thylakoid membranes. Heat shock and oxidative stress had some effect on levels of the three psbA transcripts but did not produce the responses generated by an increase in light intensity. Prolonged exposure to high light (24-h time course) was characterized by elevated levels of all psbA transcripts through maintenance of high levels of psbAII and psbAIII messages and a rebound of the psbAI transcript after its initial decline. Form II-encoding transcripts were enriched relative to those encoding form I at all high-light time points. Form II replaced form I in the thylakoid membrane at high light despite an abundance of psbAI transcript at later time points; this may be explained by the observed faster turnover of form I than form II in the membrane. We propose that form II is less susceptible to damage at high light and that this qualitative alteration, coupled with increased turnover of D1, protects the cells from photoinhibition.

Isolation and characterization of PSII core complexes from a brown alga, Laminaria saccharina

PSII-enriched particles, active for DCIP-reduction, were prepared from Laminaria saccharina chloroplasts, and PSII core complexes were further purified by ion-exchange chromatography. They contained several polypeptides, four of them cross-reacting with antibodies raised against CP47, CP43, D1 and D2 of green plants. A second chromatography was required to separate: (i) a core antenna, composed of 51 kDa polypeptide subunits, binding 11 beta-carotene, 4 chlorophyll (Chl) c and 7 fucoxanthin for 100 Chl a, and reacting with CP47 antibodies; and (ii) a reaction center complex consisting of two main polypeptides of 34 and 36 kDa. The pigment stoichiometry was of 5 Chl a and 0.5 beta-carotene for 2 pheophytin a. The 34 and 36 kDa components cross-reacted with anti-D1 and anti-D2 antibodies, respectively. The presence of cytochrome b-559 was substantiated by spectrophotometry.

Identification of the psbH gene product as a 6 kDa phosphoprotein in the cyanobacterium Synechocystis 6803

The product of the psbH gene has been identified in Synechocystis 6803 thylakoid membranes as a 6 kDa phosphoprotein. This protein becomes phosphorylated in vitro despite the fact that in cyanobacteria it is truncated at the N-terminus such that the phosphorylation site identified in the higher plant protein is missing. Phosphorylation occurred both in the light and in the dark but was inhibited by oxidising conditions, DCMU and zinc ions. The cyanobacterial 6 kDa phosphoprotein degrades when the membranes are subjected to high intensity illumination.

Flash-induced reduction of cytochrome b-559 by Q infB (sup-) in chloroplasts in the presence of protonophores

Flash-induced, fast (t 1/2 ≈ 1 ms), reversible reduction of the high potential cytochrome b-559 (cyt b-559HP) was observed in chloroplasts in the presence of 2 μM protonophore, FCCP (carbonylcyanide p-trifluoromethoxyphenylhydrazone), CCCP (carbonylcyanide 3-chlorophenylhydrazone) or SF 6847 (2,6-di-(t-butyl)-4-(2',2'-dicyanovinyl)phenol). These protonophores promote autooxidation of cyt b-559HP in the dark (Arnon and Tang 1988, Proc Natl Acad Sci USA 85: 9524). No fast photoreduction could, however, be observed if the molecules were oxidized with ferricyanide in the absence of protonophores. This suggests that the molecules must be deprotonated to be capable for fast photoreduction.Photoreduction of cyt b-559HP was largely insensitive to DBMIB (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone), but was inhibited by DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea). With a train of flashes, no oscillation could be observed in the amplitudes of photoreduction. These data strongly suggest that cyt b-559HP is reduced by the semireduced secondary quinone acceptor (QB (-)) of Photosystem 2.

Photosynthetic water oxidation: The protein framework

Approximately 20 protein subunits are associated with the PS II complex, not counting subunits of peripheral light-harvesting antenna complexes. However, it is not yet established which proteins specifically are involved in the water-oxidation process. Much evidence supports the concept that the D1/D2 reaction center heterodimer not only plays a central role in the primary photochemistry of Photosystem II, but also is involved in electron donation to P680 and in ligation of the manganese cluster. This evidence includes (a) the primary donor to P680 has been shown to be a redox-active tyrosyl residue (Tyr161) in the D1 protein, and (b) site-directed mutagenesis and computer-assisted modeling of the reaction center heterodimer have suggested several sites with a possible function in manganese ligation. These include Asp170, Gln165 and Gln189 of the D1 protein and Glu69 of the D2 protein as well as the C-terminal portion of the mature D1 protein. Also, hydrophilic loops of the chlorophyll-binding protein CP43 that are exposed at the inner thylakoid surface could be essential for the water-splitting process.In photosynthetic eukaryotes, three lumenal extrinsic proteins, PS II-O (33 kDa), PS II-P (23 kDa) and PS II-Q (16 kDa), influence the properties of the manganese cluster without being involved in the actual catalysis of water oxidation. The extrinsic proteins together may have multiple binding sites to the integral portion of PS II, which could be provided by the D1/D2 heterodimer and CP47. A major role for the PS II-O protein is to stabilize the manganese cluster. Most experimental evidence favors a connection of the PS II-P protein with binding of the Cl(-) and Ca(2+) ions required for the water oxidation, while the PS II-Q protein seems to be associated only with the Cl(-) requirement. The two latter proteins are not present in PS II of prokaryotic organisms, where their functions may be replaced by a 10-12 kDa subunit and a newly discovered low-potential cytochrome c-550.

Functional size of Photosystem II determined by radiation inactivation

The functional size of Photosystem II (PS II) was investigated by radiation inactivation. The technique provides an estimate of the functional mass required for a specific reaction and depends on irradiating samples with high energy γ-rays and assaying the remaining activity. The analysis is based on target theory that has been modified to take into account the temperature dependence of radiation inactivation of proteins. Using PS II enriched membranes isolated from spinach we determined the functional size of primary charge separation coupled to water oxidation and quinone reduction at the QB site: H2O → (Mn)4 → Yz → P680 → Pheophytin → Q → phenyl-p-benzoquinone. Radiation inactivation analysis indicates a functional mass of 88 ± 12 kDa for electron transfer from water to phenyl-p-benzoquinone. It is likely that the reaction center heterodimer polypeptides, D1 and D2, contribute approximately 70 kDa to the functional mass, in which case polypeptides adding up to approximately 20 kDa remain to be identified. Likely candidates are the α and β subunits of cytochrome b 559and the 4.5 kDa psbI gene product.

Transcriptional and posttranscriptional components of psbA response to high light intensity in Synechococcus sp. strain PCC 7942

The psbA genes, which encode the D1 protein of photosystem II, constitute a multigene family in the cyanobacterium Synechococcus sp. strain PCC 7942. Levels of messages from the three psbA genes change rapidly when cells are shifted from low-light to high-light conditions: the psbAI message level drops, whereas psbAII and psbAIII message levels increase dramatically. We examined the potential contributions of transcriptional and posttranscriptional processes in these high-light responses by subjecting cells that had been grown in a turbidostat at a standard light intensity (130 microeinsteins [microE] m-2 s-1) to either the same or a higher light intensity (500 microE m-2 s-1) in the presence or absence of rifampin. Northern (RNA blot) analysis of RNA isolated from cells subjected to high light showed that the increases in psbAII and psbAIII transcripts were blocked by rifampin. This suggests a transcriptional induction of these genes at high light intensities. Increased mRNA stability does not contribute to their accumulation in high-light conditions, since their half-life values did not increase relative to the half-lives measured at the standard light intensity. The rate of disappearance of the psbAI transcript in cells shifted to high light was diminished when either transcription or translation was blocked by rifampin or chloramphenicol, suggesting that accelerated degradation of the message requires de novo synthesis of a protein factor. When rifampin was added 10 min after the shift to high light intensity rather than before the shift, psbAI and psbAIII messages, but not the psbAII message, decayed at a faster rate. Susceptibility of the psbAIII transcript to the high-light-induced factor was also demonstrated by addition of chloramphenicol prior to the shaft to high light. psbAIII transcript levels went up more than twofold higher in chloramphenicol-treated cells than in untreated cells, whereas psbAII transcript levels were affected by the inhibitor. These experiments provide evidence that either new or increased synthesis of a degradation factor which affects a subset of Synechococcus transcripts occurs in cells subjected to high light intensity.

Effect of formate on Mössbauer parameters of the non-heme iron of PS II particles of cyanobacteria

Mössbauer spectra were measured for PSII particles having an active water-splitting system. The particles were isolated from the thermophilic cyanobacterium Synechococcus elongatus enriched in 57Fe. The Mössbauer resonance absorption spectrum is a superposition of 3 doublets with the following quadrupole splitting and chemical shift: 1, delta = 0.40, delta = 0.85; II, delta = 1.35, delta = 2.35; III, delta = 0.25, delta = 1.65. The delta and delta values of doublets I, II, III are characteristic of proteins with iron-sulphur center, non-heme iron of the reaction center of higher plants and of the oxidized cytochrome b-559. Treatment with sodium formate to remove bicarbonate affects only the doublet of non-heme iron, causing its quadrupole splitting to reduce to 1.75 and the chemical shift to reduce to 0.90. After washing out the formate, the Mössbauer spectrum of non-heme iron is restored. The data suggest that bicarbonate is a ligand for the non-heme iron of the reaction center of cyanobacteria.

Comparison of the D1/D2/cytochrome b559 reaction centre complex of photosystem two isolated by two different methods

Photosystem 2 reaction centre complexes prepared either by solubilisation with Triton X-100 and subsequent exchange into dodecyl maltoside or by a procedure involving a combination of dodecyl maltoside and LiClO4, were characterised in terms of chlorophyll a, pheophytin a, beta-carotene and cytochrome b559 content. Time-resolved chlorophyll fluorescence decay kinetics were measured using both types of complexes. Our data show that the isolated photosystem two reaction centre complex contain, for two pheophytin a molecules, close to six chlorophyll a, two beta-carotene and one cytochrome b559. No major differences were observed in the composition or the kinetic characteristics measured in the samples prepared by the different procedures. Time-resolved fluorescence measurements indicate that more than 94% of the chlorophyll a in both preparations is coupled to the reaction centre complex.

The structure and function of CPa-1 and CPa-2 in Photosystem II

This review presents a summary of recent investigations examining the structure and function of the chlorophyll-proteins CPa-1 (CP47) and CPa-2 (CP43). Comparisons of the derived amino acid sequences of these proteins suggest sites for chlorophyll binding and for interactions between these chlorophyll-proteins and other Photosystem II components. Hydropathy plot analysis of these proteins allows the formulation fo testable hypotheses concerning their topology and orientation within the photosynthetic membrane. The role of these chlorophyll-proteins as interior light-harvesting chlorophyll-a antennae for Photosystem II is examined and other possible additional roles for these important Photosystem II components are discussed.