Genetic engineering of the processing site of D1 precursor protein of photosystem II reaction center in Chlamydomonas reinhardtii

Characterization of tryptophan oxidation affecting D1 degradation by FtsH in the photosystem II quality control of chloroplasts

Photosynthesis is one of the most important reactions for sustaining our environment. Photosystem II (PSII) is the initial site of photosynthetic electron transfer by water oxidation. Light in excess, however, causes the simultaneous production of reactive oxygen species (ROS), leading to photo-oxidative damage in PSII. To maintain photosynthetic activity, the PSII reaction center protein D1, which is the primary target of unavoidable photo-oxidative damage, is efficiently degraded by FtsH protease. In PSII subunits, photo-oxidative modifications of several amino acids such as Trp have been indeed documented, whereas the linkage between such modifications and D1 degradation remains elusive. Here, we show that an oxidative post-translational modification of Trp residue at the N-terminal tail of D1 is correlated with D1 degradation by FtsH during high-light stress. We revealed that Arabidopsis mutant lacking FtsH2 had increased levels of oxidative Trp residues in D1, among which an N-terminal Trp-14 was distinctively localized in the stromal side. Further characterization of Trp-14 using chloroplast transformation in Chlamydomonas indicated that substitution of D1 Trp-14 to Phe, mimicking Trp oxidation enhanced FtsH-mediated D1 degradation under high light, although the substitution did not affect protein stability and PSII activity. Molecular dynamics simulation of PSII implies that both Trp-14 oxidation and Phe substitution cause fluctuation of D1 N-terminal tail. Furthermore, Trp-14 to Phe modification appeared to have an additive effect in the interaction between FtsH and PSII core in vivo. Together, our results suggest that the Trp oxidation at its N-terminus of D1 may be one of the key oxidations in the PSII repair, leading to processive degradation by FtsH.

Processing of D1 Protein: A Mysterious Process Carried Out in Thylakoid Lumen

In oxygenic photosynthetic organisms, D1 protein, a core subunit of photosystem II (PSII), displays a rapid turnover in the light, in which D1 proteins are distinctively damaged and immediately removed from the PSII. In parallel, as a repair process, D1 proteins are synthesized and simultaneously assembled into the PSII. On this flow, the D1 protein is synthesized as a precursor with a carboxyl-terminal extension, and the D1 processing is defined as a step for proteolytic removal of the extension by a specific protease, CtpA. The D1 processing plays a crucial role in appearance of water-oxidizing capacity of PSII, because the main chain carboxyl group at carboxyl-terminus of the D1 protein, exposed by the D1 processing, ligates a manganese and a calcium atom in the Mn₄CaO₅-cluster, a special equipment for water-oxidizing chemistry of PSII. This review focuses on the D1 processing and discusses it from four angles: (i) Discovery of the D1 processing and recognition of its importance: (ii) Enzyme involved in the D1 processing: (iii) Efforts for understanding significance of the D1 processing: (iv) Remaining mysteries in the D1 processing. Through the review, I summarize the current status of our knowledge on and around the D1 processing.

Proton transfer pathway from the oxygen-evolving complex in photosystem II substantiated by extensive mutagenesis

We report a structure-based biological approach to identify the proton-transfer pathway in photosystem II. First, molecular dynamics (MD) simulations were conducted to analyze the H-bond network that may serve as a Grotthuss-like proton conduit. MD simulations show that D1-Asp61, the H-bond acceptor of H₂O at the Mn₄CaO₅ cluster (W1), forms an H-bond via one water molecule with D1-Glu65 but not with D2-Glu312. Then, D1-Asp61, D1-Glu65, D2-Glu312, and the adjacent residues, D1-Arg334, D2-Glu302, and D2-Glu323, were thoroughly mutated to the other 19 residues, i.e., 114 Chlamydomonas chloroplast mutant cells were generated. Mutation of D1-Asp61 was most crucial. Only the D61E and D61C cells grew photoautotrophically and exhibit O₂-evolving activity. Mutations of D2-Glu312 were less crucial to photosynthetic growth than mutations of D1-Glu65. Quantum mechanical/molecular mechanical calculations indicated that in the PSII crystal structure, the proton is predominantly localized at D1-Glu65 along the H-bond with D2-Glu312, i.e., pK_a(D1-Glu65) > pK_a(D2-Glu312). The potential-energy profile shows that the release of the proton from D1-Glu65 leads to the formation of the two short H-bonds between D1-Asp61 and D1-Glu65, which facilitates downhill proton transfer along the Grotthuss-like proton conduit in the S₂ to S₃ transition. It seems possible that D1-Glu65 is involved in the dominant pathway that proceeds from W1 via D1-Asp61 toward the thylakoid lumen, whereas D2-Glu312 and D1-Arg334 may be involved in alternative pathways in some mutants.

The photosystem I assembly apparatus consisting of Ycf3-Y3IP1 and Ycf4 modules

In oxygenic photosynthesis, light energy is converted into redox energy by two photosystems (PSI and PSII). PSI forms one of the largest multiprotein complexes in thylakoid membranes consisting of a core complex, peripheral light-harvesting complexes (LHCIs) and cofactors. Although the high-resolution structure of the PSI–LHCI complex has been determined, the assembly process remains unclear due to the rapid nature of the assembly process. Here we show that two conserved chloroplast-encoded auxiliary factors, Ycf3 and Ycf4, form modules that mediate PSI assembly. The first module consists of the tetratricopeptide repeat protein Ycf3 and its interacting partner, Y3IP1, and mainly facilitates the assembly of reaction center subunits. The second module consists of oligomeric Ycf4 and facilitates the integration of peripheral PSI subunits and LHCIs into the PSI reaction center subcomplex. We reveal that these two modules are major mediators of the PSI–LHCI assembly process.

Photosystem I is a large multiprotein complex embedded in the chloroplast thylakoid membrane. Here the authors provide evidence for a modular assembly process, whereby Ycf3 facilitates assembly of the reaction center, while Ycf4 incorporates peripheral core and light harvesting complex subunits to the reaction center.

Functional Accumulation of Antenna Proteins in Chlorophyll b-Less Mutants of Chlamydomonas reinhardtii

The green alga Chlamydomonas reinhardtii contains several light-harvesting chlorophyll a/b complexes (LHC): four major LHCIIs, two minor LHCIIs, and nine LHCIs. We characterized three chlorophyll b-less mutants to assess the effect of chlorophyll b deficiency on the function, assembly, and stability of these chlorophyll a/b binding proteins. We identified point mutations in two mutants that inactivate the CAO gene responsible for chlorophyll a to chlorophyll b conversion. All LHCIIs accumulated to wild-type levels in a CAO mutant but their light-harvesting function for photosystem II was impaired. In contrast, most LHCIs accumulated to wild-type levels in the mutant and their light-harvesting capability for photosystem I remained unaltered. Unexpectedly, LHCI accumulation and the photosystem I functional antenna size increased in the mutant compared with in the wild type when grown in dim light. When the CAO mutation was placed in a yellow-in-the-dark background (yid-BF3), in which chlorophyll a synthesis remains limited in dim light, accumulation of the major LHCIIs and of most LHCIs was markedly reduced, indicating that sustained synthesis of chlorophyll a is required to preserve the proteolytic resistance of antenna proteins. Indeed, after crossing yid-BF3 with a mutant defective for the thylakoid FtsH protease activity, yid-BF3-ftsh1 restored wild-type levels of LHCI, which defines LHCI as a new substrate for the FtsH protease.

Identification and characterization of an assembly intermediate subcomplex of photosystem I in the green alga Chlamydomonas reinhardtii

Photosystem I (PSI) is a multiprotein complex consisting of the PSI core and peripheral light-harvesting complex I (LHCI) that together form the PSI-LHCI supercomplex in algae and higher plants. The supercomplex is synthesized in steps during which 12-15 core and 4-9 LHCI subunits are assembled. Here we report the isolation of a PSI subcomplex that separated on a sucrose density gradient from the thylakoid membranes isolated from logarithmic growth phase cells of the green alga Chlamydomonas reinhardtii. Pulse-chase labeling of total cellular proteins revealed that the subcomplex was synthesized de novo within 1 min and was converted to the mature PSI-LHCI during the 2-h chase period, indicating that the subcomplex was an assembly intermediate. The subcomplex was functional; it photo-oxidized P700 and demonstrated electron transfer activity. The subcomplex lacked PsaK and PsaG, however, and it bound PsaF and PsaJ weakly and was not associated with LHCI. It seemed likely that LHCI had been integrated into the subcomplex unstably and was dissociated during solubilization and/or fractionation. We, thus, infer that PsaK and PsaG stabilize the association between PSI core and LHCI complexes and that PsaK and PsaG bind to the PSI core complex after the integration of LHCI in one of the last steps of PSI complex assembly.

Biochemical and structural studies of the large Ycf4-photosystem I assembly complex of the green alga Chlamydomonas reinhardtii

Ycf4 is a thylakoid protein essential for the accumulation of photosystem I (PSI) in Chlamydomonas reinhardtii. Here, a tandem affinity purification tagged Ycf4 was used to purify a stable Ycf4-containing complex of >1500 kD. This complex also contained the opsin-related COP2 and the PSI subunits PsaA, PsaB, PsaC, PsaD, PsaE, and PsaF, as identified by mass spectrometry (liquid chromatography-tandem mass spectrometry) and immunoblotting. Almost all Ycf4 and COP2 in wild-type cells copurified by sucrose gradient ultracentrifugation and subsequent ion exchange column chromatography, indicating the intimate and exclusive association of Ycf4 and COP2. Electron microscopy revealed that the largest structures in the purified preparation measure 285 x 185 A; these particles may represent several large oligomeric states. Pulse-chase protein labeling revealed that the PSI polypeptides associated with the Ycf4-containing complex are newly synthesized and partially assembled as a pigment-containing subcomplex. These results indicate that the Ycf4 complex may act as a scaffold for PSI assembly. A decrease in COP2 to 10% of wild-type levels by RNA interference increased the salt sensitivity of the Ycf4 complex stability but did not affect the accumulation of PSI, suggesting that COP2 is not essential for PSI assembly.

Chloroplast-encoded PsbT is required for efficient biogenesis of photosystem II complex in the green alga Chlamydomonas reinhardtii

The small hydrophobic polypeptide PsbT is associated with the photosystem II (PSII) reaction center (D1/D2 heterodimer). Here, we report the effect of the deletion of PsbT on the biogenesis of PSII complex during light-induced greening of y-1 mutants of the green alga Chlamydomonas reinhardtii. The y-1 is unable to synthesize chlorophylls in the dark but do so in the light. The dark-grown y-1 cells accumulated no major PSII proteins but a small amount of PsbT. Upon illumination, PsbT was immediately synthesized while chlorophylls, major PSII proteins, and O(2)-evolving activity increased after a 1-h lag. The y-1 cells without PsbT accumulated chlorophylls and PSI protein at a similar rate, whereas the accumulation of PSII complex was specifically retarded during greening. The absence of PsbT did not affect the synthesis of PSII proteins. These results indicate that PsbT is required for the efficient biogenesis of PSII complex.

Chloroplast-encoded Polypeptide PsbT Is Involved in the Repair of Primary Electron Acceptor QA of Photosystem II during Photoinhibition in Chlamydomonas reinhardtii

PsbT is a small chloroplast-encoded hydrophobic polypeptide associated with the D1/D2 heterodimer of the photosystem II (PSII) reaction center and is required for the efficient post-translational repair of photodamaged PSII. Here we addressed that role in detail in Chlamydomonas reinhardtii wild type and DeltapsbT cells by analyzing the activities of PSII, the assembly of PSII proteins, and the redox components of PSII during photoinhibition and repair. Strong illumination of cells for 15 min decreased the activities of electron transfer through PSII and Q(A) photoreduction by 50%, and it reduced the amount of atomic manganese by 20%, but it did not affect the steady-state level of PSII proteins, photoreduction of pheophytin (pheo(D1)), and the amount of bound plastoquinone (Q(A)), indicating that the decrease in PSII activity resulted mainly from inhibition of the electron transfer from pheo(D1) to Q(A). In wild type cells, we observed parallel recovery of electron transfer activity through PSII and Q(A) photoreduction, suggesting that the recovery of Q(A) activity is one of the rate-limiting steps of PSII repair. In DeltapsbT cells, the repairs of electron transfer activity through PSII and of Q(A) photoreduction activity were both impaired, but PSII protein turnover was unaffected. Moreover, about half the Q(A) was lost from the PSII core complex during purification. Since PsbT is intimately associated with the Q(A)-binding region on D2, we propose that this polypeptide enhances the efficient recovery of Q(A) photoreduction by stabilizing the structure of the Q(A)-binding region.

Identification of tenuazonic acid as a novel type of natural photosystem II inhibitor binding in Q(B)-site of Chlamydomonas reinhardtii

Tenuazonic acid (TeA) is a natural phytotoxin produced by Alternaria alternata, the causal agent of brown leaf spot disease of Eupatorium adenophorum. Results from chlorophyll fluorescence revealed TeA can block electron flow from Q(A) to Q(B) at photosystem II acceptor side. Based on studies with D1-mutants of Chlamydomonas reinhardtii, the No. 256 amino acid plays a key role in TeA binding to the Q(B)-niche. The results of competitive replacement with [(14)C]atrazine combined with JIP-test and D1-mutant showed that TeA should be considered as a new type of photosystem II inhibitor because it has a different binding behavior within Q(B)-niche from other known photosystem II inhibitors. Bioassay of TeA and its analogues indicated 3-acyl-5-alkyltetramic and even tetramic acid compounds may represent a new structural framework for photosynthetic inhibitors.

Degenerated recognition property of a mitochondrial homing enzyme in the unicellular green alga Chlamydomonas smithii

Target sequence cleavage is the essential step for intron invasion into an intronless allele. DNA cleavage at a specific site is performed by an endonuclease, termed a homing enzyme, which is encoded by an open reading frame within the intron. The recognition properties of them have only been analyzed in vitro, using purified, recombinant homing enzyme and various mutated DNA substrates, but it is unclear whether the homing enzyme behaves similarly in vivo. To answer this question, we determined the recognition properties of I-CsmI in vivo. I-CsmI is a homing enzyme encoded by the open reading frame of the alpha-group I-intron, located in the mitochondrial apocytochrome b gene of the green alga Chlamydomonas smithii. The in vivo recognition properties of it were determined as the frequency of intron invasion into a mutated target site. For this purpose, we utilized hybrid diploid cells developed by crossing alpha-intron-plus C. smithii to intron-minus C. reinhardtii containing mutated target sequences. The intron invasion frequency was much higher than the expected from the in vitro cleavage frequency of the respective mutated substrates. Even the substrates that had very little cleavage in the in vitro experiment were efficiently invaded in vivo, and were accompanied by a large degree of coconversion. Considering the ease of the homing enzyme invading into various mutated target sequences, we propose that the principle bottleneck for lateral intron transmission is not the sequence specificity of the homing enzyme, but instead is limited by the rare occurrence of inter-specific cell fusion.

Shared molecular characteristics of successfully transformed mitochondrial genomes in Chlamydomonas reinhardtii

Three types of respiratory deficient mitochondrial strains have been reported in Chlamydomonas reinhardtii: a deficiency due to (i) two base substitutions causing an amino acid change in the apocytochrome b (COB) gene (i.e., strain named dum-15), (ii) one base deletion in the COXI gene (dum-19), or (iii) a large deletion extending from the left terminus of the genome to somewhere in the COB gene (dum-1, -14, and -16). We found that these respiratory deficient strains of C. reinhardtii can be divided into two groups: strains that are constantly transformable and those could not be transformed in our experiments. All transformable mitochondrial strains were limited to the type that has a large deletion in the left arm of the genome. For these mitochondria, transformation was successful not only with purified intact mitochondrial genomes but also with DNA-constructs containing the compensating regions. In comparison, mitochondria of all the non-transformable strains have both of their genome termini intact, leading us to speculate that mitochondria lacking their left genome terminus have unstable genomes and might have a higher potential for recombination. Analysis of mitochondrial gene organization in the resulting respiratory active transformants was performed by DNA sequencing and restriction enzyme digestion. Such analysis showed that homologous recombination occurred at various regions between the mitochondrial genome and the artificial DNA-constructs. Further analysis by Southern hybridization showed that the wild-type genome rapidly replaces the respiratory deficient monomer and dimer mitochondrial genomes, while the E. coli vector region of the artificial DNA-construct likely does not remain in the mitochondria.

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.

Does complexity constrain organelle evolution?

The evolution of eukaryotes was punctuated by invasions of the bacteria that have evolved to mitochondria and plastids. These bacterial endosymbionts founded major eukaryotic lineages by enabling them to carry out aerobic respiration and oxygenic photosynthesis. Yet, having evolved as free-living organisms, they were at first poorly adapted organelles. Although mitochondria and plastids have integrated within the physiology of eukaryotic cells, this integration has probably been constrained by the high level of complexity of their bacterial ancestors and the inability of gradual evolutionary processes to drastically alter complex systems. Here, I review complex processes that directly involve translation of plastid mRNAs and how they could constrain transfer to the nucleus of the genes encoding them.

PsbT polypeptide is required for efficient repair of photodamaged photosystem II reaction center

PsbT is a small chloroplast-encoded hydrophobic polypeptide associated with the photosystem II (PSII) core complex. A psbT-deficient mutant (Delta psbT) of the green alga Chlamydomonas reinhardtii grows photoautotrophically, whereas its growth is significantly impaired in strong light. To understand the photosensitivity of Delta psbT, we have studied the effect of strong illumination on PSII activity and proteins. It is shown that the level of PSII activity and proteins is reduced in the Delta psbT more significantly than in wild type under strong light. When recovery of the photodamaged PSII is inhibited by a chloroplast protein synthesis inhibitor, the light-induced inactivation and degradation of PSII occur similarly in wild-type and mutant cells. On the contrary, the recovery of PSII activity after partial photoinactivation is remarkably delayed in the Delta psbT cells, suggesting that PsbT is required for efficient recovery of the photodamaged PSII complex. These results therefore present the first evidence for involvement of this small PSII polypeptide in the recovery process. Partial disintegration of the purified PSII core complex and localization of PSII proteins in the resulting PSII subcore complexes have revealed that PsbT is associated with D1/D2 heterodimer. A possible role of PsbT in the recovery process is discussed.

Two functionally distinct manganese clusters formed by introducing a mutation in the carboxyl terminus of a photosystem II reaction center polypeptide, D1, of the green alga Chlamydomonas reinhardtii

To study the function of the carboxyl-terminal domain of a photosystem II (PSII) reaction center polypeptide, D1, chloroplast mutants of the green alga Chlamydomonas reinhardtii have been generated in which Leu-343 and Ala-344 have been simultaneously or individually replaced by Phe and Ser, respectively. The mutants carrying these replacements individually, L343F and A344S, showed a wild-type phenotype. In contrast, the double mutant, L343FA344S, evolved O2 at only 20-30% of the wild-type rate and was unable to grow photosynthetically. In this mutant, PSII accumulated to 60% of the wild-type level, indicating that the O2-evolving activity per PSII was reduced to approximately half that of the wild-type. However, the amount of Mn atom detected in the thylakoids suggested that a normal amount of Mn cluster was assembled. An investigation of the kinetics of flash-induced fluorescence yield decay revealed that the electron transfer from Q(-)(A) to Q(B) was not affected. When a back electron transfer from Q(-)(A) to a donor component was measured in the presence of 3-(3,4-dichlorophenol)-1,1-dimethylurea, a significantly slower component of the Q(-)(A) oxidation was detected in addition to the normal component that corresponds to the back electron transfer from the Q(-)(A) to the S(2)-state of the Mn cluster. Thermoluminescence measurements revealed that L343FA344S cells contained two functionally distinct Mn clusters. One was equivalent to that of the wild-type, while the other was incapable of water oxidation and was able to advance the transition from the S(1)-state to the S(2)-state. These results suggested that a fraction of the Mn cluster had been impaired by the L343FA344S mutation, leading to decreased O2 evolution. We concluded that the structure of the C-terminus of D1 is critical for the formation of the Mn cluster that is capable of water oxidation, in particular, transition to higher S-states.

Overexpression and characterization of carboxyl-terminal processing protease for precursor D1 protein: regulation of enzyme-substrate interaction by molecular environments

CtpA, which is classified as a novel type of serine protease with a Ser/Lys catalytic dyad, is responsible for the C-terminal processing of precursor D1 protein (pD1) of the photosystem II reaction center, a process that is indispensable for the integration of water-splitting machinery in photosynthesis. In this study, overexpression in Escherichia coli and one-step purification of spinach CtpA were carried out to analyze the characteristics of this new type of protease and to elucidate the molecular interactions in the C-terminal processing of pD1 on the thylakoid membrane. The successful accumulation of functional CtpA in E. coli may argue against the possibility, based on homology to E. coli Tsp, that the enzyme is involved in the degradation of incomplete proteins in chloroplasts, e.g. by utilizing the ssrA-tagging system. Analysis using a synthetic pD1 oligopeptide demonstrated that the enzymatic properties (including substrate recognition) of overexpressed CtpA with an extra sequence of GSHMLE at the N terminus were indistinguishable from those of the native enzyme. CtpA was insensitive to penem, which has been shown to inhibit some Ser/Lys-type proteases, suggesting that the catalytic center of CtpA is quite unique. By using the substrate in different molecular environments (i.e. synthetic pD1 oligopeptide in solution and pD1 in photosystem II-enriched thylakoid membrane), we observed a dramatic difference in the pH profile and affinity for the substrate, suggesting the presence of a specific interaction of CtpA with a factor(s) that modulates the pH dependence of proteolytic action in response to physiological conditions.

Amino acid residues involved in the coordination and assembly of the manganese cluster of photosystem II. Proton-coupled electron transport of the redox-active tyrosines and its relationship to water oxidation

The combination of site-directed mutagenesis, isotopic labeling, new magnetic resonance techniques and optical spectroscopic methods have provided new insights into cofactor coordination and into the mechanism of electron transport and proton-coupled electron transport in photosystem II. Site-directed mutations in the D1 polypeptide of this photosystem have implicated a number of histidine and carboxylate residues in the coordination and assembly of the manganese cluster, responsible for photosynthetic water oxidation. Many of these are located in the carboxy-terminal region of this polypeptide close to the processing site involved in its maturation. This maturation is a required precondition for cluster assembly. Recent proposals for the mechanism of water oxidation have directly implicated redox-active tyrosine Y(Z) in this mechanism and have emphasized the importance of the coupling of proton and electron transfer in the reduction of Y(Z)(radical) by the Mn cluster. The interaction of both homologous redox-active tyrosines Y(Z) and Y(D) with their respective homologous proton acceptors is discussed in an effort to better understand the significance of such coupling.

Competitive inhibition analysis of the enzyme-substrate interaction in the carboxy-terminal processing of the precursor D1 protein of photosystem II reaction center using substituted oligopeptides

A clear parallelism was demonstrated between the efficiency as substrate of the substituted oligopeptides corresponding to the carboxy-terminal (C-terminal) sequence of the precursor D1 protein (pD1) in the in vitro enzymatic assay and their competitive inhibitory capacity toward the proteolytic C-terminal processing of the full-length pD1 integrated in the intact photosystem II complex embedded in the thylakoid membrane of Scenedesmus obliquus LF-1 mutant, as shown e.g. by the influence of L343A, A345G and A345V substitutions and the effect of C-terminal fragments. This suggests that the basic mechanism for substrate recognition by the processing protease elucidated in the enzymatic analysis using synthetic oligopeptides is also effective in vivo, although it can sometimes be difficult to detect the consequence of amino acid substitution in the integrated systems.

Chlorophyll a availability affects psbA translation and D1 precursor processing in vivo in Synechocystis sp. PCC 6803

Transcript accumulation and translation of psbA as well as processing of the D1 precursor protein were investigated in relation to chlorophyll availability in vivo in cyanobacterial strains lacking photosystem I (PS I). The psbA transcript level was almost independent of chlorophyll availability and was approximately 3-fold lower in darkness than in continuous light (5 microE m-2 s-1). Upon illumination, it reached a steady-state level within several hours. Upon growth under light-activated heterotrophic growth conditions (LAHG) in the PS I-less strain, D1 synthesis occurred immediately upon illumination. However, in PS I-less/chlL- cells, which lacked the light-independent chlorophyll biosynthesis pathway and had very low chlorophyll levels after LAHG growth, very little D1 synthesis occurred upon illumination, and the synthesis rate increased with time. This result suggests a translational control of D1 biosynthesis related to chlorophyll availability. Upon illumination, initially a high level of the nonprocessed D1 precursor was observed by pulse labeling and immunodetection in LAHG-grown PS I-less/chlL- cells but not in PS I-less cells. A significant amount of the D1 precursor eventually was processed to mature D1, and the half-life of the D1 precursor decreased as the chlorophyll content of the cells increased. The D1 processing enzyme CtpA was found to be present at similar levels regardless illumination or chlorophyll levels. We conclude that, directly or indirectly, chlorophyll availability is needed for D1 translation as well as for efficient processing of the D1 precursor.

Rapid and discrete isolation of oxygen-evolving His-tagged photosystem II core complex from Chlamydomonas reinhardtii by Ni2+ affinity column chromatography

We have developed a simple and rapid procedure to isolate an oxygen-evolving photosystem II (PS II) core complex from Chlamydomonas reinhardtii. A His-tag made of six consecutive histidine residues was genetically attached at the carboxy terminus of D2 protein to create a metal binding site on the PS II supramolecular complex. The recombinant cells producing the His-tagged variant of D2 protein grew photoautotrophically as well as the wild-type cells. Characterization of the oxygen evolution and the thermoluminescence properties revealed that the His-tagging did not affect the functional integrity of the PS II reaction center. A PS II core complex was isolated from the detergent-solubilized thylakoids of the recombinant cells in 4 h by a single one-step Ni2+ affinity column chromatography. This preparation consists of D1, D2, CP43, CP47, 33 kDa, and a few low molecular weight proteins, and retains a high rate of oxygen-evolving activity (= 1000 micromol/mg Chl/h).

The D1 C-terminal processing protease of photosystem II from Scenedesmus obliquus. Protein purification and gene characterization in wild type and processing mutants

Polypeptide D1 of the photosystem II reaction center of oxygenic photosynthesis is expressed in precursor form (pre-D1), and it must be proteolytically processed at its C terminus to enable assembly of the manganese cluster responsible for photosynthetic water oxidation. A rapid and highly sensitive enzyme-linked immunosorbent assay-based microtiter plate method is described for assaying this D1 C-terminal processing protease. A protocol is described for the isolation and purification to homogeneity of the enzyme from the green alga, Scenedesmus obliquus. Amino acid sequence information on the purified protease was used to clone the corresponding gene, the translated sequence of which is presented. A comparison of the gene product with homologous proteases points to a region of conserved residues that likely corresponds to the active site of a new class of serine protease. The LF-1 mutant strain of Scenedesmus (isolated by Dr. Norman Bishop) is incapable of processing pre-D1. We show here that the C-terminal processing protease gene in this strain contains a single base deletion that causes a frame shift and a premature stop of translation within the likely active site of the enzyme. A suppressor strain, LF-1-RVT-1, which is photoautotrophic and capable of processing pre-D1 has a nearby single base insertion that restores the expression of active enzyme. These observations provide the first definitive proof that the enzyme isolated is responsible for in vivo proteolytic processing of pre-D1 and that no other protease can compensate for its loss.