Assembly of the Photosystem II oxygen-evolving complex is inhibited in psbA site-directed mutants of Chlamydomonas reinhardtii. Aspartate 170 of the D1 polypeptide

Alteration of the O2-Producing Mn4Ca Cluster in Photosystem II by the Mutation of a Metal Ligand

The O₂-evolving Mn₄Ca cluster in photosystem II (PSII) is arranged as a distorted Mn₃Ca cube that is linked to a fourth Mn ion (denoted as Mn4) by two oxo bridges. The Mn4 and Ca ions are bridged by residue D1-D170. This is also the only residue known to participate in the high-affinity Mn(II) site that participates in the light-driven assembly of the Mn₄Ca cluster. In this study, we use Fourier transform infrared difference spectroscopy to characterize the impact of the D1-D170E mutation. On the basis of analyses of carboxylate and carbonyl stretching modes and the O-H stretching modes of hydrogen-bonded water molecules, we show that this mutation alters the extensive network of hydrogen bonds that surrounds the Mn₄Ca cluster in the same manner as that of many other mutations. It also alters the equilibrium between conformers of the Mn₄Ca cluster in the dark-stable S₁ state so that a high-spin form of the S₂ state is produced during the S₁-to-S₂ transition instead of the low-spin form that gives rise to the S₂ state multiline electron paramagnetic resonance signal. The mutation may also change the coordination mode of the carboxylate group at position 170 to unidentate ligation of Mn4. This is the first mutation of a metal ligand in PSII that substantially impacts the spectroscopic signatures of the Mn₄Ca cluster without substantially eliminating O₂ evolution. The results have significant implications for our understanding of the roles of alternate active/inactive conformers of the Mn₄Ca cluster in the mechanism of O₂ formation.

Morpho-physiological and proteomic responses of Aegilops tauschii to imposed moisture stress

Moisture stress is the most important limitation of wheat production in the worldwide. Among the tribe Triticeae, Aegilops tauschii is one of the most valuable gene sources of resistance to abiotic stresses. In order to identify the most tolerant accession to moisture stress, and to understand its adaptive mechanisms at the molecular level, the present experiment was carried out on ten Ae. tauschii accessions under normal (95% soil pot capacity) and moisture stress (45% soil pot capacity) conditions. At the start of the heading time, the expanded flag leaves of treated and untreated plants were sampled for two-dimensional electrophoresis (2-DE) based on proteomics approach. A19 accession was less affected by the imposed moisture stress; therefore, it was used for the proteomics experiment. Among 252 protein spots which were reproducibly detected in each given 2-DE gels, 25 spots showed significant differences between the two moisture treatments; 17 spots were upregulated and 8 spots were downregulated. The identified proteins by MALDI-TOF/TOF, were allocated to seven functional protein groups, which were mainly involved in photosynthesis/respiration (28.5%), carbohydrate metabolism (14.2%), energy metabolism (7.1%), chaperone (14.2%), protein translation and processing (14.2%), repair and stability of the genome (7.1%) and unknown function (14.2%). We report this for the first time that RMI2 protein (in the group of repair and stability of the genome) was significantly changed in wheat in response to moisture stress. We believe that, the identified proteins could play important roles in acclimation and tolerance to moisture stress and provide the genetic pathways for improving tolerance to moisture stress in wheat.

Photoactivation: The Light-Driven Assembly of the Water Oxidation Complex of Photosystem II

Photosynthetic water oxidation is catalyzed by the Mn4CaO5 cluster of photosystem II. The assembly of the Mn4O5Ca requires light and involves a sequential process called photoactivation. This process harnesses the charge-separation of the photochemical reaction center and the coordination environment provided by the amino acid side chains of the protein to oxidize and organize the incoming manganese ions to form the oxo-bridged metal cluster capable of H2O-oxidation. Although most aspects of this assembly process remain poorly understood, recent advances in the elucidation of the crystal structure of the fully assembled cyanobacterial PSII complex help in the interpretation of the rich history of experiments designed to understand this process. Moreover, recent insights on the structure and stability of the constituent ions of the Mn4CaO5 cluster may guide future experiments. Here we consider the literature and suggest possible models of assembly including one involving single Mn(2+) oxidation site for all Mn but requiring ion relocation.

Label-free quantitative proteomics analysis of dormant terminal buds of poplar

Induction and break of bud dormancy are important features for perennial plants surviving extreme seasonal variations in climate. However, the molecular mechanism of the dormancy regulation, still remain poorly understood. To better understand the molecular basis of poplar bud dormancy, we used a label-free quantitative proteomics method based on nanoscale ultra performance liquid chromatography-ESI-MS(E) for investigation of differential protein expression during dormancy induction, dormancy, and dormancy break in apical buds of poplar (Populus simonii × P. nigra). Among these identified over 300 proteins during poplar bud dormancy, there are 74 significantly altered proteins, most of which involved in carbohydrate metabolism (22 %), redox regulation (19 %), amino acid transport and metabolism (10 %), and stress response (8 %). Thirty-one of these proteins were up-regulated, five were down-regulated during three phase, and thirty-eight were expressed specifically under different conditions. Pathway analysis suggests that there are still the presence of various physiological activities and a particular influence on photosynthesis and energy metabolism during poplar bud dormancy. Differential expression patterns were identified for key enzymes involved in major metabolic pathways such as glycolysis and the pentose phosphate pathway, thus manifesting the interplay of intricate molecular events in energy generation for new protein synthesis in the dormant buds. Furthermore, there are significant changes present in redox regulation and defense response proteins, for instance in peroxidase and ascorbate peroxidase. Overall, this study provides a better understanding of the possible regulation mechanisms during poplar bud dormancy.

Protein Ligation of the Photosynthetic Oxygen-Evolving Center

Photosynthetic water oxidation is catalyzed by a unique Mn(4)Ca cluster in Photosystem II. The ligation environment of the Mn(4)Ca cluster optimizes the cluster's reactivity at each step in the catalytic cycle and minimizes the release of toxic, partly oxidized intermediates. However, our understanding of the cluster's ligation environment remains incomplete. Although the recent X-ray crystallographic structural models have provided great insight and are consistent with most conclusions of earlier site-directed mutagenesis studies, the ligation environments of the Mn(4)Ca cluster in the two available structural models differ in important respects. Furthermore, while these structural models and the earlier mutagenesis studies agree on the identity of most of the Mn(4)Ca cluster's amino acid ligands, they disagree on the identity of others. This review describes mutant characterizations that have been undertaken to probe the ligation environment of the Mn(4)Ca cluster, some of which have been inspired by the recent X-ray crystallographic structural models. Many of these characterizations have involved Fourier Transform Infrared (FTIR) difference spectroscopy because of the extreme sensitivity of this form of spectroscopy to the dynamic structural changes that occur during an enzyme's catalytic cycle.

A carboxylic residue at the high-affinity, Mn-binding site participates in the binding of iron cations that block the site

The role of carboxylic residues at the high-affinity, Mn-binding site in the ligation of iron cations blocking the site [Biochemistry 41 (2000) 5854] was studied, using a method developed to extract the iron cations blocking the site. We found that specifically bound Fe(III) cations can be extracted with citrate buffer at pH 3.0. Furthermore, citrate can also prevent the photooxidation of Fe(II) cations by YZ. Participation of a COOH group(s) in the ligation of Fe(III) at the high-affinity site was investigated using 1-ethyl-3-[(3-dimethylamino)propyl] carbodiimide (EDC), a chemical modifier of carboxylic amino acid residues. Modification of the COOH groups inhibits the light-induced oxidation of exogenous Mn(II) cations by Mn-depleted photosystem II (PSII[-Mn]) membranes. The rate of Mn(II) oxidation saturates at > or = 10 microM in PSII(-Mn) membranes and > or = 500 microM in EDC-treated PSII (-Mn) samples. Intact PSII(-Mn) membranes have only one site for Mn(II) oxidation via YZ (dissociation constant, Kd = 0.64 microM), while EDC-treated PSII(-Mn) samples have two sites (Kd = 1.52 and 22 microM; the latter is the low-affinity site). When PSII(-Mn) membranes were incubated with Fe(II) before modifier treatment (to block the high-affinity site) and the blocking iron cations were extracted with citrate (pH 3.0) after modification, the membranes contained only one site (Kd = 2.3 microM) for exogenous Mn(II) oxidation by Y(Z)() radical. In this case, the rate of electron donation via YZ saturated at a Mn(II) concentration > or = 15 microM. These results indicate that the carboxylic residue participating in Mn(II) coordination and the binding of oxidized manganese cations at the HAZ site is protected from the action of the modifier by the iron cations blocking the HAZ site. We concluded that the carboxylic residue (D1 Asp-170) participating in the coordination of the manganese cation at the HAZ site (Mn4 in the tetranuclear manganese cluster [Science 303 (2004) 1831]) is also involved in the ligation of the Fe cation(s) blocking the high-affinity Mn-binding site.

No evidence from FTIR difference spectroscopy that aspartate-170 of the D1 polypeptide ligates a manganese ion that undergoes oxidation during the S0 to S1, S1 to S2, or S2 to S3 transitions in photosystem II

On the basis of mutagenesis and X-ray crystallographic studies, Asp170 of the D1 polypeptide is widely believed to ligate the (Mn)4 cluster that is located at the catalytic site of water oxidation in photosystem II. Recent proposals for the mechanism of water oxidation postulate that D1-Asp170 ligates a Mn ion that undergoes oxidation during one or more of the S0 --> S1, S1 --> S2, and S2 --> S3 transitions. To test these hypotheses, we have compared the FTIR difference spectra of the individual S state transitions in wild-type* PSII particles from the cyanobacterium Synechocystis sp. PCC 6803 with those in D1-D170H mutant PSII particles. Remarkably, our data show that the D1-D170H mutation does not significantly alter the mid-frequency regions (1800-1000 cm(-1)) of any of the FTIR difference spectra. Therefore, we conclude that the oxidation of the (Mn)4 cluster does not alter the frequencies of the carboxylate stretching modes of D1-Asp170 during the S0 --> S1, S1 --> S2, or S2 --> S3 transitions. The simplest explanation for these data is that the Mn ion that is ligated by D1-Asp170 does not increase its charge or oxidation state during any of these S state transitions. These data have profound implications for the mechanism of water oxidation. Either (1) the oxidation of the Mn ion that is ligated by D1-Asp170 occurs only during the transitory S3 --> S4 transition and serves as the critical step in the ultimate formation of the O-O bond or (2) the oxidation increments and O2 formation chemistry that occur during the catalytic cycle involve only the remaining Mn3Ca portion of the Mn4Ca cluster. Our data also show that, if the increased positive charge on the (Mn)4 cluster that is produced during the S1 --> S2 transition is delocalized over the (Mn)4 cluster, it is not delocalized onto the Mn ion that is ligated by D1-Asp170.

Does aspartate 170 of the D1 polypeptide ligate the manganese cluster in photosystem II? An EPR and ESEEM Study

Aspartate 170 of the D1 polypeptide provides part of the high-affinity binding site for the first Mn(II) ion that is photooxidized during the light-driven assembly of the (Mn)(4) cluster in photosystem II [Campbell, K. A., Force, D. A., Nixon, P. J., Dole, F., Diner, B. A., and Britt, R. D. (2000) J. Am. Chem. Soc. 122, 3754-3761]. However, despite a wealth of data on D1-Asp170 mutants accumulated over the past decade, there is no consensus about whether this residue ligates the assembled (Mn)(4) cluster. To address this issue, we have conducted an EPR and ESEEM (electron spin-echo envelope modulation) study of D1-D170H PSII particles purified from the cyanobacterium Synechocystis sp. PCC 6803. The line shapes of the S(1) and S(2) state multiline EPR signals of D1-D170H PSII particles are unchanged from those of wild-type PSII particles, and the signal amplitudes correlate approximately with the lower O(2) evolving activity of the mutant PSII particles (40-60% compared to that of the wild type). These data provide further evidence that the assembled (Mn)(4) clusters in D1-D170H cells function normally, even though the assembly of the (Mn)(4) cluster is inefficient in this mutant. In the two-pulse frequency domain ESEEM spectrum of the 9.2 GHz S(2) state multiline EPR signal of D1-D170H PSII particles, the histidyl nitrogen modulation observed at 4-5 MHz is unchanged from that of wild-type PSII particles and no significant new modulation is observed. Three scenarios are presented to explain this result. (1) D1-Asp170 ligates the assembled (Mn)(4) cluster, but the hyperfine couplings to the ligating histidyl nitrogen of D1-His170 are too large or anisotropic to be detected by ESEEM analyses conducted at 9.2 GHz. (2) D1-Asp170 ligates the assembled (Mn)(4) cluster, but D1-His170 does not. (3) D1-Asp170 does not ligate the assembled (Mn)(4) cluster.

Does histidine 332 of the D1 polypeptide ligate the manganese cluster in photosystem II? An electron spin echo envelope modulation study

The tetranuclear manganese cluster in photosystem II is ligated by one or more histidine residues, as shown by an electron spin echo envelope modulation (ESEEM) study conducted with [(15)N]histidine-labeled photosystem II particles isolated from the cyanobacterium Synechocystis sp. strain PCC 6803 [Tang, X.-S., Diner, B. A., Larsen, B. S., Gilchrist, M. L., Jr., Lorigan, G. A., and Britt, R. D. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 704-708]. One of these residues may be His332 of the D1 polypeptide. Photosystem II particles isolated from the Synechocystis mutant D1-H332E exhibit an altered S(2) state multiline EPR signal that has more hyperfine lines and narrower splittings than the corresponding signal in wild-type PSII particles [Debus, R. J., Campbell, K. A., Peloquin, J. M., Pham, D. P., and Britt, R. D. (2000) Biochemistry 39, 470-478]. These D1-H332E PSII particles are also unable to advance beyond an altered S(2)Y(Z)(*) state, and the quantum yield for forming the S(2) state is very low, corresponding to an 8000-fold slowing of the rate of Mn oxidation by Y(Z)(*). These observations are consistent with His332 being close to the Mn cluster and modulating the redox properties of both the Mn cluster and tyrosine Y(Z). To determine if D1-His332 ligates the Mn cluster, we have conducted an ESEEM study of D1-H332E PSII particles. The histidyl nitrogen modulation observed near 5 MHz in ESEEM spectra of the S(2) state multiline EPR signal of wild-type PSII particles is substantially diminished in D1-H332E PSII particles. This result is consistent with ligation of the Mn cluster by D1-His332. However, alternate explanations are possible. These are presented and discussed.

D1-Asp170 is structurally coupled to the oxygen evolving complex in photosystem II as revealed by light-induced Fourier transform infrared difference spectroscopy

We report both mid-frequency (1800-1200 cm(-)(1)) and low-frequency (670-350 cm(-)(1)) S(2)/S(1) FTIR difference spectra of photosystem II (PSII) particles isolated from wild-type and D1-D170H mutant cells of the cyanobacterium Synechocystis sp. PCC 6803. Both mid- and low-frequency S(2)/S(1) spectra of the Synechocystis wild-type PSII particles closely resemble those from spinach PSII samples, which confirms an earlier result by Noguchi and co-workers [Noguchi, T., Inoue, Y., and Tang, X.-S. (1997) Biochemistry 36, 14705-14711] and indicates that the coordination environment of the oxygen evolving complex (OEC) in Synechocystis is very similar to that in spinach. We also found that there is no appreciable difference between the mid-frequency S(2)/S(1) spectra of wild-type and of D1-D170H mutant PSII particles, from which we conclude that D1-Asp170 does not undergo a significant structural change during the S(1) to S(2) transition. This result also suggests that, if D1-Asp170 ligates Mn, it does not ligate the Mn ion that is oxidized during the S(1) to S(2) state transition. Finally, we found that a mode at 606 cm(-)(1) in the low-frequency wild-type S(2)/S(1) spectrum shifts to 612 cm(-)(1) in the D1-D170H mutant spectrum. Because this 606 cm(-)(1) mode has been previously assigned to an Mn-O-Mn cluster mode of the OEC [Chu, H.-A., Sackett, H., and Babcock, G. T. (2000) Biochemistry 39, 14371-14376], we conclude that D1-Asp170 is structurally coupled to the Mn-O-Mn cluster structure that gives rise to this band. Our results suggest that D1-Asp170 either directly ligates Mn or Ca(2+) or participates in a hydrogen bond to the Mn(4)Ca(2+) cluster. Our results demonstrate that combining FTIR difference spectroscopy with site-directed mutagenesis has the potential to provide insights into structural changes in Mn and Ca(2+) coordination environments in the different S states of the OEC.

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.

Amino acid residues that modulate the properties of tyrosine Y(Z) and the manganese cluster in the water oxidizing complex of photosystem II

The catalytic site for photosynthetic water oxidation is embedded in a protein matrix consisting of nearly 30 different polypeptides. Residues from several of these polypeptides modulate the properties of the tetrameric Mn cluster and the redox-active tyrosine residue, Y(Z), that are located at the catalytic site. However, most or all of the residues that interact directly with Y(Z) and the Mn cluster appear to be contributed by the D1 polypeptide. This review summarizes our knowledge of the environments of Y(Z) and the Mn cluster as obtained from the introduction of site-directed, deletion, and other mutations into the photosystem II polypeptides of the cyanobacterium Synechocystis sp. PCC 6803 and the green alga Chlamydomonas reinhardtii.

Histidine 332 of the D1 polypeptide modulates the magnetic and redox properties of the manganese cluster and tyrosine Y(Z) in photosystem II

An electron spin-echo envelope modulation study [Tang, X.-S., Diner, B. A., Larsen, B. S., Gilchrist, M. L., Jr., Lorigan, G. A., and Britt, R. D. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 704-708] and a recent Fourier transform infrared study [Noguchi, T., Inoue, Y., and Tang, X.-S. (1999) Biochemistry 38, 10187-10195], both conducted with [(15)N]histidine-labeled photosystem II particles, show that at least one histidine residue coordinates the O(2)-evolving Mn cluster in photosystem II. Evidence obtained from site-directed mutagenesis studies suggests that one of these residues may be His332 of the D1 polypeptide. The mutation D1-H332E is of particular interest because cells of the cyanobacterium Synechocystis sp. PCC 6803 that contain this mutation evolve no O(2) but appear to assemble Mn clusters in nearly all photosystem II reaction centers [Chu, H.-A., Nguyen, A. P. , and Debus, R. J. (1995) Biochemistry 34, 5859-5882]. Photosystem II particles isolated from the Synechocystis D1-H332E mutant are characterized in this study. Intact D1-H332E photosystem II particles exhibit an altered S(2) state multiline EPR signal that has more hyperfine lines and narrower splittings than the S(2) state multiline EPR signal observed in wild-type PSII particles. However, the quantum yield for oxidizing the S(1) state Mn cluster is very low, corresponding to an 8000-fold slowing of the rate of Mn oxidation by Y(Z)(*), and the temperature threshold for forming the S(2) state is approximately 100 K higher than in wild-type PSII preparations. Furthermore, the D1-H332E PSII particles are unable to advance beyond the Y(Z)(*)S(2) state, as shown by the accumulation of a narrow "split" EPR signal under multiple turnover conditions. In Mn-depleted photosystem II particles, charge recombination between Q(A)(*)(-) and Y(Z)(*) in D1-H332E is accelerated in comparison to wild-type, showing that the mutation alters the redox properties of Y(Z) in addition to those of the Mn cluster. These results are consistent with D1-His332 being located near the Mn-Y(Z) complex and perhaps ligating Mn.

The PsbY protein is not essential for oxygenic photosynthesis in the cyanobacterium Synechocystis sp. PCC 6803

A tetra-manganese cluster in the photosystem II (PSII) pigment-protein complex plays a critical role in the photosynthetic oxygen evolution process. PsbY, a small membrane-spanning polypeptide, has recently been suggested to provide a ligand for manganese in PSII (A.E. Gau, H.H. Thole, A. Sokolenko, L. Altschmied, R.G. Herrmann, E.K. Pistorius [1998] Mol Gen Genet 260: 56-68). We have constructed a mutant strain of the cyanobacterium Synechocystis sp. PCC 6803 with an inactivated psbY gene (sml0007). Southern-blot and polymerase chain reaction analysis showed that the mutant had completely segregated. However, the DeltapsbY mutant cells grew normally under photoautotrophic conditions. Moreover, growth of the wild-type and mutant cells were similar under high-light photoinhibition conditions, as well as in media without any added manganese, calcium, or chloride, three required inorganic cofactors for the oxygen-evolving complex of PSII. Analysis of steady-state and flash-induced oxygen evolution, fluorescence induction, and decay kinetics, and thermoluminescence profiles demonstrated that the DeltapsbY mutant cells have normal photosynthetic activities. We conclude that the PsbY protein in Synechocystis 6803 is not essential for oxygenic photosynthesis and does not provide an important binding site for manganese in the oxygen-evolving complex of PSII.

Orientation of the tetranuclear manganese cluster and tyrosine Z in the O(2)-evolving complex of photosystem II: An EPR study of the S(2)Y(Z)(*) state in oriented acetate-inhibited photosystem II membranes

Inhibitory treatment by acetate, followed by illumination and rapid freezing, is known to trap the S(2)Y(Z)(*) state of the O(2)-evolving complex (OEC) in photosystem II (PS II). An EPR spectrum of this state exhibits broad split signals due to the interaction of the tyrosyl radical, Y(Z)(*), with the S = 1/2 S(2) state of the Mn(4) cluster. We present a novel approach to analyze S(2)Y(Z)(*) spectra of one-dimensionally (1-D) oriented acetate-inhibited PS II membranes to determine the magnitude and relative orientation of the S(2)Y(Z)(*) dipolar vector within the membrane. Although there exists a vast body of EPR data on isolated spins in oriented membrane sheets, the present study is the first of its kind on dipolar-coupled electron spin pairs in such systems. We demonstrate the feasibility of the technique and establish a rigorous treatment to account for the disorder present in partially oriented 1-D membrane preparations. We find that (i) the point-dipole distance between Y(Z)(*) and the Mn(4) cluster is 7.9 +/- 0.2 A, (ii) the angle between the interspin vector and the thylakoid membrane normal is 75 degrees, (iii) the g(z)()-axis of the Mn(4) cluster is 70 degrees away from the membrane normal and 35 degrees away from the interspin vector, and (iv) the exchange interaction between the two spins is -275 x 10(-)(4) cm(-)(1), which is antiferromagnetic. Due to the sensitivity of EPR line shapes of oriented spin-coupled pairs to the interspin distance, the present study imposes a tighter constraint on the Y(Z)-Mn(4) point-dipole distance than obtained from randomly oriented samples. The geometric constraints obtained from the 1-D oriented sample are combined with published models of the structure of Mn-depleted PS II to propose a location of the Mn(4) cluster. A structure in which Y(Z) is hydrogen bonded to a manganese-bound hydroxide ligand is consistent with available data and favors maximal orbital overlap between the two redox center that would facilitate direct electron- and proton-transfer steps.

Involvement of histidine 190 on the D1 protein in electron/proton transfer reactions on the donor side of photosystem II

Flash-induced chlorophyll fluorescence kinetics from photosystem II in thylakoids from the dark-grown wild type and two site-directed mutants of the D1 protein His190 residue (D1-H190) in Chlamydomonas reinhardtii have been characterized. Induction of the chlorophyll fluorescence on the first flash, reflecting electron transport from YZ to P680(+), exhibited a strong pH dependence with a pK of 7.6 in the dark-grown wild type which lacks the Mn cluster. The chlorophyll fluorescence decay, measured in the presence of DCMU, which reflects recombination between QA- and YZox, was also pH-dependent with a similar pK of 7.5. These results indicate participation by the same base, which is suggested to be D1-H190, in oxidation and reduction of YZ in forward electron transfer and recombination pathways, respectively. This hypothesis was tested in the D1-H190 mutants. Induction of chlorophyll fluorescence in these H190 mutants has been observed to be inefficient due to slow electron transfer from YZ to P680(+) [Roffey, R. A., et al. (1994) Biochim. Biophys. Acta 1185, 257-270]. We show that this reaction is pH-dependent, with a pK of 8. 1, and at pH >/=9, the fluorescence induction is efficient in the H190 mutants, suggesting direct titration of YZ. The efficient oxidation of YZ ( approximately 70% at pH 9.0) at high pH was confirmed by kinetic EPR measurements. In contrast to the wild type, the H190 mutants show little or no observable fluorescence decay. Our data suggest that H190 is an essential component in the electron transfer reactions in photosystem II and acts as a proton acceptor upon YZ oxidation. In the H190 mutants, this reaction is inefficient and YZ oxidation only occurs at elevated pHs when YZ itself probably is deprotonated. We also propose that H190 is able to return a proton to YZox during electron recombination from QA- in a reaction which does not take place in the D1-H190 mutants.

Effects of carboxyl amino acid modification on the properties of the high-affinity, manganese-binding site in photosystem II

Our previous work using the "diphenylcarbazide (DPC)-inhibition assay" has identified four amino acid (two carboxyls and two histidyls) ligands to four Mn2+ bound with high affinity on Tris-washed photosystem II (PSII) membrane fragments [Preston and Seibert (1991) Biochemistry 30, 9615-9624, 9625-9633]. One of the ligands binds a photooxidizable Mn, specifically, and the others bind either nonphotooxidizable Mn2+, Zn2+, or Co2+ [Ghirardi et al. (1996) Biochemistry 35, 1820-1828]. The current paper shows the following: (a) the high-affinity photooxidizable Mn, which donates to the oxidized primary PSII donor (YZ*), is bound to a carboxyl residue with a KM = 1.5 microM or Kd = 0.94 microM in the absence of DPC, and a Ki = 1.3 microM in the presence of DPC (both steady-state and flash approaches were used); (b) if this carboxyl is chemically modified using 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide hydrochloride (EDC), Mn2+ is photooxidized at a lower affinity (Kd = 25 microM) site that does not involve carboxyl ligands; (c) low-affinity Mn is photooxidized (possibly by YD*, the oxidized form of the alternative PSII donor) with a KM = 220 microM at a completely different site that also requires a carboxyl ligand; (d) photooxidation of high-affinity DPC by YZ* with a KM of 40-42 microM or Kd of 49-58 microM occurs at a site that does not require carboxyl residues; (e) photooxidation of low-affinity DPC with a KM = 1200 microM occurs at a site (possibly near YD) that is not affected by carboxyl modification with EDC. Due to the similarities between the binding of the high-affinity photooxidizable Mn to EDC-treated membranes and to PSII complexes from Asp170D1 mutants [Nixon and Diner (1992) Biochemistry 31, 942-948], we identify its carboxyl residue ligand as Asp170 on D1, one of the reaction-center proteins. The second carboxyl ligand identified using the DPC-inhibition assay binds Mn (but not a photooxidizable one), Zn, or Co ions. At least one of the two histidyl ligands (either His337 on D1 or another unidentified histidyl) that bind nonphotooxidizable, high-affinity Mn2+ also binds Zn2+ and Co2+.

Mutation of residue threonine-2 of the D2 polypeptide and its effect on photosystem II function in Chlamydomonas reinhardtii

The D2 polypeptide of the photosystem II (PSII) complex in the green alga Chlamydomonas reinhardtii is thought to be reversibly phosphorylated. By analogy to higher plants, the phosphorylation site is likely to be at residue threonine-2 (Thr-2). We have investigated the role of D2 phosphorylation by constructing two mutants in which residue Thr-2 has been replaced by either alanine or serine. Both mutants grew photoautotrophically at wild-type rates, and noninvasive biophysical measurements, including the decay of chlorophyll fluorescence, the peak temperature of thermoluminescence bands, and rates of oxygen evolution, indicate little perturbation to electron transfer through the PSII complex. The susceptibility of mutant PSII to photoinactivation as measured by the light-induced loss of PSII activity in whole cells in the presence of the protein-synthesis inhibitors chloramphenicol or lincomycin was similar to that of wild type. These results indicate that phosphorylation at Thr-2 is not required for PSII function or for protection from photoinactivation. In control experiments the phosphorylation of D2 in wild-type C. reinhardtii was examined by 32P labeling in vivo and in vitro. No evidence for the phosphorylation of D2 in the wild type could be obtained. [14C]Acetate-labeling experiments in the presence of an inhibitor of cytoplasmic protein synthesis also failed to identify phosphorylated (D2.1) and nonphosphorylated (D2.2) forms of D2 upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Our results suggest that the existence of D2 phosphorylation in C. reinhardtii is still in question.

The lumenal loop connecting transmembrane helices I and II of the D1 polypeptide is important for assembly of the photosystem two complex

Current structural models indicate that the D1 and D2 polypeptides of the Photosystem two reaction center complex (PS II RC) each span the thylakoid membrane five times. In order to assess the importance of the lumenal extrinsic loop that connects transmembrane helices I and II of D1 we have constructed five deletion mutants and two double mutants in the cyanobaterium Synechocystic sp. PCC 6803. Four of the deletion mutants (Δ59-65, Δ69-74, Δ79-86 and Δ109-110) are obligate photoheterotrophs unable to accumulate D1 in the membrane as assayed by immunoblotting experiments or pulse-labelling experiments using [(35)S]-methionine. In contrast deletion mutant Δ100 which lacks A100 behaved very similarly to the WT control strain in terms of photoautotrophic growth rate, saturated rates of oxygen evolution, flash-induced oxygen evolution, fluorescence induction and decay, and thermoluminescence. Δ100 is the first example of an internal deletion on the lumenal side of the D1 polypeptide that is benign to photosystem two function. Double mutant D103G/E104A also behaves similarly to the WT control strain leading to the conclusion that residues D103 and E104 are unlikely to be involved in ligating the metal ions Mn or Ca(2+), which are needed for photosynthetic oxygen evolution. Double mutant, G109A/G110A, was constructed to assess the significance of this GlyGly motif which is also conserved in the L subunit of purple bacterial reaction centres. The G109A/G110A mutant is able to evolve oxygen at approximately 50-70% of WT rates but is unable to grow phatoautotrophically apparently because of an enhanced sensitivity to photoinactivation than the WT control strain. A photoautotropic revertant was isolated from this strain and shown to result from a mutation that restored the WT codon at position 109. Pulse-chase experiments in cells using [(35)S]-methionine showed that resistance to photoinhibition in the revertant correlated with an enhanced rate of incorporation of D1 into the membrane compared to mutant G109A/G110A. The sensitivity to photoinhibition shown by the G109A/G110A mutant is therefore consistent with a perturbation to the D1 repair cycle possibly at the level of D1 synthesis or incorporation of D1 into the PS II complex.

Donor-side photoinhibition in photosystem II from Chlamydomonas reinhardtii upon mutation of tyrosine-Z in the D1 polypeptide to phenylalanine

When tyrosine-Z of the D1-polypeptide of the photosystem II from Chlamydomonas reinhardtii was changed to phenylalanine, the rapid donor to P680+ was lost, and P680+ accumulated on illumination. The rapid donation from tyrosine-Z was replaced by a slow electron transfer from an endogenous donor. Spectrophotometric measurements showed that carotenoids and chlorophylls were bleached by the P680+ either directly or indirectly upon illumination. The carotenoid bleaching was inhibited in the presence of SOD or catalase, but the reaction did not require molecular oxygen as an electron acceptor. These observations led us to conclude that active oxygen radicals, possibly hydroxyl radicals, take part in the destruction of carotenoids in the Y161F mutant. Possible mechanisms for the destruction are discussed.

Photosystem II of green plants: topology of core pigments and redox cofactors as inferred from electrochromic difference spectra

Three electrochromic difference spectra induced by the deposition of (1) a negative charge on the primary quinone acceptor, Q(A), (2) a positive charge on (or near) Tyr161 of the D1 subunit (Y(Z)), and (3) a positive charge on the manganese cluster were determined at room temperature in photosystem II (PSII) core particles from pea. They were deconvoluted into Gaussian components by Powell's numerical optimization procedure. All three spectra were fitted by four components, which we assigned to the Q(y) absorption bands of two chlorophyll a molecules of the primary donor P, the accessory chlorophyll a, and the pheophytin a molecules on the D1 subunit. On the basis of the electrochromic properties of chlorins and our data, we suggest an arrangement of pigments and redox cofactors in PSII that differs from current structural models, which have been shaped like the reaction centers (RC) of purple bacteria. Our model is compatible with sequence data, with the spectroscopic and electrochemical properties of chlorophyll a and pheophytin a, and with the extremely positive redox potential of water oxidation. We conclude the following: (1) P is formed from two orthogonally oriented chlorophyll a molecules that peak at 681 and 677 nm. (2) The accessory chlorophyll a on D1 is oriented perpendicular to the membrane, with ring V pointing to Q(A). It is presumably attached to His118 of D1. (3) The mutual arrangement of pheophytin a on the D1 subunit and Q(A) differs from that of their counterparts in bacterial RC. (4) The manganese cluster is located out of the axis that is formed by Y(Z) (Tyr161 of D1), P, and Y(D) (Tyr161 of D2).

Interactions between diphenylcarbazide, zinc, cobalt, and manganese on the oxidizing side of photosystem II

The inhibition of DPC-mediated DCIP photoreduction by exogenous MnCl2 in Tris-treated photosystem II (PSII) membrane fragments has been used to probe for amino acids on the PSII reaction center proteins, including D1His337, that provide ligands for binding manganese [Preston, C., & Seibert, M. (1990) in Current Research in Photosynthesis (Baltscheffsky, M., Ed.) Vol. I, pp 925-928, Kluwer Academic Publishers, Dordrecht, The Netherlands; Preston, C., & Seibert, M. (1991) Biochemistry 30, 9615-9624 and 9625-9633]. At a concentration of 200 microM, DPC is photooxidized at both a high-affinity and a low-affinity site in PSII at approximately the same initial rate. Addition of 10 microM MnCl2 noncompetitively inhibits DPC photooxidation at the high-affinity site, with a Ki of 1.5 microM, causing a decrease of about 50% in the overall DCIP photoreduction rate. The high-affinity site for Mn binding was deconvoluted into four independent components. In earlier work, the inhibition was attributed to the tight association of either Mn2+ or Mn3+ with the PSII membrane. We report here that inhibition of DPC photooxidation may involve two different types of high-affinity, Mn-binding components: (a) one that is specific for Mn, and (b) others that bind Mn, but may also bind additional divalent cations, such as Zn and Co, that are not photooxidized by PSII. These conclusions are based on the observations that (a) DPC photooxidation can be inhibited by Zn2+ and Co2+; (b) Zn2+ and Co2+ interact with Mn2+ in a nonmutually exclusive manner, suggesting that they may share some binding components with Mn2+; (c) high-affinity Mn2+ (but not Zn2+ or Co2+) inhibition of DPC photooxidation is accompanied by nondecaying fluorescence emission, following a single saturating flash, indicating efficient electron donation by Mn2+ to YZ+; (d) Mn2+ photooxidation in the presence of DPC is not inhibited by Zn2+ or Co2+; and (e) kinetic modeling of the interaction between high-affinity Mn2+ and DPC in PSII indicates inhibition of steady-state Mn2+ photooxidation by DPC, but allows for a single photooxidation of Mn2+. We conclude that Mn inhibition of DPC photooxidation can be used to identify Mn-binding sites of physiological importance, and suggest that the Mn-specific component of the high-affinity, Mn-binding site involves the ligand to the first Mn bound during photoactivation (i.e., Asp170 on D1, as found by other investigators).

The deletion of petG in Chlamydomonas reinhardtii disrupts the cytochrome bf complex

The 4-kDa protein encoded by chloroplast petG copurifies with the cytochrome bf complex of spinach and is found in a number of other photosynthetic organisms, including the eukaryotic alga Chlamydomonas reinhardtii. To determine whether petG is involved in the function or assembly of the cytochrome bf complex, the gene was cloned from C. reinhardtii, excised from the DNA fragment, and replaced with a spectinomycin resistance cassette. A petG deletion strain of C. reinhardtii was then obtained by biolistic transformation. The resulting homoplasmic petG deletion strains are unable to grow photosynthetically, and immunoblot analysis shows markedly decreased levels of cytochrome b6, cytochrome f, the Rieske iron-sulfur protein, and subunit IV. To verify that this phenotype was due to the removal of petG, we also constructed a strain with a deletion in the open reading frame (ORF56), which is found 25 base pairs downstream of petG. The ORF56 deletion strain grew photosynthetically and had wild-type levels of the four major cytochrome bf subunits. We conclude that the absence of the PetG protein affects either the assembly or stability of the cytochrome bf complex in C. reinhardtii.

Genetic engineering of thylakoid protein complexes by chloroplast transformation in Chlamydomonas reinhardtii

Chloroplast transformation of Chlamydomonas reinhardtii has developed into a powerful tool for studying the structure, function and assembly of thylakoid protein complexes in a eukaryotic organism. In this article we review the progress that is being made in the development of procedures for efficient chloroplast transformation. This focuses on the development of selectable markers and the use of Chlamydomonas mutants, individually lacking thylakoid protein complexes, as recipients. Chloroplast transformation has now been used to engineer all four major thylakoid protein complexes, photosystem II, photosystem I, cytochrome b 6/f and ATP synthase. These results are discussed with an emphasis on new insights into assembly and function of these complexes in chloroplasts as compared with their prokaryotic counterparts.