Functional analysis of combinatorial mutants altered in a conserved region in loop E of the CP47 protein in Synechocystis sp. PCC 6803

Current status of the role of Cl(-) ion in the oxygen-evolving complex

This minireview summarizes the current state of knowledge concerning the role of Cl(-) in the oxygen-evolving complex (OEC) of photosystem II (PSII). The model that proposes that Cl(-) is a Mn ligand is discussed in light of more recent work. Studies of Cl(-) specificity, stoichiometry, kinetics, and retention by extrinsic polypeptides are discussed, as are the results that fail to detect Cl(-) ligation to Mn and results that show a lack of a requirement for Cl(-) in PSII-catalyzed H(2)O oxidation. Mutagenesis experiments in cyanobacteria and higher plants that produce evidence for a correlation between Cl(-) retention and stable interactions among intrinsic and extrinsic polypeptides are summarized, and spectroscopic data on the interaction between PSII and Cl(-) are discussed. Lastly, the question of the site of Cl(-) action in PSII is discussed in connection with the current crystal structures of the enzyme.

Perturbations at the chloride site during the photosynthetic oxygen-evolving cycle

Photosystem II (PSII) catalyzes the oxidation of water to O2 at the manganese-containing, oxygen-evolving complex (OEC). Photoexcitation of PSII results in the oxidation of the OEC; four sequential oxidation reactions are required for the generation and release of molecular oxygen. Therefore, with flash illumination, the OEC cycles among five Sn states. Chloride depletion inhibits O2 evolution. However, the binding site of chloride in the OEC is not known, and the role of chloride in oxygen evolution has not as yet been elucidated. We have employed reaction-induced FT-IR spectroscopy and selective flash excitation, which cycles PSII samples through the S state transitions. On the time scale employed, these FT-IR difference spectra reflect long-lived structural changes in the OEC. Bromide substitution supports oxygen evolution and was used to identify vibrational bands arising from structural changes at the chloride-binding site. Contributions to the vibrational spectrum from bromide-sensitive bands were observed on each flash. Sulfate treatment led to an elimination of oxygen evolution activity and of the FT-IR spectra assigned to the S3 to S0 (third flash) and S0 to S1 transitions (fourth flash). However, sulfate treatment changed, but did not eliminate, the FT-IR spectra obtained with the first and second flashes. Solvent isotope exchange in chloride-exchanged samples suggests flash-dependent structural changes, which alter protein dynamics during the S state cycle.

The mystery of oxygen evolution: analysis of structure and function of photosystem II, the water-plastoquinone oxido-reductase

Photosystem II (PS II) of thylakoid membrane of photosynthetic organisms has drawn attention of researchers over the years because it is the only system on Earth that provides us with oxygen that we breathe. In the recent past, structure of PS II has been the focus of research in plant science. The report of X-ray crystallographic structure of PS II complex by the research groups of James Barber and So Iwata in UK is a milestone in the area of research in photosynthesis. It follows the pioneering and elegant work from the laboratories of Horst Witt and W. Saenger in Germany, and J. Shen in Japan. It is time to analyze the historic events during the long journey made by the researchers to arrive at this point. This review makes an attempt to critically review the growth of the advancement of concepts and knowledge on the photosystem in the background of technological development. We conclude the review with perspectives on research and technology that should reveal the complete story of PS II of thylakoid in the future.

Heterogeneity of the mechanism of water splitting in photosystem II

We measured the temperature dependence of oxygen evolution in thylakoids from tobacco using mass spectrometry and high resolution polarography. We determined the initial S-state distribution and the efficiency of the transition between these states including the probability of the O(2) yield through a fast mode. We observed discontinuous changes of the parameters at the temperatures 11 degrees C, 15 degrees C and 21 degrees C. Due to the mass spectroscopy data we think that the irregularity observed at 11 degrees C is due to conformational changes within the water catalytic site. We show that the different contributions of the slow and fast modes of oxygen evolution and of the water molecule exchange are correlated and that their behavior can be explained in terms of the H(2)O accessibility to the water splitting enzyme.

The oxidation state of the photosystem II manganese cluster influences the structure of manganese stabilizing protein

Exposure of photosystem II membranes to trypsin that has been treated to inhibit chymotrypsin activity produces limited hydrolysis of manganese stabilizing protein. Exposure to chymotrypsin under the same conditions yields substantial digestion of the protein. Further probing of the unusual insensitivity of manganese stabilizing protein to trypsin hydrolysis reveals that increasing the temperature from 4 to 25 degrees C will cause some acceleration in the rate of proteolysis. However, addition of low (100 microM) concentrations of NH2OH, that are sufficient to reduce, but not destroy, the photosystem II Mn cluster, causes a change in PS II-bound manganese stabilizing protein that causes it to be rapidly digested by trypsin. Immunoblot analyses with polyclonal antibodies directed against the N-terminus of the protein, or against the entire sequence show that trypsin cleavage produces two distinct peptide fragments estimated to be in the 17-20 kDa range, consistent with proposals that there are 2 mol of the protein/mol photosystem II. The correlation of trypsin sensitivity with Mn redox state(s) in photosystem II suggest that manganese stabilizing protein may interact either directly with Mn, or alternatively, that the polypeptide is bound to another protein of the photosystem II reaction center that is intimately involved in binding and redox activity of Mn.

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.

Targeted random mutagenesis to identify functionally important residues in the D2 protein of photosystem II in Synechocystis sp. strain PCC 6803

To identify important residues in the D2 protein of photosystem II (PSII) in the cyanobacterium Synechocystis sp. strain PCC 6803, we randomly mutagenized a region of psbDI (coding for a 96-residue-long C-terminal part of D2) with sodium bisulfite. Mutagenized plasmids were introduced into a Synechocystis sp. strain PCC 6803 mutant that lacks both psbD genes, and mutants with impaired PSII function were selected. Nine D2 residues were identified that are important for PSII stability and/or function, as their mutation led to impairment of photoautotrophic growth. Five of these residues are likely to be involved in the formation of the Q(A)-binding niche; these are Ala249, Ser254, Gly258, Ala260, and His268. Three others (Gly278, Ser283, and Gly288) are in transmembrane alpha-helix E, and their alteration leads to destabilization of PSII but not to major functional alterations of the remaining centers, indicating that they are unlikely to interact directly with cofactors. In the C-terminal lumenal tail of D2, only one residue (Arg294) was identified as functionally important for PSII. However, from the number of mutants generated it is likely that most or all of the 70 residues that are susceptible to bisulfite mutagenesis have been altered at least once. The fact that mutations in most of these residues have not been picked up by our screening method suggests that these mutations led to a normal photoautotrophic phenotype. A novel method of intragenic complementation in Synechocystis sp. strain PCC 6803 was developed to facilitate genetic analysis of psbDI mutants containing several amino acid changes in the targeted domain. Recombination between genome copies in the same cell appears to be much more prevalent in Synechocystis sp. strain PCC 6803 than was generally assumed.

Combinatorial mutagenesis and pseudorevertant analysis to characterize regions in loop E of the CP47 protein in Synechocystis sp. PCC 6803

Deletion of the I265-F268 and T271-K277 regions in the large lumenally exposed loop of the CP47 protein are known to lead to a loss of photoautotrophic growth. Here, these regions have been investigated by combinatorial mutagenesis and pseudorevertant mapping. No single amino-acid residue in the I265-F268 region was found to be critical for function, but a large hydrophobic residue at position 267 and preferentially an aromatic residue at position 268 appeared to be required for photoautotrophic growth. Starting from an obligate photoheterotrophic mutant lacking the T271-K277 region, photoautotrophic pseudorevertants were generated with short in-frame tandem repeats near the site of the original deletion, partially or fully restoring the length of the original protein. These pseudorevertants were sensitive to oxygen indicating that the T271-K277 region may provide PS II stability and/or protection against oxygen-dependent photoinactivation. Pseudorevertants with much improved photoautotrophic growth were also generated for one of the combinatorial mutants in the I265-F268 region. Surprisingly, the secondary mutations in these pseudorevertants mapped to the ferrochelatase gene. We speculate that the secondary mutation in ferrochelatase gene resulted in altered ferrochelatase activity. Decreased heme (phycobilin) biosynthesis and/or increased chlorophyll biosynthesis could then lead to improved PS II performance of the combinatorial CP47 mutant.

Revealing the structure of the photosystem II chlorophyll binding proteins, CP43 and CP47

A review of the structural properties of the photosystem II chlorophyll binding proteins, CP47 and CP43, is given and a model of the transmembrane helical domains of CP47 has been constructed. The model is based on (i) the amino acid sequence of the spinach protein, (ii) an 8 A three-dimensional electron density map derived from electron crystallography and (iii) the structural homology which the membrane spanning region of CP47 shares with the six N-terminal transmembrane helices of the PsaA/PsaB proteins of photosystem I. Particular emphasis has been placed on the position of chlorophyll molecules assigned in the 8 A three-dimensional map of CP47 (K.-H. Rhee, E.P. Morris, J. Barber, W. Kühlbrandt, Nature 396 (1998) 283-286) relative to histidine residues located in the transmembrane regions of this protein which are likely to form axial ligands for chlorophyll binding. Of the 14 densities assigned to chlorophyll, the model predicted that five have their magnesium ions within 4 A of the imidazole nitrogens of histidine residues. For the remaining seven histidine residues the densities attributed to chlorophylls were within 4-8 A of the imidazole nitrogens and thus too far apart for direct ligation with the magnesium ion within the tetrapyrrole head group. Improved structural resolution and reconsiderations of the orientation of the porphyrin rings will allow further refinement of the model.

Probing the CD lumenal loop region of the D2 protein of photosystem II in Synechocystis sp. strain PCC 6803 by combinatorial mutagenesis

The CD lumenal loop region of the photosystem II reaction center protein D2 contains residues involved in oxygen evolution. Since detailed structural information about this region is unavailable, an M13-based combinatorial mutagenesis approach was used to investigate structure-function relationships in this vital region of D2 in Synechocystis sp. strain PCC 6803. The CD loop coding region contains close to 100 nucleotides, and for effective mutagenesis, it was subdivided into four regions of seven to eight codons. A gain-of-function selection protocol was employed such that all mutants that were selected contained a functional D2 protein. In this way, conservation patterns of residues along with numbers and types of amino acid substitutions accommodated at each position for each set of mutants would indicate which residues in the CD loop may play important structural and functional roles. Results of this study have substantiated the importance of residues previously studied by site-directed mutagenesis such as Arg180 and His189 and have identified other previously unremarkable residues in the CD loop (such as Ser166, Phe169, and Ala170) that cannot be replaced by many other residues. In addition, the pliability of the CD loop was further tested using deletion and D1-D2 substitution constructs in M13. This showed that the length of the loop was important to its function, and in two cases, D2 could accommodate homologous sequences from D1, which forms a heterodimer with D2 in photosystem II, but not the other way around. This study of the CD loop in D2 provides valuable clues regarding the structural and functional requirements of the region.

Is the manganese stabilizing 33 kDa protein of photosystem II attaining a 'natively unfolded' or 'molten globule' structure in solution?

This study compares the properties of the extrinsic 33 kDa subunit acting as 'manganese stabilizing protein' (MSP) of the water oxidizing complex with characteristic features of proteins that are known to attain a 'natively unfolded' or a 'molten globule' structure. The analysis leads to the conclusion that the MSP in solution is most likely a 'molten globule' with well defined compact regions of beta structure. The possible role of these structural peculiarities of MSP in solution for its function as important constituent of the WOC is discussed.

Accumulation of pre-apocytochrome f in a Synechocystis sp. PCC 6803 mutant impaired in cytochrome c maturation

Cytochrome c maturation involves heme transport and covalent attachment of heme to the apoprotein. The 5' end of the ccsB gene, which is involved in the maturation process and resembles the ccs1 gene from Chlamydomonas reinhardtii, was replaced by a chloramphenicol resistance cartridge in the cyanobacterium Synechocystis sp. PCC 6803. The resulting Delta(M1-A24) mutant lacking the first 24 ccsB codons grew only under anaerobic conditions. The mutant retained about 20% of the wild-type amount of processed cytochrome f with heme attached, apparently assembled in a functional cytochrome b(6)f complex. Moreover, the mutant accumulated unprocessed apocytochrome f in its membrane fraction. A pseudorevertant was isolated that regained the ability to grow under aerobic conditions. The locus of the second-site mutation was mapped to ccsB, and the mutation resulted in the formation of a new potential start codon in the intergenic region, between the chloramphenicol resistance marker and ccsB, in frame with the remaining part of ccsB. In this pseudorevertant the amount of holocyt f increased, whereas that of unprocessed apocytochrome f decreased. We suggest that the original deletion mutant Delta(M1-A24) expresses an N-terminally truncated version of the protein. The stable accumulation of unprocessed apocytochrome f in membranes of the Delta(M1-A24) mutant may be explained by its association with truncated and only partially functional CcsB protein resulting in protection from degradation. Our attempt to delete the first 244 codons of ccsB in Synechocystis sp. PCC 6803 was not successful, suggesting that this would lead to a lack of functional cytochrome b(6)f complex. The results suggest that the CcsB protein is an apocytochrome chaperone, which together with CcsA may constitute part of cytochrome c lyase.