Functional characterization of mutant strains of the cyanobacterium Synechocystis sp. PCC 6803 lacking short domains within the large, lumen-exposed loop of the chlorophyll protein CP47 in photosystem II

The lipoproteins of cyanobacterial photosystem II

Photosystem II (PSII) complexes from cyanobacteria and plants perform water splitting and plastoquinone reduction and yet have a different complement of lumenal extrinsic proteins. Whereas PSII from all organisms has the PsbO extrinsic protein, crystal structures of PSII from cyanobacteria have PsbV and PsbU while green algae and higher plants instead contain the extrinsic PsbP and PsbQ subunits. Proteomic studies in Synechocystis sp. PCC 6803 identified three further extrinsic proteins in the thylakoid lumen that are associated with cyanobacterial PSII and these are predicted to attach to the thylakoid membrane via a lipidated N-terminus. These proteins are cyanobacterial homologues to the PsbP and PsbQ subunits as well as to Psb27, an additional extrinsic protein associated with "inactive" photosystems that lack the other extrinsic polypeptides. The PsbQ homologue is not present in Prochlorococcus species but otherwise these proteins have been identified in most cyanobacteria although our phylogenetic analyses identified some strains that lack an apparent motif for lipidation in one or other of these subunits. Over the past decade the physiological function of these additional lipoproteins has been investigated in several cyanobacterial strains and recently the structures for each have been solved. This review will evaluate the physiological and structural results obtained for these lipid-attached extrinsic proteins and in silico protein docking of these proteins to PSII centers will be presented.

Effect of protein modification by malondialdehyde on the interaction between the oxygen-evolving complex 33 kDa protein and photosystem II core proteins

Previously we observed that the oxygen-evolving complex 33 kDa protein (OEC33) which stabilizes the Mn cluster in photosystem II (PSII), was modified with malondialdehyde (MDA), an end-product of peroxidized polyunsaturated fatty acids, and the modification increased in heat-stressed plants (Yamauchi et al. 2008). In this study, we examined whether the modification of OEC33 with MDA affects its binding to the PSII complex and causes inactivation of the oxygen-evolving complex. Purified OEC33 and PSII membranes that had been removed of extrinsic proteins of the oxygen-evolving complex (PSIIOEE) of spinach (Spinacia oleracea) were separately treated with MDA. The binding was diminished when both OEC33 and PSIIOEE were modified, but when only OEC33 or PSIIOEE was treated, the binding was not impaired. In the experiment using thylakoid membranes, release of OEC33 from PSII and corresponding loss of oxygen-evolving activity were observed when thylakoid membranes were treated with MDA at 40 degrees C but not at 25 degrees C. In spinach leaves treated at 40 degrees C under light, maximal efficiency of PSII photochemistry (F(v)/F(m) ratio of chlorophyll fluorescence) and oxygen-evolving activity decreased. Simultaneously, MDA contents in heat-stressed leaves increased, and OEC33 and PSII core proteins including 47 and 43 kDa chlorophyll-binding proteins were modified with MDA. In contrast, these changes were to a lesser extent at 40 degrees C in the dark. These results suggest that MDA modification of PSII proteins causes release of OEC33 from PSII and it is promoted in heat and oxidative conditions.

Identification and characterization of a cytochrome b559 Synechocystis 6803 mutant spontaneously generated from DCMU-inhibited photoheterotrophical growth conditions

We identified a spontaneously generated mutant from Synechocystis sp. PCC6803 wild-type cells grown in BG-11 agar plates containing 5 mM Glu and 10 microM DCMU. This mutant carries an R7L mutation on the alpha-subunit of cyt b559 in photosystem II (PSII). In the recent 2.9 A PSII crystal structural model, the side chain of this arginine residue is in close contact with the heme propionates of cyt b559. We called this mutant WR7Lalpha cyt b559. This mutant grew at about the same rate as wild-type cells under photoautotrophical conditions but grew faster than wild-type cells under photoheterotrophical conditions. In addition, 77 K fluorescence and 295 K chlorophyll a fluorescence spectral results indicated that the energy delivery from phycobilisomes to PSII reaction centers was partially inhibited or uncoupled in this mutant. Moreover, WR7Lalpha cyt b559 mutant cells were more susceptible to photoinhibition than wild-type cells under high light conditions. Furthermore, our EPR results indicated that in a significant fraction of mutant reaction centers, the R7Lalpha cyt b559 mutation induced the displacement of one of the axial histidine ligands to the heme of cyt b559. On the basis of these results, we propose that the Arg7Leu mutation on the alpha-subunit of cyt b559 alters the interaction between the APC core complex and PSII reaction centers, which reduces energy delivery from the antenna to the reaction center and thus protects mutant cells from DCMU-induced photo-oxidative stress.

Structures and functions of the extrinsic proteins of photosystem II from different species

This minireview presents a summary of information available on the variety and binding properties of extrinsic proteins that form the oxygen-evolving complex of photosystem II (PSII) of cyanobacteria, red alga, diatom, green alga, euglena, and higher plants. In addition, the structure and function of extrinsic PsbO, PsbV, and PsbU proteins are summarized based on the crystal structure of thermophilic cyanobacterial PSII together with biochemical and genetic studies from various organisms.

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.

Dynamics of electron transfer in photosystem II

Photosystem II, being a constituent of light driven photosynthetic apparatus, is a highly organized pigment-protein-lipid complex. The arrangement of PSII active redox cofactors insures efficiency of electron transfer within it. Donation of electrons extracted from water by the oxygen evolving complex to plastoquinones requires an additional activation energy. In this paper we present theoretical discussion of the anharmonic fluctuations of the protein-lipid matrix of PSII and an experimental evidence showing that the fluctuations are responsible for coupling of its donor and acceptor side. We argue that the fast collective motions liberated at temperatures higher that 200 K are crucial for the two final steps of the water splitting cycle and that one can distinguish three different dynamic regimes of PSII action which are controlled by the timescales of forward electron transfer, which vary with temperature. The three regimes of the dynamical behavior are related to different spatial domains of PSII.

Characterization of the water oxidizing complex of photosystem II of the Chl d-containing cyanobacterium Acaryochloris marina via its reactivity towards endogenous electron donors and acceptors

Acaroychloris (A.) marina is a unique oxygen evolving organism that contains a large amount of chlorophyll d (Chl d) and only very few Chl a molecules. This feature raises questions on the nature of the photoactive pigment, which supports light-induced oxidative water splitting in Photosystem II (PS II). In this study, flash-induced oxygen evolution patterns (FIOPs) were measured to address the question whether the S(i) state transition probabilities and/or the redox-potentials of the water oxidizing complex (WOC) in its different S(i) states are altered in A. marina cells compared to that of spinach thylakoids. The analysis of the obtained data within the framework of different versions of the Kok model reveals that in light activated A. marina cells the miss probability is similar compared to spinach thylakoids. This finding indicates that the redox-potentials and kinetics within the WOC, of the reaction center (P680) and of Y(Z) are virtually the same for both organisms. This conclusion is strongly supported by lifetime measurements of the S(2) and S(3) states. Virtually identical time constants were obtained for the slow phase of deactivation. Kinetic differences in the fast phase of S(2) and S(3) decay between A. marina cells and spinach thylakoids reflect a shift of the E(m) of Y(D)/Y(D)(ox) to lower values in the former compared to the latter organisms, as revealed by the observation of an opposite change in the kinetics of S(0) oxidation to S(1) by Y(D)(ox). A slightly increased double hit probability in A. marina cells is indicative of a faster Q(A)(-) to Q(B) electron transfer in these cells compared to spinach thylakoids.

EPR kinetic studies of the S−1 state in spinach thylakoids

The Y(Z)* decay kinetics in a formal S(-1) state, regarded as a reduced state of the oxygen evolving complex, was determined using time-resolved EPR spectroscopy. This S(-1) state was generated by biochemical treatment of thylakoid membranes with hydrazine. The steady-state oxygen evolution of the sample was used to optimize the biochemical procedure for performing EPR experiments. A high yield of the S(-1) state was generated as judged by the two-flash delay in the first maximum of oxygen evolution in Joliot flash-type experiments. We have shown that the Y(Z)* re-reduction rate by the S(-1) state is much slower than that of any other S-state transition in hydrazine-treated samples. This slow reduction rate in the S(-1) to S(0) transition, which is in the order of the S(3) to S(0) transition rate, suggests that this transition is accompanied by some structural rearrangements. Possible explanations of this unique, slow reduction rate in the S(-1) to S(0) transition are considered, in light of earlier observations by others on hydrazine/hydroxylamine reduced PS II samples.

Over-reduced states of the Mn-cluster in cucumber leaves induced by dark-chilling treatment

Oxygen evolution is inhibited when leaves of chilling-sensitive plants like cucumber are treated at 0 degrees C in the dark. The activity is restored by moderate illumination at room temperature. We examined the changes in the redox state of the Mn-cluster in cucumber leaves in the processes of dark-chilling inhibition and subsequent light-induced reactivation by means of thermoluminescence (TL). A TL B-band arising from S(2)Q(B)(-) charge recombination in PSII was observed upon single-flash illumination of untreated leaves, whereas four flashes were required to yield the B-band after dark-chilling treatment for 24 h. This three-step delay indicates that over-reduced states of the Mn-cluster such as the S(-2) state were formed during the treatment. Fitting analysis of the flash-number dependence of the TL intensities showed that the Mn-cluster was more reduced with a longer period of the treatment and that S(-3) was the lowest S-state detectable in the dark-chilled leaves. Measurements of the Mn content by atomic absorption spectroscopy showed that Mn atoms were gradually released from PSII during the dark-chilling treatment but re-bound to PSII by illumination at 30 degrees C. Thus, dark-chilling inhibition of oxygen evolution can be ascribed to the disintegration of the Mn-cluster due to its over-reduction. The observation of the S(-3) state in the present in vivo system strongly suggests that S(-3), which has been observed only by addition of exogenous reductants into in vitro preparations, is indeed a redox intermediate of the Mn-cluster in the processes of its disintegration and photoactivation.

D1' centers are less efficient than normal photosystem II centers

One prominent difference between the photosystem II (PSII) reaction center protein D1' in Synechocystis 6803 and normal D1 is the replacement of Phe-186 in D1 with leucine in D1'. Mutants of Synechocystis 6803 producing only D1', or containing engineered D1 proteins with Phe-186 substitutions, were analyzed by 77 K fluorescence emission spectra, chlorophyll a fluorescence induction yield and decay kinetics, and flash-induced oxygen evolution. Compared to D1-containing PSII centers, D1' centers exhibited a 50% reduction in variable chlorophyll a fluorescence yield, while the flash-induced O(2) evolution pattern was unaffected. In the F186 mutants, both the P680(+)/Q(A)(-) recombination and O(2) oscillation pattern were noticeably perturbed.

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.

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.

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.

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.

Engineering of N-terminal threonines in the D1 protein impairs photosystem II energy transfer in Synechocystis 6803

Mutants of the cyanobacterium Synechocystis sp. PCC 6803 with N-terminal changes in the photosystem (PSII) II D1 protein were analysed by flash-induced oxygen evolution, chlorophyll a fluorescence decay kinetics and 77 K fluorescence emission spectra. The data presented here show that mutations of the Thr-2, Thr-3 and Thr-4 in D1 do not influence the oxygen evolution. A perturbation on the acceptor side was observed and the importance of the N-terminal threonines for an efficient energy transfer between the phycobilisome and PSII and for stability of the PSII complex was demonstrated.

STRUCTURE AND MEMBRANE ORGANIZATION OF PHOTOSYSTEM II IN GREEN PLANTS

Photosystem II (PSII) is the pigment protein complex embedded in the thylakoid membrane of higher plants, algae, and cyanobacteria that uses solar energy to drive the photosynthetic water-splitting reaction. This chapter reviews the primary, secondary, tertiary, and quaternary structures of PSII as well as the function of its constituent subunits. The understanding of in vivo organization of PSII is based in part on freeze-etched and freeze-fracture images of thylakoid membranes. These images show a resolution of about 40-50 A and so provide information mainly on the localization, heterogeneity, dimensions, and shapes of membrane-embedded PSII complexes. Higher resolution of about 15-40 A has been obtained from single particle images of isolated PSII complexes of defined and differing subunit composition and from electron crystallography of 2-D crystals. Observations are discussed in terms of the oligomeric state and subunit organization of PSII and its antenna components.

Identification of domains on the 43 kDa chlorophyll-carrying protein (CP43) that are shielded from tryptic attack by binding of the extrinsic 33 kDa protein with photosystem II complex

The structural association of the spinach 33 kDa extrinsic protein with the 43 kDa chlorophyll-carrying protein (CP43) in oxygen-evolving photosystem II (PS II) complexes was investigated by comparing the peptide mappings and N-terminal sequences of the trypsin-digested products of NaCl-washed PS II membranes, which bind the 33 kDa protein, with those of CaCl2-washed PS II membranes, which lack the 33 kDa protein. (1) Peptide from N-terminus to Arg26 of CP43, which is exposed to stromal side, was digested in both PS II membranes, independent of binding of the 33 kDa protein. (2) Peptide bond of Arg357-Phe358 located in the large extrinsic loop E of CP43, which is exposed to lumenal side, was cleaved by trypsin in CaCl2-washed PS II membranes but not in NaCl-washed PS II membranes. This indicates that the region around Arg357-Phe358 in loop E of CP43 is shielded from tryptic attack by binding of the 33 kDa protein to PS II. (3) Trypsin treatment of CaCl2-washed PS II membranes also cleaved peptide bond between Lys457 and Gly458 in C-terminal region of CP43, while no cleavage of this region was detected by trypsin treatment of NaCl-washed PS II membranes. This implies that a conformational change of the C-terminal region of CP43 which is exposed to stromal side occurred upon removal of the 33 kDa protein, which makes the C-terminal region accessible to trypsin. (4) Release of peptide from Gln60 to C-terminus of the alpha-subunit of cytochrome b-559 was detected only in trypsin treatment of CaCl2-washed PS II membranes, indicating that the C-terminal region of this subunit is shielded from tryptic attack by binding of the 33 kDa protein. (5) The PS II membranes, in which Arg357-Phe358, Lys457-Gly458 of CP43 and the C-terminal part of the cytochrome b-559 alpha-subunit had been cleaved by trypsin, was no longer able to bind the 33 kDa protein. This strongly suggests that a domain in loop E of CP43 and/or the C-terminal region of the cytochrome b-559 alpha-subunit are necessary for binding of the extrinsic 33 kDa protein to PS II.

Intermolecular and intramolecular interactions of the 33-kDa protein in photosystem II

Intermolecular and intramolecular interactions of the extrinsic 33-kDa protein in photosystem II were investigated by cross-linking with a water-soluble carbodiimide as cross-linking agent. This zero-length cross-linker is known to cross-link the 33-kDa protein to the chlorophyll-alpha-binding protein CP47 [Bricker, T. M., Odom, W. R. & Queirolo, C. B. (1988) FEBS Lett. 231, 111-117; Enami, I., Kaneko, M., Kitamura, N., Koike, H., Sonoike, K., Inoue, Y & Katoh, S. (1991) Biochim. Biophys. Acta 1960, 224-232]. In this work, cross-linking was observed not only to CP47 but also to a small intrinsic subunit. In addition, through the use of a high-resolution SDS-gel system, three intramolecular cross-linked products of the 33-kDa protein were detected. To search for additional cross-linking sites that might not be accessible to the cross-linker in intact photosystem II, the isolated 33-kDa protein was activated for cross-linking and subsequently bound to CaCl2-washed photosystem II. In the complementary experiment, CaCl2-washed photosystem II was activated, then reconstituted with the 33-kDa protein. The results of the cross-linking reactions demonstrated that all carboxylic acid groups involved in cross-linking were located on the 33-kDa protein and all primary amines were located on intrinsic membrane proteins. No cross-linking other than those observed in cross-linking experiments with intact photosystem II were observed. This indicated that the 33-kDa protein is bound to CP47 and a small subunit but not to the photosystem II reaction centre. This observation is consistent with the finding that cross-linking was independent of the presence or absence of the manganese cluster. Possible residues on the 33-kDa protein and CP47 involved in cross-linking are suggested.

Site-directed mutagenesis of the CP 47 protein of photosystem II: 167W in the lumenally exposed loop C is required for photosystem II assembly and stability

The intrinsic chlorophyll-protein CP 47 is a component of photosystem II which functions in both light-harvesting and oxygen evolution. Using site-directed mutagenesis we have produced the mutant W167S which lies in loop C of CP 47. This strain exhibited a 75% loss in oxygen evolution activity and grew extremely slowly in the absence of glucose. Examination of normalized oxygen evolution traces indicated that the mutant was susceptible to photoinactivation. Analysis of the variable fluorescence yield indicated that the mutant accumulated very few functional PS II reaction centers. This was confirmed by immunoblotting experiments. Interestingly, when W167S was grown in the presence of 20 microM DCMU, the mutant continued to exhibit these defects. These results indicate that tryptophan 167 in loop C of CP 47 is important for the assembly and stability of the PS II reaction center.

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.

Thermoluminescence from the photosynthetic apparatus

One of the fundamental discoveries of W. Arnold was the detection of thermally stimulated light emission from preilluminated photosynthetic material (Arnold and Sherwood (1957) Proc Natl Acad Sci USA 43: 105-114). This phenomenon, called thermoluminescence (TL), is characteristic of a wide range of materials (semiconductors, minerals, inorganic and organic crystals, and complex biological systems such as the photosynthetic apparatus) which share the common ability of storing radiant energy in thermally stabilized trap states.The original discovery of TL in dried chloroplasts later proved to be a phenomenon common to all photosynthetic organisms: photosynthetic bacteria, cyanobacteria, algae and higher plants. Following the pioneering work of Arnold, considerable effort has been devoted to identification and characterization of photosynthetic TL components. This work has firmly established the participation of various redox states of the water-oxidizing complex and the quinone electron acceptors of Photosystem II in the generation of photosynthetic glow curves. Since TL characteristics are very sensitive to subtle changes in redox properties of the involved electron transport components, the TL method has become a powerful tool in probing a wide range of PS II redox reactions. In this paper, we will review the impact of Arnold's work in initiating and promoting TL studies in photosynthesis and will cover the most important developments of this field of research until the present day.

A null mutant of Synechococcus sp. PCC7942 deficient in the sulfolipid sulfoquinovosyl diacylglycerol

The sulfolipid 6-sulfo-alpha-D-quinovosyldiacylglycerol is associated with the thylakoid membranes of many photosynthetic organisms. Previously, genes involved in sulfolipid biosynthesis have been characterized only in the purple bacterium Rhodobacter sphaeroides. Unlike plants and cyanobacteria, photosynthesis in this bacterium is anoxygenic due to the lack of a water splitting photosystem II. To test the function of sulfolipid in an organism with oxygenic photosynthesis, we isolated and inactivated a sulfolipid gene of the cyanobacterium Synechococcus sp. PCC7942. Extensive analysis of the sulfolipid-deficient null mutant revealed subtle changes in photosynthesis related biochemistry of O2. In addition, a slight increase in the variable room temperature chlorophyll fluorescence yield was observed. Regardless of these changes, it seems unlikely that sulfolipid is an essential constituent of a functional competent water oxidase or the core antenna complex of photosystem II. However, reduced growth of the mutant under phosphate-limiting conditions supports the hypothesis that sulfolipid acts as a surrogate for anionic phospholipids under phosphate-limiting growth conditions.

Random chemical mutagenesis of a specific psbDI region coding for a lumenal loop of the D2 protein of photosystem II in Synechocystis sp. PCC 6803

To identify amino acid residues of the D2 protein that are critical fo r functional photosystem II (PS II), sodium bisulfite was utilized for in vitro random mutagenesis of the psbDI gene from Synechocystis sp. PCC 6803. Sodium bisulfite reacts specifically with cytosine in single-stranded regions of DNA and does not attack double-stranded DNA. Using a hybrid plasmid that was single-stranded in the region to be mutagenized and that was double-stranded elsewhere, mutations were targeted to a specific psbDI region coding for the lumenal A-B loop of the D2 protein. Several mutants were isolated with a total of 15 different amino acid changes in the loop. The majority of these mutations did not result in a loss of photoautotrophic growth or in significantly altered PS II function. However, mutation of Glu-69 to Lys, Ser-79 to Phe, and Ser-88 to Phe were found to influence photosystem II activity; the importance of the latter two residues for proper PS II function was unexpected. Cells carrying the double mutation S79F/S88F in D2 did not grow photoautotrophically and had no functionally active PS II centers. The single mutant S79F was also incapable of photoautotrophic growth, but displayed reasonably stable oxygen evolution, while PS II function in the single mutant S88F appeared to be close to normal. Because of the more pronounced phenotype of the S79F/S88F strain as compared to the single mutants, both Ser residues appear to affect stable assembly and function of the PS II complex. The mechanism by which the S79F mutant loses photoautotrophic growth remains to be established. However, these results show the potential of targeted random mutagenesis to identify functionally important residues in selected regions of proteins.

Involvement of the CP47 protein in stabilization and photoactivation of a functional water-oxidizing complex in the cyanobacterium Synechocystis sp. PCC 6803

Oscillation patterns of the oxygen yield per flash induced by a train of single-turnover flashes were measured as a function of dark incubation and different pre-illumination conditions in several autotrophic mutant strains of Synechocystis sp. PCC 6803 carrying short deletions within the large, lumen-exposed hydrophilic region (loop E) of the chlorophyll a-binding photosystem II protein CP47. A physiological and biochemical characterization of these mutant strains has been presented previously [Eaton-Rye, J. J., & Vermaas, W. F. J. (1991) Plant Mol. Biol. 17, 1165-1177; Haag, E., Eaton-Rye, J. J., Renger, G., & Vermaas, W. F. J. (1993) Biochemistry 32, 4444-4454], and some functional properties were described recently [Gleiter, H. M., Haag, E., Shen, J.-R., Eaton-Rye, J. J., Inoue, Y., Vermaas, W. F. J., & Renger, G. (1994) Biochemistry 33, 12063-12071]. The present study shows that in several mutants the water-oxidizing complex (WOC) became inactivated during prolonged dark incubation, whereas the WOC of the wild-type strain remained active. The rate and extent of the inactivation in the mutants depend on the domain of loop E, where 3-8 amino acid residues were deleted. The most pronounced effects are observed in mutants delta(A373-D380) and delta(R384-V392). A competent WOC can be restored from the fully inactivated state by illumination with short saturating flashes. The number of flashes required for this process strongly depends on the site at which a deletion has been introduced into loop E. Again, the most prominent effects were found in mutants delta(A373-D380) and delta(R384-V392). Interestingly, the number of flashes required for activation was reduced by more than an order of magnitude in both mutants by the addition of 10 mM CaCl2 to the cell suspension. On the basis of a model for photoactivation proposed by Tamura and Cheniae (1987) [Biochim. Biophys. Acta 890, 179-194], a scheme is presented for the processes of dark inactivation and photoactivation in these mutants. The results presented here corroborate an important role of the large hydrophilic domain (loop E) of CP47 in a functional and stable WOC.