Fluorescence induction transients indicate dissociation of Photosystem II from the phycobilisome during the State-2 transition in the cyanobacterium Synechococcus 6301

State transitions in cyanobacteria studied with picosecond fluorescence at room temperature

Cyanobacteria can rapidly regulate the relative activity of their photosynthetic complexes photosystem I and II (PSI and PSII) in response to changes in the illumination conditions. This process is known as state transitions. If PSI is preferentially excited, they go to state I whereas state II is induced either after preferential excitation of PSII or after dark adaptation. Different underlying mechanisms have been proposed in literature, in particular i) reversible shuttling of the external antenna complexes, the phycobilisomes, between PSI and PSII, ii) reversible spillover of excitation energy from PSII to PSI, iii) a combination of both and, iv) increased excited-state quenching of the PSII core in state II. Here we investigated wild-type and mutant strains of Synechococcus sp. PCC 7942 and Synechocystis sp. PCC 6803 using time-resolved fluorescence spectroscopy at room temperature. Our observations support model iv, meaning that increased excited-state quenching of the PSII core occurs in state II thereby balancing the photochemistry of photosystems I and II.

Radioprotective role of cyanobacterial phycobilisomes

Cyanobacteria are thought to be responsible for pioneering dioxygen production and the so-called "Great Oxygenation Event" that determined the formation of the ozone layer and the ionosphere restricting ionizing radiation levels reaching our planet, which increased biological diversity but also abolished the necessity of radioprotection. We speculated that ancient protection mechanisms could still be present in cyanobacteria and studied the effect of ionizing radiation and space flight during the Foton-M4 mission on Synechocystis sp. PCC6803. Spectral and functional characteristics of photosynthetic membranes revealed numerous similarities of the effects of α-particles and space flight, which both interrupted excitation energy transfer from phycobilisomes to the photosystems and significantly reduced the concentration of phycobiliproteins. Although photosynthetic activity was severely suppressed, the effect was reversible, and the cells could rapidly recover from the stress. We suggest that the actual existence and the uncoupling of phycobilisomes may play a specific role not only in photo-, but also in radioprotection, which could be crucial for the early evolution of Life on Earth.

Features of Protein-Protein Interactions in the Cyanobacterial Photoprotection Mechanism

Photoprotective mechanisms of cyanobacteria are characterized by several features associated with the structure of their water-soluble antenna complexes - the phycobilisomes (PBs). During energy transfer from PBs to chlorophyll of photosystem reaction centers, the "energy funnel" principle is realized, which regulates energy flux due to the specialized interaction of the PBs core with a quenching molecule capable of effectively dissipating electron excitation energy into heat. The role of the quencher is performed by ketocarotenoid within the photoactive orange carotenoid protein (OCP), which is also a sensor for light flux. At a high level of insolation, OCP is reversibly photoactivated, and this is accompanied by a significant change in its structure and spectral characteristics. Such conformational changes open the possibility for protein-protein interactions between OCP and the PBs core (i.e., activation of photoprotection mechanisms) or the fluorescence recovery protein. Even though OCP was discovered in 1981, little was known about the conformation of its active form until recently, as well as about the properties of homologs of its N and C domains. Studies carried out during recent years have made a breakthrough in understanding of the structural-functional organization of OCP and have enabled discovery of new aspects of the regulation of photoprotection processes in cyanobacteria. This review focuses on aspects of protein-protein interactions between the main participants of photoprotection reactions and on certain properties of representatives of newly discovered families of OCP homologs.

Coupled rows of PBS cores and PSII dimers in cyanobacteria: symmetry and structure

Phycobilisome (PBS) is a giant water-soluble photosynthetic antenna transferring the energy of absorbed light mainly to the photosystem II (PSII) in cyanobacteria. Under the low light conditions, PBSs and PSII dimers form coupled rows where each PBS is attached to the cytoplasmic surface of PSII dimer, and PBSs come into contact with their face surfaces (state 1). The model structure of the PBS core that we have developed earlier by comparison and combination of different fine allophycocyanin crystals, as reported in Zlenko et al. (Photosynth Res 130(1):347-356, 2016b), provides a natural way of the PBS core face-to-face stacking. According to our model, the structure of the protein-protein contact between the neighboring PBS cores in the rows is the same as the contact between the APC hexamers inside the PBS core. As a result, the rates of energy transfer between the cores can occur, and the row of PBS cores acts as an integral PBS "supercore" providing energy transfer between the individual PBS cores. The PBS cores row pitch in our elaborated model (12.4 nm) is very close to the PSII dimers row pitch obtained by the electron microscopy (12.2 nm) that allowed to unite a model of the PBS cores row with a model of the PSII dimers row. Analyzing the resulting model, we have determined the most probable locations of ApcD and ApcE terminal emitter subunits inside the bottom PBS core cylinders and also revealed the chlorophyll molecules of PSII gathering energy from the PBS.

Photosystem II Activity of Wild Type Synechocystis PCC 6803 and Its Mutants with Different Plastoquinone Pool Redox States

To assess the role of redox state of photosystem II (PSII) acceptor side electron carriers in PSII photochemical activity, we studied sub-millisecond fluorescence kinetics of the wild type Synechocystis PCC 6803 and its mutants with natural variability in the redox state of the plastoquinone (PQ) pool. In cyanobacteria, dark adaptation tends to reduce PQ pool and induce a shift of the cyanobacterial photosynthetic apparatus to State 2, whereas illumination oxidizes PQ pool, leading to State 1 (Mullineaux, C. W., and Holzwarth, A. R. (1990) FEBS Lett., 260, 245-248). We show here that dark-adapted Ox(-) mutant with naturally reduced PQ is characterized by slower QA(-) reoxidation and O2 evolution rates, as well as lower quantum yield of PSII primary photochemical reactions (Fv/Fm) as compared to the wild type and SDH(-) mutant, in which the PQ pool remains oxidized in the dark. These results indicate a large portion of photochemically inactive PSII reaction centers in the Ox(-) mutant after dark adaptation. While light adaptation increases Fv/Fm in all tested strains, indicating PSII activation, by far the greatest increase in Fv/Fm and O2 evolution rates is observed in the Ox(-) mutant. Continuous illumination of Ox(-) mutant cells with low-intensity blue light, that accelerates QA(-) reoxidation, also increases Fv/Fm and PSII functional absorption cross-section (590 nm); this effect is almost absent in the wild type and SDH(-) mutant. We believe that these changes are caused by the reorganization of the photosynthetic apparatus during transition from State 2 to State 1. We propose that two processes affect the PSII activity during changes of light conditions: 1) reversible inactivation of PSII, which is associated with the reduction of electron carriers on the PSII acceptor side in the dark, and 2) PSII activation under low light related to the increase in functional absorption cross-section at 590 nm.

Photosystem activity and state transitions of the photosynthetic apparatus in cyanobacterium Synechocystis PCC 6803 mutants with different redox state of the plastoquinone pool

To better understand how photosystem (PS) activity is regulated during state transitions in cyanobacteria, we studied photosynthetic parameters of photosystem II (PSII) and photosystem I (PSI) in Synechocystis PCC 6803 wild type (WT) and its mutants deficient in oxidases (Ox(-)) or succinate dehydrogenase (SDH(-)). Dark-adapted Ox(-) mutant, lacking the oxidation agents, is expected to have a reduced PQ pool, while in SDH(-) mutant the PQ pool after dark adaptation will be more oxidized due to partial inhibition of the respiratory chain electron carriers. In this work, we tested the hypothesis that control of balance between linear and cyclic electron transport by the redox state of the PQ pool will affect PSII photosynthetic activity during state transition. We found that the PQ pool was reduced in Ox(-) mutant, but oxidized in SDH(-) mutant after prolonged dark adaptation, indicating different states of the photosynthetic apparatus in these mutants. Analysis of variable fluorescence and 77K fluorescence spectra revealed that the WT and SDH(-) mutant were in State 1 after dark adaptation, while the Ox(-) mutant was in State 2. State 2 was characterized by ~1.5 time lower photochemical activity of PSII, as well as high rate of P700 reduction and the low level of P700 oxidation, indicating high activity of cyclic electron transfer around PSI. Illumination with continuous light 1 (440 nm) along with flashes of light 2 (620 nm) allowed oxidation of the PQ pool in the Ox(-) mutant, thus promoting it to State 1, but it did not affect PSII activity in dark adapted WT and SDH(-) mutant. State 1 in the Ox(-) mutant was characterized by high variable fluorescence and P700(+) levels typical for WT and the SDH(-) mutant, indicating acceleration of linear electron transport. Thus, we show that PSII of cyanobacteria has a higher photosynthetic activity in State 1, while it is partially inactivated in State 2. This process is controlled by the redox state of PQ in cyanobacteria through enhancement/inhibition of electron transport on the acceptor side of PSII.

Elevated CO2 causes changes in the photosynthetic apparatus of a toxic cyanobacterium, Cylindrospermopsis raciborskii

We studied the physiological acclimation of growth, photosynthesis and CO2-concentrating mechanism (CCM) in Cylindrospermopsis raciborskii exposed to low (present day; L-CO2) and high (1300ppm; H-CO2) pCO2. Results showed that under H-CO2 the cell specific division rate (μc) was higher and the CO2- and light-saturated photosynthetic rates (Vmax and Pmax) doubled. The cells' photosynthetic affinity for CO2 (K0.5CO2) was halved compared to L-CO2 cultures. However, no significant differences were found in dark respiration rates (Rd), pigment composition and light harvesting efficiency (α). In H-CO2 cells, non-photochemical quenching (NPQ), associated with state transitions of the electron transport chain (ETC), was negligible. Simultaneously, a reorganisation of PSII features including antenna connectivity (JconPSIIα), heterogeneity (PSIIα/β) and effective absorption cross sectional area (σPSIIα/β) was observed. In relation to different activities of the CCM, our findings suggest that for cells grown under H-CO2: (1) there is down-regulation of CCM activity; (2) the ability of cells to use the harvested light energy is altered; (3) the occurrence of state transitions is likely to be associated with changes of electron flow (cyclic vs linear) through the ETC; (4) changes in PSII characteristics are important in regulating state transitions.

Oxidation-reduction signalling components in regulatory pathways of state transitions and photosystem stoichiometry adjustment in chloroplasts

State transitions and photosystem stoichiometry adjustment are two oxidation-reduction (redox)-regulated acclimatory responses in photosynthesis. State transitions are short-term adaptations that, in chloroplasts, involve reversible post-translational modification by phosphorylation of light-harvesting complex II (LHC II). Photosystem stoichiometry adjustments are long-term responses involving transcriptional regulation of reaction centre genes. Both responses are initiated by changes in light quality and are regulated by the redox state of plastoquinone (PQ). The LHC II kinase involved in the state 2 transition is a serine/threonine kinase known as STT7 in Chlamydomonas, and as STN7 in Arabidopsis. The phospho-LHC II phosphatase that produces the state 1 transition is a PP2C-type protein phosphatase currently termed both TAP38 and PPH1. In plants and algae, photosystem stoichiometry adjustment is governed by a modified two-component sensor kinase of cyanobacterial origin - chloroplast sensor kinase (CSK). CSK is a sensor of the PQ redox state. Chloroplast sigma factor 1 (SIG1) and plastid transcription kinase (PTK) are the functional partners of CSK in chloroplast gene regulation. We suggest a signalling pathway for photosystem stoichiometry adjustment. The signalling pathways of state transitions and photosystem stoichiometry adjustments are proposed to be distinct, with the two pathways sensing PQ redox state independently of each other.

Regulation of cyclic electron transport through photosystem I in cyanobacterium Synechocystis sp. PCC 6803 mutants deficient in respiratory dehydrogenases

The rate of PSI mediated cyclic electron transport was studied in wild type and mutant cells of Synechocystis sp. PCC 6803 deficient in NDH-1 (M55) or succinate dehydrogenase (SDH(-)) that are responsible for the dark reduction of the plastoquinone pool. Kinetics of P700 photooxidation and P700(+) dark reduction in the presence of 5·10(-5) M 3-(3,4-dichlorophenyl)-1,1-dimethylurea have been registered as light induced absorbance changes at 810 nm resulting from illumination of cells with 730-nm actinic light for 1 sec. It is shown that in the absence of dehydrogenases the rate of dark reduction of P700(+) in both mutants did not decrease but even increased in NDH-1-less mutant cells as compared with the rate in wild type cells. Dibromothymoquinone drastically reduced the rate of P700(+) dark reduction both in wild type and in mutant cells. Thus, the cyclic electron transfer from ferredoxin through the plastoquinone pool to P700(+), which is independent from dehydrogenases, takes place in all the types of cells. Preillumination of cells of wild type and both mutants for 30 min or anaerobic conditions resulted in delay of P700 photooxidation and acceleration of P700(+) dark reduction, while the level of photosynthesis and respiration terminal acceptors (NAD(P)(+) and oxygen) decreased. It appears that the rate of P700 photooxidation and P700(+) dark reduction in cyclic electron transport in Synechocystis wild type and mutant cells is determined by the level of NADP+ and oxygen in stroma. A possible approach to evaluation of the levels of these acceptors in vivo is proposed, based on kinetic curve parameters of P700 photoconversions induced by 730-nm light with 1-sec duration.

Fluorescence changes accompanying short-term light adaptations in photosystem I and photosystem II of the cyanobacterium Synechocystis sp. PCC 6803 and phycobiliprotein-impaired mutants: State 1/State 2 transitions and carotenoid-induced quenching of phycobilisomes

The features of the two types of short-term light-adaptations of photosynthetic apparatus, State 1/State 2 transitions, and non-photochemical fluorescence quenching of phycobilisomes (PBS) by orange carotene-protein (OCP) were compared in the cyanobacterium Synechocystis sp. PCC 6803 wild type, CK pigment mutant lacking phycocyanin, and PAL mutant totally devoid of phycobiliproteins. The permanent presence of PBS-specific peaks in the in situ action spectra of photosystem I (PSI) and photosystem II (PSII), as well as in the 77 K fluorescence excitation spectra for chlorophyll emission at 690 nm (PSII) and 725 nm (PSI) showed that PBS are constitutive antenna complexes of both photosystems. The mutant strains compensated the lack of phycobiliproteins by higher PSII content and by intensification of photosynthetic linear electron transfer. The detectable changes of energy migration from PBS to the PSI and PSII in the Synechocystis wild type and the CK mutant in State 1 and State 2 according to the fluorescence excitation spectra measurements were not registered. The constant level of fluorescence emission of PSI during State 1/State 2 transitions and simultaneous increase of chlorophyll fluorescence emission of PSII in State 1 in Synechocystis PAL mutant allowed to propose that spillover is an unlikely mechanism of state transitions. Blue-green light absorbed by OCP diminished the rout of energy from PBS to PSI while energy migration from PBS to PSII was less influenced. Therefore, the main role of OCP-induced quenching of PBS is the limitation of PSI activity and cyclic electron transport under relatively high light conditions.

Dark-to-light transition in Synechococcus sp. PCC 7942 cells studied by fluorescence kinetics assesses plastoquinone redox poise in the dark and photosystem II fluorescence component and dynamics during state 2 to state 1 transition

We investigated the dark-to-light transition in Synechococcus sp. PCC 7942 cells by a detailed analysis of fluorescence transients induced by strong red light. The transients, recorded with high data-acquisition, revealed all the steps of the fast (OJIP; 10(-5)-1 s) and slow phase (PSM(T); 1-10(3) s), kinetically distinguished with precision. Focusing on the OJIP-rise, we show, for the first time, how the variable to initial fluorescence ratio and the relative height of J-level can serve as indexes of the plastoquinone redox poise and the established state in the dark; hence, differences among cyanobacteria can be recognised in a simple way. Applying intermittent illumination (20-s light pulses separated by 10-s dark intervals) to induce dark-to-light transition and analysing the individual transients, we establish a method by which we determine the fluorescence component not originating from photosystem (PS) II and we assess PSII dynamics during state 2 to state 1 transition. The development of photochemical and non-photochemical quenching is also discussed, as well as evidences favouring the mobile antenna model.

RATE OF O2 PRODUCTION DERIVED FROM PULSE-AMPLITUDE-MODULATED FLUORESCENCE: TESTING THREE BIOOPTICAL APPROACHES AGAINST MEASURED O2 -PRODUCTION RATE(1)

Light absorption by phytoplankton is both species specific and affected by photoacclimational status. To estimate oxygenic photosynthesis from pulse-amplitude-modulated (PAM) fluorescence, the amount of quanta absorbed by PSII needs to be quantified. We present here three different biooptical approaches to estimate the fraction of light absorbed by PSII: (1) the factor 0.5, which implies that absorbed light is equally distributed among PSI and PSII; (2) the fraction of chl a in PSII, determined as the ratio between the scaled red-peak fluorescence excitation and the red absorption peak; and (3) the measure of light absorbed by PSII, determined from the scaling of the fluorescence excitation spectra to the absorption spectra by the "no-overshoot" procedure. Three marine phytoplankton species were used as test organisms: Prorocentrum minimum (Pavill.) J. Schiller (Dinophyceae), Prymnesium parvum cf. patelliferum (J. C. Green, D. J. Hibberd et Pienaar) A. Larsen (Haptophyceae), and Phaeodactylum tricornutum Bohlin (Bacillariophyceae). Photosynthesis versus irradiance (P vs. E) parameters calculated using the three approaches were compared with P versus E parameters obtained from simultaneously measured rates of oxygen production. Generally, approach 1 underestimated, while approach 2 overestimated the gross O2 -production rate calculated from PAM fluorescence. Approach 3, in principle the best approach to estimate quanta absorbed by PSII, was also superior according to observations. Hence, we recommend approach 3 for estimation of gross O2 -production rates based on PAM fluorescence measurements.

Changes in cyclic and respiratory electron transport by the movement of phycobilisomes in the cyanobacterium Synechocystis sp. strain PCC 6803

Phycobilisomes (PBS) are the major accessory light-harvesting complexes in cyanobacteria and their mobility affects the light energy distribution between the two photosystems. We investigated the effect of PBS mobility on state transitions, photosynthetic and respiratory electron transport, and various fluorescence parameters in Synechocystis sp. strain PCC 6803, using glycinebetaine to immobilize and couple PBS to photosystem II (PSII) or photosystem I (PSI) by applying under far-red or green light, respectively. The immobilization of PBS at PSII inhibited the increase in cyclic electron flow, photochemical and non-photochemical quenching, and decrease in respiration that occurred during the movement of PBS from PSII to PSI. In contrast, the immobilization of PBS at PSI inhibited the increase in respiration and photochemical quenching and decrease in cyclic electron flow and non-photochemical quenching that occurred when PBS moved from PSI to PSII. Linear electron transport did not change during PBS movement but increased or decreased significantly during longer illumination with far-red or green light, respectively. This implies that PBS movement is completed in a short time but it takes longer for the overall photosynthetic reactions to be tuned to a new state.

Light-induced excitation energy redistribution in Spirulina platensis cells: “spillover” or “mobile PBSs”?

State transitions induced by light and redox were investigated by observing the 77 K fluorescence spectra for the intact cells of Spirulina platensis. To clarify if phycobilisomes (PBSs) take part in the state transition, the contributions of PBSs to light-induced state transition were studied in untreated cells and the cells treated by betaine which fixed PBSs firmly on the thylakoid membranes. It was observed that the betaine-treated cells did not show any light-induced state transition. This result definitely confirmed that the light-induced excitation energy regulation between the two photosystems is mainly dependent on a spatial movement of PBSs on the thylakoid membranes, which makes PBS cores partially decoupled from photosystem II (PSII) while PBS rods more strongly coupled with photosystem I (PSI) during the transition from state 1 to state 2. On the other hand, an energy exchange between the two photosystems was observed in both untreated and betaine-treated cells during redox-induced state transition. These observations suggested that two different mechanisms were involved in the light-induced state transition and the redox-induced one. The former involves only a physical movement of PBSs, while the latter involves not only the movement of PBS but also energy spillover from PSII to PSI. A model for light-induced state transition was proposed based on the current results as well as well known knowledge.

Kinetic analyses of state transitions of the cyanobacterium Synechococcus sp. PCC 7002 and its mutant strains impaired in electron transport

The state transitions of the cyanobacterium Synechococcus sp. PCC 7002 and of three mutant strains, which were impaired in PsaE-dependent cyclic electron transport (psaE(-)), respiratory electron transport (ndhF(-)) and both activities (psaE(-)ndhF(-)), were analyzed. Dark incubation of the wild type and psaE(-) cells led to a transition to state 2, while the ndhF(-) strains remained in state 1 after dark incubation. The ndhF(-) cells adapted to state 2 when the cells were incubated under anaerobic conditions or in the presence of potassium cyanide; these results suggest that the ndhF(-) cells were inefficient in performing state 1 to state 2 transitions in the dark unless cytochrome oxidase activity was inhibited. In the state 2 to state 1 transition of wild-type cells induced by light in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), there was still a significant reduction of the interphotosystem electron carriers by both respiration and cyclic electron flow around PSI. Kinetic analysis of the state 2 to state 1 transition shows that, in the absence of PSII activity, the relative contribution to the reduced state of the interphotosystem electron carriers by respiratory and cyclic electron transfer is about 72% and 28%, respectively. The state 2 to state 1 transition was prevented by the cytochrome b(6)f inhibitor 2,5-dibromo-3-methyl-6-isopropylbenzoquinone (DBMIB). On the other hand, the state 1 to state 2 transition was induced by DBMIB with half times of approximately 8 s in all strains. The externally added electron acceptor 2,5-dimethyl-benzoquinone (DMBQ) induced a state 2 to state 1 transition in the dark and this transition could be prevented by DBMIB. The light-induced oxidation of P700 showed that approximately 50% of PSI could be excited by 630-nm light absorbed by phycobilisomes (PBS) under state 2 conditions. P700 oxidation measurements with light absorbed by PBS also showed that the dark-induced state 1 to state 2 transition occurred in wild-type cells but not in the ndhF(-) cells. The possible mechanism for sensing an imbalanced light regime in cyanobacterial state transitions is discussed.

Characterization of the non-photochemical quenching of chlorophyll fluorescence that occurs during the active accumulation of inorganic carbon in the cyanobacterium Synechococcus PCC 7942

Previous work has shown that the maximum fluorescence yield from PS 2 of Synechococcus PCC 7942 occurs when the cells are at the CO2 compensation point. The addition of inorganic carbon (Ci), as CO2 or HCO3 (-), causes a lowering of the fluorescence yield due to both photochemical (qp) and non-photochemical (qN) quenching. In this paper, we characterize the qN that is induced by Ci addition to cells grown at high light intensities (500 μmol photons m(-2) s(-1)). The Ci-induced qN was considerably greater in these cells than in cells grown at low light intensities (50 μmol photons m(-2) s(-1)), when assayed at a white light (WL) intensity of 250 μmol photons m(-2) s(-1). In high-light grown cells we measured qN values as high as 70%, while in low-light grown cells the qN was about 16%. The qN was relieved when cells regained the CO2 compensation point, when cells were illuminated by supplemental far-red light (FRL) absorbed mainly by PS 1, or when cells were illuminated with increased WL intensities. These characteristics indicate that the qN was not a form of energy quenching (qE). Supplemental FRL illumination caused significant enhancement of photosynthetic O2 evolution that could be correlated with the changes in qp and qN. The increases in qp induced by Ci addition represent increases in the effective quantum yield of PS 2 due to increased levels of oxidized QA. The increase in qN induced by Ci represents a decrease in PS 2 activity related to decreases in the potential quantum yield. The lack of diagnostic changes in the 77 K fluorescence emission spectrum argue against qN being related to classical state transitions, in which the decrease in potential quantum yield of PS 2 is due either to a decrease in absorption cross-section or by increased 'spill-over' of excitation energy to PS 1. Both the Ci-induced qp (t 0.5<0.5 s) and qN (t 0.5≃1.6 s) were rapidly relieved by the addition of DCMU. The two time constants give further support for two separate quenching mechanisms. We have thus characterized a novel form of qN in cyanobacteria, not related to state transitions or energy quenching, which is induced by the addition of Ci to cells at the CO2-compensation point.

Photodynamic effects of hypericin on photosynthetic electron transport and fluorescence of Anacystis nidulans (Synechococcus 6301)

We investigated the photodynamic action of hypericin, a natural naphthodianthrone, on photosynthetic electron transport and fluorescence of the cyanobacterium Anacystis nidulans (Synechococcus 6301). The most drastic effect was the inactivation of photosynthetic oxygen evolution in the presence of the electron acceptor phenyl-p-benzoquinone in aerobic cells which required 1 hypericin/5 chlorophyll a for half-maximal effect. Anaerobic A. nidulans was only partially inactivated and variable chlorophyll a fluorescence remained unperturbed suggesting that photoreaction center II was not a target. Further, hypericin, stimulated photoinduced oxygen uptake in the presence of methylviologen in aerobic cells. This action was less specific than the inactivation of oxygen evolution (1 hypericin/0.5-0.7 chlorophyll a for half-maximal effect). Results point to the involvement of molecular oxygen in two ways. Type I mechanism (Henderson BW and Dougherty TJ (1992) Photochem Photobiol 55: 145-157) in which ground state oxygen reacts with excited substrate triplets appears probable for the inactivation of oxygen evolution. On the other hand, Type II mechanism in which excited oxygen singlets react with ground state substrate molecules appears probable in the stimulation of methylviologen mediated oxygen uptake.

Functional analysis of the iron-stress induced CP 43' polypeptide of PS II in the cyanobacterium Synechococcus sp. PCC 7942

Under conditions of iron-stress, the Photosystem II associated chlorophyll a protein complex designated CP 43', which is encoded by the isiA gene, becomes the major pigment-protein complex in Synechococcus sp. PCC 7942. The isiB gene, which is located immediately downstream of isiA, encodes the protein flavodoxin, which can functionally replace ferredoxin under conditions of iron stress. We have constructed two cyanobacterial insertion mutants which are lacking (i) the CP 43' apoprotein (designated isiA (-)) and (ii) flavodoxin (designated isiB (-)). The function of CP 43' was studied by comparing the cell characteristics, PS II functional absorption cross-sections and Chl a fluorescence parameters from the wild-type, isiA (-) and isiB (-) strains grown under iron-stressed conditions. In all strains grown under iron deprivation, the cell number doubling time was maintained despite marked changes in pigment composition and other cell characteristics. This indicates that iron-starved cells remained viable and that their altered phenotype suggests an adequate acclimation to low iron even in absence of CP 43' and/or flavodoxin. Under both iron conditions, no differences were detected between the three strains in the functional absorption crossection of PS II determined from single turnover flash saturation curves of Chl a fluorescence. This demonstrates that CP 43' is not part of the functional light-harvesting antenna for PS II. In the wild-type and the isiB (-) strain grown under iron-deficient conditions, CP 43' was present in the thylakoid membrane as an uncoupled Chl-protein complex. This was indicated by (1) an increase of the yield of prompt Chl a fluorescence (Fo) and (2) the persistence after PS II trap closure of a fast fluorescence decay component showing a maximum at 685 nm.

State adaptations in the cyanobacterium Synechcoccus 6301 (PCC): Dependence on light intensity or spectral composition?

A profile of high light to intense self-shading conditions was constructed using a white light source and cultures of the cyanobacterium Synechococcus 6301; this profile approximates to a natural self-shading gradient of decreasing light intensity and PS II/PS I excitation ratio. Samples of S.6301 were placed along this profile and allowed to state adapt. To separate the effects of light intensity and wavelength on state adaptation, samples were also placed in a shade profile produced by a white light source and neutral density filters. After adaptation, samples were fixed in their resulting state by the addition of glutaraldehyde, and fluorescence measurements were made at 35° C or -160 °C. It is concluded: 1. Under conditions of deep shade (<5 μmol m(-2)s(-1) PAR) and weak shade (>200 μmol m(-2)s(-1) PAR), cells adapt to a low PS II fluorescence state (state 2); in moderate shade (20-60 μmol m(-2)s(-1)PAR) cells adapt to a high PS II fluorescence state (state 1). We suggest these findings provide evidence for the operation of different factors on the control of state adaptations in cyanobacteria; one set operates at low light and another at high light intensities. 2. Under conditions of self-shading, there is little evidence to support the contention that state adaptations in cyanobacteria are produced by wavelength-dependent changes in the PS II/PS I excitation ratio, instead, it appaers they are produced by changes in the intensity of incident irradiation. 3. The observed fluorescence changes do not appear to involve major changes in the phycobilisome sensitisation of PS II and PS I. Instead, it appears that these changes are effected by alterations in Φ(F) of PS II (i.e. changes in PS II excitation density caused by alterations in the rate constants controlling spillover to PS I, photochemistry, fluorescence emission or thermal deactivation.

Cyanobacterial thylakoid membrane proteins are reversibly phosphorylated under plastoquinone-reducing conditions in vitro

Reversible, light-dependent protein phosphorylation was observed in isolated thylakoid membranes of the cyanobacterium Synechococcus 6301. A polypeptide of 15 kDa in particular was phosphorylated under plastoquinone-reducing conditions and was not phosphorylated under plastoquinone-oxidising conditions. Phosphorylation and dephosphorylation reactions involving this and several other membrane polypeptides showed sensitivity to inhibitors of protein kinases and phosphatases. Changes in phosphorylation state correlated with changes in low temperature fluorescence emission characteristic of changes in excitation energy distribution between the photosystems. The 15 kDa phosphopolypeptide is likely to be involved directly in light state adaptations in cyanobacteria.

State 1-state 2 adaptation in the cyanobacteria Synechocystis PCC 6714 wild type and Synechocystis PCC 6803 wild type and phycocyanin-less mutant

The mechanism of excitation energy distribution between the two photosystems (state transitions) is studied in Synechocystis 6714 wild type and in wild type and a mutant lacking phycocyanin of Synechocystis 6803. (i) Measurements of fluorescence transients and spectra demonstrate that state transitions in these cyanobacteria are controlled by changes in the efficiency of energy transfer from PS II to PS I (spillover) rather than by changes in association of the phycobilisomes to PS II (mobile antenna model). (ii) Ultrastructural study (freeze-fracture) shows that in the mutant the alignment of the PS II associated EF particles is prevalent in state 1 while the conversion to state 2 results in randomization of the EF particle distribution, as already observed in the wild type (Olive et al. 1986). In the mutant, the distance between the EF particle rows is smaller than in the wild type, probably because of the reduced size of the phycobilisomes. Since a parallel increase of spillover is not observed we suggest that the probability of excitation transfer between PS II units and between PS II and PS I depends on the mutual orientation of the photosystems rather than on their distance. (iii) Measurements of the redox state of the plastoquinone pool in state 1 obtained by PS I illumination and in state 2 obtained by various treatments (darkness, anaerobiosis and starvation) show that the plastoquinone pool is oxidized in state 1 and reduced in state 2 except in starved cells where it is still oxidized. In the latter case, no important decrease of ATP was observed. Thus, we propose that in Synechocystis the primary control of the state transitions is the redox state of a component of the cytochrome b 6/f complex rather than that of the plastoquinone pool.

Differences in the capacity for radiationless energy dissipation in the photochemical apparatus of green and blue-green algal lichens associated with differences in carotenoid composition

Green algal lichens, which were able to form zeaxanthin rapidly via the de-epoxidation of violaxanthin, exhibited a high capacity to dissipate excess excitation energy nonradiatively in the antenna chlorophyll as indicated by the development of strong nonphotochemical quenching of chlorophyll fluorescence (FM, the maximum yield of fluorescence induced by pulses of saturating light) and, to a lesser extent, FO (the yield of instantaneous fluorescence). Blue-green algal lichens which did not contain any zeaxanthin were incapable of such radiationless energy dissipation and were unable to maintain the acceptor of photosystem II in a low reduction state upon exposure to excessive photon flux densities (PFD). Furthermore, following treatment of the thalli with an inhibitor of the violaxanthin de-epoxidase, dithiothreitol, the response of green algal lichens to light became very similar to that of the blue-green algal lichens. Conversely, blue-green algal lichens which had accumulated some zeaxanthin following long-term exposure to higher PFDs exhibited a response to light which was intermediate between that of zeaxanthin-free blue-green algal lichens and zeaxanthin-containing green algal lichens. Zeaxanthin can apparently be formed in blue-green algal lichens (which lack the xanthophyll epoxides, i.e. violaxanthin and antheraxanthin) as part of the normal biosynthetic pathway which leads to a variety of oxygenated derivatives of β-carotene during exposure to high light over several days. We conclude that the pronounced difference in the capacity for photoprotective energy dissipation in the antenna chlorophyll between (zeaxanthin-containing0 green algal lichens and (zeaxanthin-free) blue-green algal lichens is related to the presence or absence of zeaxanthin, and that this difference can explain the greater susceptibility to high-light stress in lichens with blue-green phycobionts.

State 1-State 2 transitions in the cyanobacterium Synechococcus 6301 are controlled by the redox state of electron carriers between Photosystems I and II

The mechanism by which state 1-state 2 transitions in the cyanobacterium Synechococcus 6301 are controlled was investigated by examining the effects of a variety of chemical and illumination treatments which modify the redox state of the plastoquinone pool. The extent to which these treatments modify excitation energy distribution was determined by 77K fluorescence emission spectroscopy. It was found that treatment which lead to the oxidation of the plastoquinone pool induce a shift towards state 1 whereas treatments which lead to the reduction of the plastoquinone pool induce a shift towards state 2. We therefore propose that state transitions in cyanobacteria are triggered by changes in the redox state of plastoquinone or a closely associated electron carrier. Alternative proposals have included control by the extent of cyclic electron transport around PS I and control by localised electrochemical gradients around PS I and PS II. Neither of these proposals is consistent with the results reported here.

State transitions, photosystem stoichiometry adjustment and non-photochemical quenching in cyanobacterial cells acclimated to light absorbed by photosystem I or photosystem II

Cells of the cyanobacterium Synechococcus 6301 were grown in yellow light absorbed primarily by the phycobilisome (PBS) light-harvesting antenna of photosystem II (PS II), and in red light absorbed primarily by chlorophyll and, therefore, by photosystem I (PS I). Chromatic acclimation of the cells produced a higher phycocyanin/chlorophyll ratio and higher PBS-PS II/PS I ratio in cells grown under PS I-light. State 1-state 2 transitions were demonstrated as changes in the yield of chlorophyll fluorescence in both cell types. The amplitude of state transitions was substantially lower in the PS II-light grown cells, suggesting a specific attenuation of fluorescence yield by a superimposed non-photochemical quenching of excitation. 77 K fluorescence emission spectra of each cell type in state 1 and in state 2 suggested that state transitions regulate excitation energy transfer from the phycobilisome antenna to the reaction centre of PS II and are distinct from photosystem stoichiometry adjustments. The kinetics of photosystem stoichiometry adjustment and the kinetics of the appearance of the non-photochemical quenching process were measured upon switching PS I-light grown cells to PS II-light, and vice versa. Photosystem stoichiometry adjustment was complete within about 48 h, while the non-photochemical quenching occurred within about 25 h. It is proposed that there are at least three distinct phenomena exerting specific effects on the rate of light absorption and light utilization by the two photoreactions: state transitions; photosystem stoichiometry adjustment; and non-photochemical excitation quenching. The relationship between these three distinct processes is discussed.

Protein phosphorylation and control of excitation energy transfer in photosynthetic purple bacteria and cyanobacteria

The function of phosphorylation of light-harvesting polypeptides is well characterised in chloroplasts of green plants, but the prokaryotic cyanobacteria and purple photosynthetic bacteria have quite different light-harvesting polypeptides whose structure and function cannot be controlled in precisely the same way. Nevertheless, cyanobacteria show light-dependent phosphorylation of membrane polypeptides associated with photosystem II and with the light-harvesting phycobilisome, and purple bacteria show light-dependent phosphorylation of low molecular-weight chromatophore membrane polypeptides. In both cases membrane protein phosphorylation is associated with functional changes observed by chlorophyll fluorescence spectroscopy or chlorophyll fluorescence induction kinetics. Here we report on our recent protein sequence and other data concerning the identities of these phosphoproteins. We also discuss the significance of these findings for regulation by protein phosphorylation of photosynthesis in prokaryotes.

State transitions in a phycobilisome-less mutant of the cyanobacterium Synechococcus sp. PCC 7002

State transitions were investigated in the cyanobacterium Synechococcus sp. PCC 7002 in both wild-type cells and mutant cells lacking phycobilisomes. Preillumination in the presence of DCMU induced State 1 and dark-adaptation induced State 2 in both wild-type and mutant cells as determined by 77 K fluorescence emission spectroscopy. Light-induced transitions were observed in the wild-type after preferential excitation of phycocyanin (State 2) or preferential excitation of Chl a (State 1). Light-induced transitions were also observed in the phycobilisome-less mutant after preferential excitation of short-wavelength Chl a (State 2) or carotenoids and long-wavelength Chl a (State 1). We conclude that the mechanism of the light-state transition in cyanobacteria does not require the presence of the phycobilisome. Our results contradict proposed models for the state transition, which require phosphorylation of, and an active role for, the phycobilisome.