Significant energy transfer from CpcG2-phycobilisomes to photosystem I in the cyanobacterium Synechococcus sp. PCC 7002 in the absence of ApcD-dependent state transitions

Hemidiscoidal phycobilisomes (PBS), the major light harvesting complexes of photosynthesis in most cyanobacteria, are composed of rods and cores, which are linked by the linker CpcG1 (L(RC)). Another type of PBS, CpcG2-PBS exits and their function in energy transfer has not been fully understood. We measured growth rates, absorption cross-sections and quantum efficiency of photosystem I in mutant strains of Synechococcus PCC sp. 7002 lacking the linker CpcG2. Our results showed that energy transfer from CpcG2-PBS to PSI in the absence of state transitions could be significant under PBS-absorbing light and energy transfer from two types of PBS is independent to each other. Evidence also suggested that CpcG2 anchors the CpcG2-PBS to thylakoid membranes.     

Distinct roles of CpcG1-phycobilisome and CpcG2-phycobilisome in state transitions in a cyanobacterium <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803

State transitions in cyanobacteria regulate the relative energy transfer from phycobilisome to photosystem I and II. Although it has been shown that phycobilisome mobility is essential for phycobilisome-dependent state transitions, the biochemical mechanism is not known. Previously we reported that two distinct forms of phycobilisome are assembled with different CpcG copies, which have been referred to as “rod-core linker,” in a cyanobacterium Synechocystis sp. PCC 6803. CpcG2-phycobilisome is devoid of a typical central core, while CpcG1-phycobilisome is equivalent to the conventional phycobilisome supercomplex. Here, we demonstrated that the cpcG1 disruptant has a severe specific defect in the phycobilisome-dependent state transition. However, fluorescence recovery after photobleaching measurements showed no obvious difference in phycobilisome mobility between the wild type and the cpcG1 disruptant. This suggests that both CpcG1 and CpcG2 phycobilisomes have an unstable interaction with the reaction centres. However, only CpcG1 phycobilisomes are involved in state transitions. This suggests that state transitions require the phycobilisome core.     

A newly developed SGB buffer greatly enhances energy transfer efficiency from phycobilisomes to photosystem II in cyanobacteria in vitro

The transfer of light energy from phycobilisomes (PBS) to photosystem II (PSII) reaction centers is vital for photosynthesis in cyanobacteria and red algae. To investigate the relationship between PBS and PSII and to optimize the energy transfer efficiency from PBS to PSII, isolation of the PBS-PSII supercomplex is necessary. SPC (sucrose/phosphate/citrate) is a conventional buffer for isolating PBS-PSII supercomplex in cyanobacteria. However, the energy transfer occurring in the supercomplex is poor. Here, we developed a new buffer named SGB by adding 1M glycinebetaine and additional sucrose to SPC buffer. Compared to SPC, the newly developed SGB buffer greatly enhanced the associated populations of PBS with thylakoid membranes and PSII and further improved the energy transfer efficiency from PBS to PSII reaction centers in cyanobacteria in vitro. Therefore, we conclude that SGB is an excellent buffer for isolating the PBS-PSII supercomplex and for enhancing the energy transfer efficiency from PBS to PSII reaction centers in cyanobacteria in vitro.     

Phycobilisomes from the mutant cyanobacterium Synechocystis sp. PCC 6803 missing chromophore domain of ApcE

Phycobilisome (PBS) is a giant photosynthetic antenna associated with the thylakoid membranes of cyanobacteria and red algae. PBS consists of two domains: central core and peripheral rods assembled of disc-shaped phycobiliprotein aggregates and linker polypeptides. The study of the PBS architecture is hindered due to the lack of the data on the structure of the large ApcE-linker also called L_CM. ApcE participates in the PBS core stabilization, PBS anchoring to the photosynthetic membrane, transfer of the light energy to chlorophyll, and, very probably, the interaction with the orange carotenoid protein (OCP) during the non-photochemical PBS quenching. We have constructed the cyanobacterium Synechocystis sp. PCC 6803 mutant lacking 235 N-terminal amino acids of the chromophorylated PBL_CM domain of ApcE. The altered fluorescence characteristics of the mutant PBSs indicate that the energy transfer to the terminal emitters within the mutant PBS is largely disturbed. The PBSs of the mutant become unable to attach to the thylakoid membrane, which correlates with the identified absence of the energy transfer from the PBSs to the photosystem II. At the same time, the energy transfer from the PBS to the photosystem I was registered in the mutant cells and seems to occur due to the small cylindrical CpcG2-PBSs formation in addition to the conventional PBSs. In contrast to the wild type Synechocystis, the OCP-mediated non-photochemical PBS quenching was not registered in the mutant cells. Thus, the PBL_CM domain takes part in formation of the OCP binding site in the PBS.     

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.     

Far-red light-regulated efficient energy transfer from phycobilisomes to photosystem I in the red microalga Galdieria sulphuraria and photosystems-related heterogeneity of phycobilisome population

Phycobilisomes (PBS) are the major photosynthetic antenna complexes in cyanobacteria and red algae. In the red microalga Galdieria sulphuraria, action spectra measured separately for photosynthetic activities of photosystem I (PSI) and photosystem II (PSII) demonstrate that PBS fraction attributed to PSI is more sensitive to stress conditions and upon nitrogen starvation disappears from the cell earlier than the fraction of PBS coupled to PSII. Preillumination of the cells by actinic far-red light primarily absorbed by PSI caused an increase in the amplitude of the PBS low-temperature fluorescence emission that was accompanied by the decrease in PBS region of the PSI 77 K fluorescence excitation spectrum. Under the same conditions, fluorescence excitation spectrum of PSII remained unchanged. The amplitude of P700 photooxidation in PBS-absorbed light at physiological temperature was found to match the fluorescence changes observed at 77 K. The far-red light adaptations were reversible within 2-5min. It is suggested that the short-term fluorescence alterations observed in far-red light are triggered by the redox state of P700 and correspond to the temporal detachment of the PBS antenna from the core complexes of PSI. Furthermore, the absence of any change in the 77 K fluorescence excitation cross-section of PSII suggests that light energy transfer from PBS to PSI in G. sulphuraria is direct and does not occur through PSII. Finally, a novel photoprotective role of PBS in red algae is discussed.     

A small multigene family encodes the rod-core linker polypeptides of Anabaena sp. PCC7120 phycobilisomes.

The cpc operon of Anabaena sp. PCC7120 is shown to encode ten genes: 5'-cpcB-cpcA-cpcC-cpcD-cpcE-cpcF- cpcG1-cpcG2-cpcG3-cpcG4-3'. The 3' portion of this operon includes four tandemly repeated genes encoding phycocyanin (PC)-associated, rod-core linker polypeptides of the phycobilisomes (PBS). The products of these four genes are most similar at their N termini, and overall are 50-61% identical and 68-76% similar to one another. The four CpcG proteins of Anabaena sp. PCC7120 are 41-47% identical and 62-65% similar to the single CpcG rod-core linker protein in Synechococcus sp. PCC7002. The N-terminal domains of the polypeptides are also more distantly related to the conserved domains of other types of rod-linker polypeptides associated with PC, phycoerythrin, and allophycocyanin (AP). Three of these rod-core linker proteins (CpcG1, CpcG2, and CpcG4) were demonstrated to occur in isolated PBS by N-terminal amino acid sequence analyses. These results indicate that previously proposed models for the PBS of Anabaena sp. are incorrect. It is suggested that the PBS of Anabaena sp. have eight peripheral rods, each of which interacts with the AP of the core via a specific rod-core linker (CpcG) polypeptide.     

Extensive remodeling of the photosynthetic apparatus alters energy transfer among photosynthetic complexes when cyanobacteria acclimate to far-red light

Some cyanobacteria remodel their photosynthetic apparatus by a process known as Far-Red Light Photoacclimation (FaRLiP). Specific subunits of the phycobilisome (PBS), photosystem I (PSI), and photosystem II (PSII) complexes produced in visible light are replaced by paralogous subunits encoded within a conserved FaRLiP gene cluster when cells are grown in far-red light (FRL; λ = 700–800 nm). FRL-PSII complexes from the FaRLiP cyanobacterium, Synechococcus sp. PCC 7335, were purified and shown to contain Chl a, Chl d, Chl f, and pheophytin a, while FRL-PSI complexes contained only Chl a and Chl f. The spectroscopic properties of purified photosynthetic complexes from Synechococcus sp. PCC 7335 were determined individually, and energy transfer kinetics among PBS, PSII, and PSI were analyzed by time-resolved fluorescence (TRF) spectroscopy. Direct energy transfer from PSII to PSI was observed in cells (and thylakoids) grown in red light (RL), and possible routes of energy transfer in both RL- and FRL-grown cells were inferred. Three structural arrangements for RL-PSI were observed by atomic force microscopy of thylakoid membranes, but only arrays of trimeric FRL-PSI were observed in thylakoids from FRL-grown cells. Cells grown in FRL synthesized the FRL-specific complexes but also continued to synthesize some PBS and PSII complexes identical to those produced in RL. Although the light-harvesting efficiency of photosynthetic complexes produced in FRL might be lower in white light than the complexes produced in cells acclimated to white light, the FRL-complexes provide cells with the flexibility to utilize both visible and FRL to support oxygenic photosynthesis.This article is part of a Special Issue entitled Light harvesting, edited by Dr. Roberta Croce.     

Redox signalling and the structural basis of regulation of photosynthesis by protein phosphorylation

In photosynthesis in chloroplasts and cyanobacteria, redox control of thylakoid protein phosphorylation regulates distribution of absorbed excitation energy between the two photosystems. When electron transfer through chloroplast photosystem II (PSII) proceeds at a rate higher than that through photosystem I (PSI), chemical reduction of a redox sensor activates a thylakoid protein kinase that catalyses phosphorylation of light-harvesting complex II (LHCII). Phosphorylation of LHCII increases its affinity for PSI and thus redistributes light-harvesting chlorophyll to PSI at the expense of PSII. This short-term redox signalling pathway acts by means of reversible, post-translational modification of pre-existing proteins. A long-term equalisation of the rates of light utilisation by PSI and PSII also occurs: by means of adjustment of the stoichiometry of PSI and PSII. It is likely that the same redox sensor controls both state transitions and photosystem stoichiometry. A specific mechanism for integration of these short- and long-term adaptations is proposed. Recent evidence shows that phosphorylation of LHCII causes a change in its 3-D structure, which implies that the mechanism of state transitions in chloroplasts involves control of recognition of PSI and PSII by LHCII. The distribution of LHCII between PSII and PSI is therefore determined by the higher relative affinity of phospho-LHCII for PSI, with lateral movement of the two forms of the LHCII being simply a result of their diffusion within the membrane plane. Phosphorylation-induced dissociation of LHCII trimers may induce lateral movement of monomeric phospho-LHCII, which binds preferentially to PSI. After dephosphorylation, monomeric, unphosphorylated LHCII may trimerize at the periphery of PSII.     

Fluorescence changes accompanying short-term light adaptations in photosystem I and photosystem II of the cyanobacterium <Emphasis Type="Italic">Synechocystis</Emphasis> 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.     

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.     

Effect of partial or complete elimination of light‐harvesting complexes on the surface electric properties and the functions of cyanobacterial photosynthetic membranes

Influence of the modification of the cyanobacterial light‐harvesting complex [i.e. phycobilisomes (PBS)] on the surface electric properties and the functions of photosynthetic membranes was investigated. We used four PBS mutant strains of Synechocystis sp. PCC6803 as follows: PAL (PBS‐less), CK (phycocyanin‐less), BE (PSII‐PBS‐less) and PSI‐less/apcE^? (PSI‐less with detached PBS). Modifications of the PBS content lead to changes in the cell morphology and surface electric properties of the thylakoid membranes as well as in their functions, such as photosynthetic oxygen‐evolving activity, P700 kinetics and energy transfer between the pigment–protein complexes. Data reveal that the complete elimination of PBS in the PAL mutant causes a slight decrease in the electric dipole moments of the thylakoid membranes, whereas significant perturbations of the surface charges were registered in the membranes without assembled PBS–PSII macrocomplex (BE mutant) or PSI complex (PSI‐less mutant). These observations correlate with the detected alterations in the membrane structural organization. Using a polarographic oxygen rate electrode, we showed that the ratio of the fast to the slow oxygen‐evolving PSII centers depends on the partial or complete elimination of light‐harvesting complexes, as the slow operating PSII centers dominate in the PBS‐less mutant and in the mutant with detached PBS.     

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.     

Distinct roles of CpcG1 and CpcG2 in phycobilisome assembly in the cyanobacterium Synechocystis sp. PCC 6803

Structural role of the second copy of the rod–core linker CpcG, which was found by genome analysis, was studied in Synechocystis sp. PCC 6803 by gene disruption and fractionation of phycobilisome (sub)complexes. Disruption of cpcG2 (sll1471) resulted in a marked decrease in phycocyanin content both in the background of wild-type and cpcG1 (slr2051)-disruptant. The unique phycocyanin rod–CpcG2 complex without the major allophycocyanin components was isolated from the cpcG1-disruptant. By fluorescence analysis, it was proposed that CpcG2 protein connects the rods with a minor allophycocyanin component, to support energy transfer to Photosystem I.     

Protective Action of Spermine and Spermidine against Photoinhibition of Photosystem I in Isolated Thylakoid Membranes

The photo-stability of photosystem I (PSI) is of high importance for the photosynthetic processes. For this reason, we studied the protective action of two biogenic polyamines (PAs) spermine (Spm) and spermidine (Spd) on PSI activity in isolated thylakoid membranes subjected to photoinhibition. Our results show that pre-loading thylakoid membranes with Spm and Spd reduced considerably the inhibition of O₂ uptake rates, P700 photooxidation and the accumulation of superoxide anions (O₂ ⁻) induced by light stress. Spm seems to be more effective than Spd in preserving PSI photo-stability. The correlation of the extent of PSI protection, photosystem II (PSII) inhibition and O₂ ⁻ generation with increasing Spm doses revealed that PSI photo-protection is assumed by two mechanisms depending on the PAs concentration. Given their antioxidant character, PAs scavenge directly the O₂ ⁻ generated in thylakoid membranes at physiological concentration (1 mM). However, for non-physiological concentration, the ability of PAs to protect PSI is due to their inhibitory effect on PSII electron transfer.     

Solar water splitting Pt-nanoparticle photosystem I thylakoid systems: Catalyst identification,location and oligomeric structure

Photosynthetic conversion of light energy into chemical energy occurs in sheet-like membrane-bound compartments called thylakoids and is mediated by large integral membrane protein-pigment complexes called reaction centers (RCs). Oxygenic photosynthesis of higher plants, cyanobacteria and algae requires the symbiotic linking of two RCs, photosystem II (PSII) and photosystem I (PSI), to split water and assimilate carbon dioxide. Worldwide there is a large research investment in developing RC-based hybrids that utilize the highly evolved solar energy conversion capabilities of RCs to power catalytic reactions for solar fuel generation. Of particular interest is the solar-powered production of H₂, a clean and renewable energy source that can replace carbon-based fossil fuels and help provide for ever-increasing global energy demands. Recently, we developed thylakoid membrane hybrids with abiotic catalysts and demonstrated that photosynthetic Z-scheme electron flow from the light-driven water oxidation at PSII can drive H₂ production from PSI. One of these hybrid systems was created by self-assembling Pt-nanoparticles (PtNPs) with the stromal subunits of PSI that extend beyond the membrane plane in both spinach and cyanobacterial thylakoids. Using PtNPs as site-specific probe molecules, we report the electron microscopic (EM) imaging of oligomeric structure, location and organization of PSI in thylakoid membranes and provide the first direct visualization of photosynthetic Z-scheme solar water-splitting biohybrids for clean H₂ production.     

Excitation energy transfer in native and unstacked thylakoid membranes studied by low temperature and ultrafast fluorescence spectroscopy

In this work, the transfer of excitation energy was studied in native and cation-depletion induced, unstacked thylakoid membranes of spinach by steady-state and time-resolved fluorescence spectroscopy. Fluorescence emission spectra at 5 K show an increase in photosystem I (PSI) emission upon unstacking, which suggests an increase of its antenna size. Fluorescence excitation measurements at 77 K indicate that the increase of PSI emission upon unstacking is caused both by a direct spillover from the photosystem II (PSII) core antenna and by a functional association of light-harvesting complex II (LHCII) to PSI, which is most likely caused by the formation of LHCII-LHCI-PSI supercomplexes. Time-resolved fluorescence measurements, both at room temperature and at 77 K, reveal differences in the fluorescence decay kinetics of stacked and unstacked membranes. Energy transfer between LHCII and PSI is observed to take place within 25 ps at room temperature and within 38 ps at 77 K, consistent with the formation of LHCII-LHCI-PSI supercomplexes. At the 150–160 ps timescale, both energy transfer from LHCII to PSI as well as spillover from the core antenna of PSII to PSI is shown to occur at 77 K. At room temperature the spillover and energy transfer to PSI is less clear at the 150 ps timescale, because these processes compete with charge separation in the PSII reaction center, which also takes place at a timescale of about 150 ps.     

Biogenesis of Thylakoid Membranes in Chlamydomonas reinhardtii y1 (A Kinetic Study of Initial Greening)

Initiation of thylakoid membrane assembly was examined in degreened cells of Chlamydomonas reinhardtii y1 cells depleted of thylakoid membranes and photosynthetic activity by growth in the dark for 3 to 4 d. Photoreductive activities of photosystem II (PSII) and photosystem I (PSI) increased with no apparent lag when degreened cells were exposed to light at 38[deg]C. However, fluorescence transients induced by actinic light, which reflect the functional state of PSII, changed only slightly during the first 2 h of greening. When these cells were treated with 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) or saturating light, fluorescence increased commensurate with the cellular content of chlorophyll. In similar experiments with greening cells of C. reinhardtii CC-2341 (ac-u-g-2.3), a PSI-minus strain, fluorescence increased with chlorophyll without treatment with DCMU. These data suggested that fluorescence of initial PSII centers in greening y1 cells was quenched by activity of PSI. Continuous monitoring of fluorescence in the presence or absence of DCMU showed that assembly of quenched PSII centers occurred within seconds after exposure of y1 cells to light. These results are consistent with initial assembly of PSI and PSII within localized domains, where their proximity allows efficient energy coupling.     

Photosynthetic pigment localization and thylakoid membrane morphology are altered in Synechocystis 6803 phycobilisome mutants

Cyanobacteria are oxygenic photosynthetic prokaryotes that are the progenitors of the chloroplasts of algae and plants. These organisms harvest light using large membrane-extrinsic phycobilisome antenna in addition to membrane-bound chlorophyll-containing proteins. Similar to eukaryotic photosynthetic organisms, cyanobacteria possess thylakoid membranes that house photosystem (PS) I and PSII, which drive the oxidation of water and the reduction of NADP+, respectively. While thylakoid morphology has been studied in some strains of cyanobacteria, the global distribution of PSI and PSII within the thylakoid membrane and the corresponding location of the light-harvesting phycobilisomes are not known in detail, and such information is required to understand the functioning of cyanobacterial photosynthesis on a larger scale. Here, we have addressed this question using a combination of electron microscopy and hyperspectral confocal fluorescence microscopy in wild-type Synechocystis species PCC 6803 and a series of mutants in which phycobilisomes are progressively truncated. We show that as the phycobilisome antenna is diminished, large-scale changes in thylakoid morphology are observed, accompanied by increased physical segregation of the two photosystems. Finally, we quantified the emission intensities originating from the two photosystems in vivo on a per cell basis to show that the PSI:PSII ratio is progressively decreased in the mutants. This results from both an increase in the amount of photosystem II and a decrease in the photosystem I concentration. We propose that these changes are an adaptive strategy that allows cells to balance the light absorption capabilities of photosystems I and II under light-limiting conditions.     

Requirement of carotene isomerization for the assembly of photosystem II in Synechocystis sp. PCC 6803

Carotene isomerase mutant (crtH mutant) cells of Synechocystis sp. PCC 6803 can accumulate beta-carotene under light conditions. However, the mutant cells grown under a light-activated heterotrophic growth condition contained detectable levels of neither beta-carotene nor D1 protein of the photosystem (PS) II reaction center, and no oxygen-evolving activity of PSII was detected. beta-Carotene and D1 protein appeared and a high level of PSII activity was detected after the cells were transferred to a continuous light condition. The PSI activities of thylakoid membranes from mutant cells were almost the same as those of thylakoid membranes from wild-type cells, both before and after transfer to the continuous light condition. These results suggest that beta-carotene is required for the assembly of PSII but not for that of PSI.