Light-induced energy dissipation in iron-starved cyanobacteria: roles of OCP and IsiA proteins

Photophysiological expressions of iron stress in phytoplankton

Iron is essential for all life, but it is particularly important to photoautotrophs because of the many iron-dependent electron transport components in photosynthetic membranes. Since the proliferation of oxygenic photosynthesis in the Archean ocean, iron has been a scarce commodity, and it is now recognized as a limiting resource for phytoplankton over broad expanses of the open ocean and even in some coastal/continental shelf waters. Iron stress does not impair photochemical or carbon fixation efficiencies, and in this respect it resembles the highly tuned photosynthetic systems of steady-state macronutrient-limited phytoplankton. However, iron stress does present unique photophysiological challenges, and phytoplankton have responded to these challenges through major architectural changes in photosynthetic membranes. These evolved responses include overexpression of photosynthetic pigments and iron-economic pathways for ATP synthesis, and they result in diagnostic fluorescence properties that allow a broad appraisal of iron stress in the field and even the detection of iron stress from space.

Site of non-photochemical quenching of the phycobilisome by orange carotenoid protein in the cyanobacterium Synechocystis sp. PCC 6803

In cyanobacteria, the thermal dissipation of excess absorbed energy at the level of the phycobilisome (PBS)-antenna is triggered by absorption of strong blue-green light by the photoactive orange carotenoid protein (OCP). This process known as non-photochemical quenching, whose molecular mechanism remains in many respects unclear, is revealed in vivo as a decrease in phycobilisome fluorescence. In vitro reconstituted system on the interaction of the OCP and the PBS isolated from the cyanobacterium Synechocystis sp. PCC 6803 presents evidence that the OCP is not only a photosensor, but also an effecter that makes direct contacts with the PBS and causes dissipation of absorbed energy. To localize the site(s) of quenching, we have analyzed the role of chromophorylated polypeptides of the PBS using PBS-deficient mutants in conjunction with in vitro systems of assembled PBS and of isolated components of the PBS core. The results demonstrated that L(CM), the core-membrane linker protein and terminal emitter of the PBS, could act as the docking site for OCP in vitro. The ApcD and ApcF terminal emitters of the PBS core are not directly subjected to quenching. The data suggests that there could be close contact between the phycocyanobilin chromophore of L(CM) and the 3'-hydroxyechinenone chromophore present in OCP and that L(CM) could be involved in OCP-induced quenching. According to the reduced average life-time of the PBS-fluorescence and linear dependence of fluorescence intensity of the PBS on OCP concentration, the quenching has mostly dynamic character. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Unique properties vs. common themes: the atypical cyanobacterium Gloeobacter violaceus PCC 7421 is capable of state transitions and blue-light-induced fluorescence quenching

The atypical unicellular cyanobacterium Gloeobacter violaceus PCC 7421, which diverged very early during the evolution of cyanobacteria, can be regarded as a key organism for understanding many structural, functional, regulatory and evolutionary aspects of oxygenic photosynthesis. In the present work, the performance of two basic photosynthetic adaptation/protection mechanisms, common to all other oxygenic photoautrophs, had been challenged in this ancient cyanobacterium which lacks thylakoid membranes: state transitions and non-photochemical fluorescence quenching. Both low temperature fluorescence spectra and room temperature fluorescence transients show that G. violaceus is capable of performing state transitions similar to evolutionarily more recent cyanobacteria, being in state 2 in darkness and in state 1 upon illumination by weak blue or far-red light. Compared with state 2, variable fluorescence yield in state 1 is strongly enhanced (almost 80%), while the functional absorption cross-section of PSII is only increased by 8%. In contrast to weak blue light, which enhances fluorescence yield via state 1 formation, strong blue light reversibly quenches Chl fluorescence in G. violaceus. This strongly suggests regulated heat dissipation which is triggered by the orange carotenoid protein whose presence was directly proven by immunoblotting and mass spectrometry in this primordial cyanobacterium. The results are discussed in the framework of cyanobacterial evolution.

THE CYANOBACTERIAL CHLOROPHYLL-BINDING-PROTEIN ISIA ACTS TO INCREASE THE IN VIVO EFFECTIVE ABSORPTION CROSS-SECTION OF PSI UNDER IRON LIMITATION(1)

Iron availability limits primary production in >30% of the world's oceans; hence phytoplankton have developed acclimation strategies. In particular, cyanobacteria express IsiA (iron-stress-induced) under iron stress, which can become the most abundant chl-binding protein in the cell. Within iron-limited oceanic regions with significant cyanobacterial biomass, IsiA may represent a significant fraction of the total chl. We spectroscopically measured the effective cross-section of the photosynthetic reaction center PSI (σPSI ) in vivo and biochemically quantified the absolute abundance of PSI, PSII, and IsiA in the model cyanobacterium Synechocystis sp. PCC 6803. We demonstrate that accumulation of IsiA results in a ∼60% increase in σPSI , in agreement with the theoretical increase in cross-section based on the structure of the biochemically isolated IsiA-PSI supercomplex from cyanobacteria. Deriving a chl budget, we suggest that IsiA plays a primary role as a light-harvesting antenna for PSI. On progressive iron-stress in culture, IsiA continues to accumulate without a concomitant increase in σPSI , suggesting that there may be a secondary role for IsiA. In natural populations, the potential physiological significance of the uncoupled pool of IsiA remains to be established. However, the functional role as a PSI antenna suggests that a large fraction of IsiA-bound chl is directly involved in photosynthetic electron transport.

ApcD, ApcF and ApcE are not required for the Orange Carotenoid Protein related phycobilisome fluorescence quenching in the cyanobacterium Synechocystis PCC 6803

In cyanobacteria, strong blue-green light induces a photoprotective mechanism involving an increase of energy thermal dissipation at the level of phycobilisome (PB), the cyanobacterial antenna. This leads to a decrease of the energy arriving to the reaction centers. The photoactive Orange Carotenoid Protein (OCP) has an essential role in this mechanism. The binding of the red photoactivated OCP to the core of the PB triggers energy and PB fluorescence quenching. The core of PBs is constituted of allophycocyanin trimers emitting at 660 or 680nm. ApcD, ApcF and ApcE are the responsible of the 680nm emission. In this work, the role of these terminal emitters in the photoprotective mechanism was studied. Single and double Synechocystis PCC 6803 mutants, in which the apcD or/and apcF genes were absent, were constructed. The Cys190 of ApcE which binds the phycocyanobilin was replaced by a Ser. The mutated ApcE attached an unusual chromophore emitting at 710nm. The activated OCP was able to induce the photoprotective mechanism in all the mutants. Moreover, in vitro reconstitution experiments showed similar amplitude and rates of fluorescence quenching. Our results demonstrated that ApcD, ApcF and ApcE are not required for the OCP-related fluorescence quenching and they strongly suggested that the site of quenching is one of the APC trimers emitting at 660nm. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

In vitro reconstitution of the cyanobacterial photoprotective mechanism mediated by the Orange Carotenoid Protein in Synechocystis PCC 6803

In conditions of fluctuating light, cyanobacteria thermally dissipate excess absorbed energy at the level of the phycobilisome, the light-collecting antenna. The photoactive Orange Carotenoid Protein (OCP) and Fluorescence Recovery Protein (FRP) have essential roles in this mechanism. Absorption of blue-green light converts the stable orange (inactive) OCP form found in darkness into a metastable red (active) form. Using an in vitro reconstituted system, we studied the interactions between OCP, FRP, and phycobilisomes and demonstrated that they are the only elements required for the photoprotective mechanism. In the process, we developed protocols to overcome the effect of high phosphate concentrations, which are needed to maintain the integrity of phycobilisomes, on the photoactivation of the OCP, and on protein interactions. Our experiments demonstrated that, whereas the dark-orange OCP does not bind to phycobilisomes, the binding of only one red photoactivated OCP to the core of the phycobilisome is sufficient to quench all its fluorescence. This binding, which is light independent, stabilizes the red form of OCP. Addition of FRP accelerated fluorescence recovery in darkness by interacting with the red OCP and destabilizing its binding to the phycobilisome. The presence of phycobilisome rods renders the OCP binding stronger and allows the isolation of quenched OCP-phycobilisome complexes. Using the in vitro system we developed, it will now be possible to elucidate the quenching process and the chemical nature of the quencher.

The small CAB-like proteins of the cyanobacterium Synechocystis sp. PCC 6803: their involvement in chlorophyll biogenesis for Photosystem II

The five small CAB-like proteins (ScpA-E) of the cyanobacterium Synechocystis sp. PCC 6803 belong to the family of stress-induced light-harvesting-like proteins, but are constitutively expressed in a mutant deficient of Photosystem I (PSI). Using absorption, fluorescence and thermoluminescence measurements this PSI-less strain was compared with a mutant, in which all SCPs were additionally deleted. Depletion of SCPs led to structural rearrangements in Photosystem II (PSII): less photosystems were assembled; and in these, the Q(B) site was modified. Despite the lower amount of PSII, the SCP-deficient cells contained the same amount of phycobilisomes (PBS) as the control. Although the excess PBS were functionally disconnected, their fluorescence was quenched under high irradiance by the activated Orange Carotenoid Protein (OCP). Additionally the amount of OCP, but not of the iron-stress induced protein (isiA), was higher in this SCP-depleted mutant compared with the control. As previously described, the lack of SCPs affects the chlorophyll biosynthesis (Vavilin, D., Brune, D. C., Vermaas, W. (2005) Biochim Biophys Acta 1708, 91-101). We demonstrate that chlorophyll synthesis is required for efficient PSII repair and that it is partly impaired in the absence of SCPs. At the same time, the amount of chlorophyll also seems to influence the expression of ScpC and ScpD.

The orange carotenoid protein in photoprotection of photosystem II in cyanobacteria

Photoprotective mechanisms have evolved in photosynthetic organisms to cope with fluctuating light conditions. Under high irradiance, the production of dangerous oxygen species is stimulated and causes photo-oxidative stress. One of these photoprotective mechanisms, non photochemical quenching (qE), decreases the excess absorbed energy arriving at the reaction centers by increasing thermal dissipation at the level of the antenna. In this review we describe results leading to the discovery of this process in cyanobacteria (qE(cya)), which is mechanistically distinct from its counterpart in plants, and recent progress in the elucidation of this mechanism. The cyanobacterial photoactive soluble orange carotenoid protein is essential for the triggering of this photoprotective mechanism. Light induces structural changes in the carotenoid and the protein leading to the formation of a red active form. The activated red form interacts with the phycobilisome, the cyanobacterial light-harvesting antenna, and induces a decrease of the phycobilisome fluorescence emission and of the energy arriving to the reaction centers. The orange carotenoid protein is the first photoactive protein to be identified that contains a carotenoid as the chromophore. Moreover, its photocycle is completely different from those of other photoactive proteins. A second protein, called the Fluorescence Recovery Protein encoded by the slr1964 gene in Synechocystis PCC 6803, plays a key role in dislodging the red orange carotenoid protein from the phycobilisome and in the conversion of the free red orange carotenoid protein to the orange, inactive, form. This protein is essential to recover the full antenna capacity under low light conditions after exposure to high irradiance. This article is part of a Special Issue entitled: Photosystem II.

Surplus photosynthetic antennae complexes underlie diagnostics of iron limitation in a cyanobacterium

Chlorophyll fluorescence from phytoplankton provides a tool to assess iron limitation in the oceans, but the physiological mechanism underlying the fluorescence response is not understood. We examined fluorescence properties of the model cyanobacterium Synechocystis PCC6803 and a ΔisiA knock-out mutant of the same species grown under three culture conditions which simulate nutrient conditions found in the open ocean: (1) nitrate and iron replete, (2) limiting-iron and high-nitrate, representative of natural high-nitrate, low-chlorophyll regions, and (3) iron and nitrogen co-limiting. We show that low variable fluorescence, a key diagnostic of iron limitation, results from synthesis of antennae complexes far in excess of what can be accommodated by the iron-restricted pool of photosynthetic reaction centers. Under iron and nitrogen co-limiting conditions, there are no excess antennae complexes and variable fluorescence is high. These results help to explain the well-established fluorescence characteristics of phytoplankton in high-nutrient, low-chlorophyll ocean regions, while also accounting for the lack of these properties in low-iron, low-nitrogen regions. Importantly, our results complete the link between unique molecular consequences of iron stress in phytoplankton and global detection of iron stress in natural populations from space.

Structural determinants underlying photoprotection in the photoactive orange carotenoid protein of cyanobacteria

The photoprotective processes of photosynthetic organisms involve the dissipation of excess absorbed light energy as heat. Photoprotection in cyanobacteria is mechanistically distinct from that in plants; it involves the orange carotenoid protein (OCP), a water-soluble protein containing a single carotenoid. The OCP is a new member of the family of blue light-photoactive proteins; blue-green light triggers the OCP-mediated photoprotective response. Here we report structural and functional characterization of the wild type and two mutant forms of the OCP, from the model organism Synechocystis PCC6803. The structural analysis provides high resolution detail of the carotenoid-protein interactions that underlie the optical properties of the OCP, unique among carotenoid-proteins in binding a single pigment per polypeptide chain. Collectively, these data implicate several key amino acids in the function of the OCP and reveal that the photoconversion and photoprotective responses of the OCP to blue-green light can be decoupled.

Identification of a protein required for recovery of full antenna capacity in OCP-related photoprotective mechanism in cyanobacteria

High light can be lethal for photosynthetic organisms. Similar to plants, most cyanobacteria protect themselves from high irradiance by increasing thermal dissipation of excess absorbed energy. The photoactive soluble orange carotenoid protein (OCP) is essential for the triggering of this photoprotective mechanism. Light induces structural changes in the carotenoid and the protein, leading to the formation of a red active form. Through targeted gene interruption we have now identified a protein that mediates the recovery of the full antenna capacity when irradiance decreases. In Synechocystis PCC 6803, this protein, which we called the fluorescence recovery protein (FRP), is encoded by the slr1964 gene. Homologues of this gene are present in all of the OCP-containing strains. The FRP is a 14-kDa protein, strongly attached to the membrane, which interacts with the active red form of the OCP. In vitro this interaction greatly accelerates the conversion of the red OCP form to the orange form. We propose that in vivo, FRP plays a key role in removing the red OCP from the phycobilisome and in the conversion of the free red OCP to the orange inactive form. The discovery of FRP and its characterization are essential elements in the understanding of the OCP-related photoprotective mechanism in cyanobacteria.

Light-dependent electrogenic activity of cyanobacteria

BACKGROUND

Cyanobacteria account for 20-30% of Earth's primary photosynthetic productivity and convert solar energy into biomass-stored chemical energy at the rate of approximately 450 TW [1]. These single-cell microorganisms are resilient predecessors of all higher oxygenic phototrophs and can be found in self-sustaining, nitrogen-fixing communities the world over, from Antarctic glaciers to the Sahara desert [2].

METHODOLOGY/PRINCIPAL FINDINGS

Here we show that diverse genera of cyanobacteria including biofilm-forming and pelagic strains have a conserved light-dependent electrogenic activity, i.e. the ability to transfer electrons to their surroundings in response to illumination. Naturally-growing biofilm-forming photosynthetic consortia also displayed light-dependent electrogenic activity, demonstrating that this phenomenon is not limited to individual cultures. Treatment with site-specific inhibitors revealed the electrons originate at the photosynthetic electron transfer chain (P-ETC). Moreover, electrogenic activity was observed upon illumination only with blue or red but not green light confirming that P-ETC is the source of electrons. The yield of electrons harvested by extracellular electron acceptor to photons available for photosynthesis ranged from 0.05% to 0.3%, although the efficiency of electron harvesting likely varies depending on terminal electron acceptor.

CONCLUSIONS/SIGNIFICANCE

The current study illustrates that cyanobacterial electrogenic activity is an important microbiological conduit of solar energy into the biosphere. The mechanism responsible for electrogenic activity in cyanobacteria appears to be fundamentally different from the one exploited in previously discovered electrogenic bacteria, such as Geobacter, where electrons are derived from oxidation of organic compounds and transported via a respiratory electron transfer chain (R-ETC) [3], [4]. The electrogenic pathway of cyanobacteria might be exploited to develop light-sensitive devices or future technologies that convert solar energy into limited amounts of electricity in a self-sustainable, CO(2)-free manner.

Identification of components associated with thermal acclimation of photosystem II in Synechocystis sp. PCC6803

BACKGROUND

Photosystem II (PSII) is the most thermally sensitive component of photosynthesis. Thermal acclimation of this complex activity is likely to be critically important to the ability of photosynthetic organisms to tolerate temperature changes in the environment.

METHODOLOGY/FINDINGS

We have analysed gene expression using whole-genome microarrays and monitored alterations in physiology during acclimation of PSII to elevated growth temperature in Synechocystis sp. PCC 6803. PSII acclimation is complete within 480 minutes of exposure to elevated temperature and is associated with a highly dynamic transcriptional response. 176 genes were identified and classified into seven distinct response profile groups. Response profiles suggest the existence of an early transient phase and a sustained phase to the acclimation response. The early phase was characterised by induction of general stress response genes, including heat shock proteins, which are likely to influence PSII thermal stability. The sustained phase consisted of acclimation-specific alterations that are involved in other cellular processes. Sustained responses included genes involved in phycobillisome structure and modification, photosynthesis, respiration, lipid metabolism and motility. Approximately 60% of genes with sustained altered expression levels have no known function. The potential role of differentially expressed genes in thermotolerance and acclimation is discussed. We have characterised the acclimation physiology of selected gene 'knockouts' to elucidate possible gene function in the response.

CONCLUSIONS/SIGNIFICANCE

All mutants show lower PSII rates under normal growth conditions. Basal PSII thermotolerance was affected by mutations in clpB1, cpcC2, hspA, htpG and slr1674. Final PSII thermotolerance was affected by mutations in cpcC2, hik34, hspA and hypA1, suggesting that these gene products play roles in long-term thermal acclimation of PSII.

Mutants of Anabaena sp. PCC 7120 lacking alr1690 and alpha-furA antisense RNA show a pleiotropic phenotype and altered photosynthetic machinery

Fur proteins are global regulators present in all prokaryotes. In Anabaena sp. PCC 7120 FurA controls iron uptake and modulates an important set of genes related primarily to photosynthesis, nitrogen metabolism and oxidative stress defense. Expression of furA is tuned by the cis-acting antisense alpha-furA RNA that is co-transcribed with the outer-membrane protein Alr1690. Disruption of the alpha-furA-alr1690 message produces the iron-deficient JAH3 mutant that lacks Alr1690 and shows enhanced expression of FurA. JAH3 cells present severe structural disorders related to the number, organization and density of photosynthetic membranes. Quantitative analysis of the fluorescence induction shows that the mutation affects the J-I and I-P phases and causes important alterations in the photosynthetic apparatus, leading to lower photosynthetic performance indexes. These results reveal that expression of the alpha-furA-alr1690 message is required for maintenance of a proper thylakoid arrangement, efficient regulation of iron uptake and optimal yield of the photosynthetic machinery.

The photoactive orange carotenoid protein and photoprotection in cyanobacteria

Photoprotective mechanisms have been evolved by photosynthetic organisms to cope with fluctuating high light conditions. One of these mechanisms downregulates photosynthesis by increasing thermal dissipation of the energy absorbed by the photosystem II antenna. While this process has been well studied in plants, the equivalent process in cyanobacteria was only recently discovered. In this chapter we describe the results leading to its discovery and the more recent advances in the elucidation of this mechanism. The light activation of a soluble carotenoid protein, the orange carotenoid protein (OCP), binding hydroxyechinenone, is the key inducer of this photoprotective mechanism. Light causes structural changes within both the carotenoid and the protein, leading to the conversion of an orange inactive form into a red active form. The activated red form induces an increase of energy dissipation leading to a decrease in the fluorescence of the phycobilisomes, the cyanobacterial antenna, and thus of the energy arriving to the reaction centers. The OCP, which senses light and triggers photoprotection, is a unique example of a photoactive protein containing a carotenoid as the photoresponsive chromophore.

A mechanism of energy dissipation in cyanobacteria

When grown under a variety of stress conditions, cyanobacteria express the isiA gene, which encodes the IsiA pigment-protein complex. Overexpression of the isiA gene under iron-depletion stress conditions leads to the formation of large IsiA aggregates, which display remarkably short fluorescence lifetimes and thus a strong capacity to dissipate energy. In this work we investigate the underlying molecular mechanism responsible for chlorophyll fluorescence quenching. Femtosecond transient absorption spectroscopy allowed us to follow the process of energy dissipation in real time. The light energy harvested by chlorophyll pigments migrated within the system and eventually reaches a quenching site where the energy is transferred to a carotenoid-excited state, which dissipates it by decaying to the ground state. We compare these findings with those obtained for the main light-harvesting complex in green plants (light-harvesting complex II) and artificial light-harvesting antennas, and conclude that all of these systems show the same mechanism of energy dissipation, i.e., one or more carotenoids act as energy dissipators by accepting energy via low-lying singlet-excited S(1) states and dissipating it as heat.

Ecological genomics of marine picocyanobacteria

Marine picocyanobacteria of the genera Prochlorococcus and Synechococcus numerically dominate the picophytoplankton of the world ocean, making a key contribution to global primary production. Prochlorococcus was isolated around 20 years ago and is probably the most abundant photosynthetic organism on Earth. The genus comprises specific ecotypes which are phylogenetically distinct and differ markedly in their photophysiology, allowing growth over a broad range of light and nutrient conditions within the 45 degrees N to 40 degrees S latitudinal belt that they occupy. Synechococcus and Prochlorococcus are closely related, together forming a discrete picophytoplankton clade, but are distinguishable by their possession of dissimilar light-harvesting apparatuses and differences in cell size and elemental composition. Synechococcus strains have a ubiquitous oceanic distribution compared to that of Prochlorococcus strains and are characterized by phylogenetically discrete lineages with a wide range of pigmentation. In this review, we put our current knowledge of marine picocyanobacterial genomics into an environmental context and present previously unpublished genomic information arising from extensive genomic comparisons in order to provide insights into the adaptations of these marine microbes to their environment and how they are reflected at the genomic level.

The dynamics of photosynthesis

Despite recent elucidation of the three-dimensional structure of major photosynthetic complexes, our understanding of light energy conversion in plant chloroplasts and microalgae under physiological conditions requires exploring the dynamics of photosynthesis. The photosynthetic apparatus is a flexible molecular machine that can acclimate to metabolic and light fluctuations in a matter of seconds and minutes. On a longer time scale, changes in environmental cues trigger acclimation responses that elicit intracellular signaling between the nucleo-cytosol and chloroplast resulting in modification of the biogenesis of the photosynthetic machinery. Here we attempt to integrate well-established knowledge on the functional flexibility of light-harvesting and electron transfer processes, which has greatly benefited from genetic approaches, with data derived from the wealth of recent transcriptomic and proteomic studies of acclimation responses in photosynthetic eukaroytes.

Photoprotection in cyanobacteria: regulation of light harvesting

To cope with a rapidly fluctuating light environment, vascular plants and algae have evolved a photoprotective mechanism that serves to downregulate the transfer of excitation energy in the light-harvesting complexes to the photosynthetic reaction centers. This process dissipates excess excitation energy in the chlorophyll pigment bed by a nonradiative pathway. Since this pathway competes with and therefore quenches chlorophyll fluoresence in a nonphotochemical manner, it has been termed Non-photochemical Quenching (NPQ). For many years, cyanobacteria were not considered capable of performing NPQ as a photoprotective mechanism. Instead, the redistribution of the phycobilisome (PBS) light-harvesting antenna between reaction centers by a process called state transitions was considered the major means of regulating the utilization of harvested light energy. Recently, it was demonstrated that cyanobacteria are able to use NPQ as one component of their photoprotective strategies. Cyanobacteria exhibit significant NPQ during nutrient-replete growth, but it becomes a more prominent means of managing absorbed excitation energy when the cells experience iron starvation. Rapid progress in understanding the molecular mechanism of cyanobacterial NPQ has revealed a process that is very distinct from the functionally analogous process in plants and algae. Cyanobacterial NPQ involves the absorption of blue light by a carotenoid binding protein, termed the Orange Carotenoid Protein, and most likely involves quenching in the PBS core. In this study, we summarize work leading to the discovery of NPQ in cyanobacteria and the elucidation of molecular mechanisms associated with this important photoprotective process.

Light-induced energetic decoupling as a mechanism for phycobilisome-related energy dissipation in red algae: a single molecule study

BACKGROUND

Photosynthetic organisms have developed multiple protective mechanisms to prevent photodamage in vivo under high-light conditions. Cyanobacteria and red algae use phycobilisomes (PBsomes) as their major light-harvesting antennae complexes. The orange carotenoid protein in some cyanobacteria has been demonstrated to play roles in the photoprotective mechanism. The PBsome-itself-related energy dissipation mechanism is still unclear.

METHODOLOGY/PRINCIPAL FINDINGS

Here, single-molecule spectroscopy is applied for the first time on the PBsomes of red alga Porphyridium cruentum, to detect the fluorescence emissions of phycoerythrins (PE) and PBsome core complex simultaneously, and the real-time detection could greatly characterize the fluorescence dynamics of individual PBsomes in response to intense light.

CONCLUSIONS/SIGNIFICANCE

Our data revealed that strong green-light can induce the fluorescence decrease of PBsome, as well as the fluorescence increase of PE at the first stage of photobleaching. It strongly indicated an energetic decoupling occurring between PE and its neighbor. The fluorescence of PE was subsequently observed to be decreased, showing that PE was photobleached when energy transfer in the PBsomes was disrupted. In contrast, the energetic decoupling was not observed in either the PBsomes fixed with glutaraldehyde, or the mutant PBsomes lacking B-PE and remaining b-PE. It was concluded that the energetic decoupling of the PBsomes occurs at the specific association between B-PE and b-PE within the PBsome rod. Assuming that the same process occurs also at the much lower physiological light intensities, such a decoupling process is proposed to be a strategy corresponding to PBsomes to prevent photodamage of the photosynthetic reaction centers. Finally, a novel photoprotective role of gamma-subunit-containing PE in red algae was discussed.

Protective dissipation of excess absorbed energy by photosynthetic apparatus of cyanobacteria: role of antenna terminal emitters

Two mechanisms of photoprotective dissipation of the excessively absorbed energy by photosynthetic apparatus of cyanobacteria are described that divert energy from reaction centers. Energy dissipation, monitored as nonphotochemical fluorescence quenching, occurs at different steps of energy transfer within the phycobilisomes or core antenna of photosystem I. Although these mechanisms differ significantly, in both cases, energy dissipates mainly from terminal emitters: allophycocyanin B or core membrane linker protein (L(CM)) in phycobilisomes, or the longest-wavelength chlorophylls in photosystem I antenna. It is supposed that carotenoid-induced energy dissipation in phycobilisomes is triggered by light-induced transformation of the nonquenched state of antenna into quenched state due to conformation changes caused by orange carotinoid-binding protein (OCP)-phycobilisome interaction. Fluorescence of the longest-wavelength chlorophylls of photosystem I antenna is strongly quenched by P700 cation radical or by P700 triplet state, dependent on redox state of the acceptor side cofactors of photosystem I.

A photoactive carotenoid protein acting as light intensity sensor

Intense sunlight is dangerous for photosynthetic organisms. Cyanobacteria, like plants, protect themselves from light-induced stress by dissipating excess absorbed energy as heat. Recently, it was discovered that a soluble orange carotenoid protein, the OCP, is essential for this photoprotective mechanism. Here we show that the OCP is also a member of the family of photoactive proteins; it is a unique example of a photoactive protein containing a carotenoid as the photoresponsive chromophore. Upon illumination with blue-green light, the OCP undergoes a reversible transformation from its dark stable orange form to a red "active" form. The red form is essential for the induction of the photoprotective mechanism. The illumination induces structural changes affecting both the carotenoid and the protein. Thus, the OCP is a photoactive protein that senses light intensity and triggers photoprotection.

In vitro characterization of a carotenoid cleavage dioxygenase from Nostoc sp. PCC 7120 reveals a novel cleavage pattern, cytosolic localization and induction by highlight

Carotenoid oxygenases catalyse the cleavage of C-C double bonds forming apocarotenoids, a diverse group of compounds, including retinoids and the precursors of some phytohormones. Some apocarotenoids, like beta-ionone (C(13)), are ecologically important volatiles released by plants and cyanobacteria. In this work, we elucidated the activity of the Nostoccarotenoid cleavage dioxygenase (NosCCD, previously named NSC1) using synthetic and cyanobacterial substrates. NosCCD converted bicyclic and monocyclic xanthophylls, including myxoxanthophylls, glycosylated carotenoids that are essential for thylakoid and cell wall structure. The products identified revealed two different cleavage patterns. The first is observed with bicyclic xanthophylls and is identical with that of plant orthologues, while the second is novel and occurs upon cleavage of monocyclic substrates at the C9-C10 and C7'-C8' double bonds. These properties enable the enzyme to produce a plenitude of different C(10) and C(13) apocarotenoids. Expression analyses indicated a role of NosCCD in response to highlight stress. Western blot analyses of Nostoc cells revealed NosCCD as a soluble enzyme in the cytosol, which also accomodates NosCCD substrates. Incubation of the corresponding fraction with synthetic substrates revealed the activity of the native enzyme and confirmed its induction by highlight.

Phycobilisome-reaction centre interaction in cyanobacteria

The phycobilisome is a remarkable light-harvesting antenna that combines high efficiency with functional flexibility and the ability to capture light across a broad spectral range. A combination of biochemical, structural and spectroscopic studies has given an excellent picture of the structure and function of isolated phycobilisomes. However, we still know remarkably little about the interaction of the phycobilisome with the thylakoid membrane and the reaction centres. This article will discuss the various current ideas about this question and explain the things we need to know more about. As a working model, I propose that the phycobilisome is attached to the membrane by multiple weak charge-charge interactions with lipid head-groups and/or proteins, and that the core-membrane linker polypeptide ApcE provides a flexible surface allowing interaction with multiple membrane components.

Photoprotection in cyanobacteria: the orange carotenoid protein (OCP)-related non-photochemical-quenching mechanism

Plants and algae have developed multiple protective mechanisms to survive under high light conditions. Thermal dissipation of excitation energy in the membrane-bound chlorophyll-antenna of photosystem II (PSII) decreases the energy arriving at the reaction center and thus reduces the generation of toxic photo-oxidative species. This process results in a decrease of PSII-related fluorescence emission, known as non-photochemical quenching (NPQ). It has always been assumed that cyanobacteria, the progenitor of the chloroplast, lacked an equivalent photoprotective mechanism. Recently, however, evidence has been presented for the existence of at least three distinct mechanisms for dissipating excess absorbed energy in cyanobacteria. One of these mechanisms, characterized by a blue-light-induced fluorescence quenching, is related to the phycobilisomes, the extramembranal antenna of cyanobacterial PSII. In this photoprotective mechanism the soluble carotenoid-binding protein (OCP) encoded by the slr1963 gene in Synechocystis sp. PCC 6803, of previously unknown function, plays an essential role. The amount of energy transferred from the phycobilisomes to the photosystems is reduced and the OCP acts as the photoreceptor and as the mediator of this antenna-related process. These are novel roles for a soluble carotenoid protein.