Fluorescence quenching of the phycobilisome terminal emitter LCM from the cyanobacterium Synechocystis sp. PCC 6803 detected in vivo and in vitro

Gradual Response of Cyanobacterial Thylakoids to Acute High-Light Stress-Importance of Carotenoid Accumulation

Light plays an essential role in photosynthesis; however, its excess can cause damage to cellular components. Photosynthetic organisms thus developed a set of photoprotective mechanisms (e.g., non-photochemical quenching, photoinhibition) that can be studied by a classic biochemical and biophysical methods in cell suspension. Here, we combined these bulk methods with single-cell identification of microdomains in thylakoid membrane during high-light (HL) stress. We used Synechocystis sp. PCC 6803 cells with YFP tagged photosystem I. The single-cell data pointed to a three-phase response of cells to acute HL stress. We defined: (1) fast response phase (0–30 min), (2) intermediate phase (30–120 min), and (3) slow acclimation phase (120–360 min). During the first phase, cyanobacterial cells activated photoprotective mechanisms such as photoinhibition and non-photochemical quenching. Later on (during the second phase), we temporarily observed functional decoupling of phycobilisomes and sustained monomerization of photosystem II dimer. Simultaneously, cells also initiated accumulation of carotenoids, especially ɣ–carotene, the main precursor of all carotenoids. In the last phase, in addition to ɣ-carotene, we also observed accumulation of myxoxanthophyll and more even spatial distribution of photosystems and phycobilisomes between microdomains. We suggest that the overall carotenoid increase during HL stress could be involved either in the direct photoprotection (e.g., in ROS scavenging) and/or could play an additional role in maintaining optimal distribution of photosystems in thylakoid membrane to attain efficient photoprotection.

Toxic and protective mechanisms of cyanobacterium Synechocystis sp. in response to titanium dioxide nanoparticles

An increasing production and use of titanium dioxide nanoparticles (TiO₂ NPs) pose a huge threat to phytoplankton since they are largely released into aquatic environments, which represent a sink for TiO₂ NPs. However, toxicity and protective mechanisms of cyanobacteria in response to TiO₂ NPs remain elusive. Here we investigated toxic effects of two sizes of TiO₂ NPs (50 and 10 nm) and one bulk TiO₂ (200 nm) on a cyanobacterium, Synechocystis sp. and their possible protective mechanisms. We found that 10 nm TiO₂ NPs caused significant growth and photosynthesis inhibition in Synechocystis sp. cells, largely reflected in decreased growth rate (38%), operational PSII quantum yields (40%), phycocyanin (51%) and allophycocyanin (63%), and increased reactive oxygen species content (245%), superoxide dismutase activity (46%). Also, transcriptomic analysis of Synechocystis sp. exposure to 10 nm TiO₂ NPs showed the up-regulation of D1 and D2 protein genes (psbA and psbD), ferredoxin gene (petF) and F-type ATPase genes (e.g., atpB), and the down-regulation of psbM and psb28-2 in PS II. We further proposed a conceptual model to explore possible toxic and protective mechanisms for Synechocystis sp. under TiO₂ nanoparticle exposure. This study provides mechanistic insights into our understanding of Synechocystis sp. responses to TiO₂ NPs. This is essential for more accurate environmental risk assessment approaches of nanoparticles in aquatic ecosystems by governmental environmental agencies worldwide.

Binding of red form of Orange Carotenoid Protein (OCP) to phycobilisome is not sufficient for quenching

The Orange Carotenoid Protein (OCP) is responsible for photoprotection in many cyanobacteria. Absorption of blue light drives the conversion of the orange, inactive form (OCP^O) to the red, active form (OCP^R). Concomitantly, the N-terminal domain (NTD) and the C-terminal domain (CTD) of OCP separate, which ultimately leads to the formation of a quenched OCP^R-PBS complex. The details of the photoactivation of OCP have been intensely researched. Binding site(s) of OCP^R on the PBS core have also been proposed. However, the post-binding events of the OCP^R-PBS complex remain unclear. Here, we demonstrate that PBS-bound OCP^R is not sufficient as a PBS excitation energy quencher. Using site-directed mutagenesis, we generated a suite of single point mutations at OCP Leucine 51 (L51) of Synechocystis 6803. Steady-state and time-resolved fluorescence analyses demonstrated that all mutant proteins are unable to quench the PBS fluorescence, owing to either failed OCP binding to PBS, or, if bound, an OCP-PBS quenching state failed to form. The SDS-PAGE and Western blot analysis support that the L51A (Alanine) mutant binds to the PBS and therefore belongs to the second category. We hypothesize that upon binding to PBS, OCP^R likely reorganizes and adopts a new conformational state (OCP^3rd) different than either OCP^O or OCP^R to allow energy quenching, depending on the cross-talk between OCP^R and its PBS core-binding counterpart.

Rates and pathways of energy migration from the phycobilisome to the photosystem II and to the orange carotenoid protein in cyanobacteria

The phycobilisome (PBS) is the cyanobacterial antenna complex which transfers absorbed light energy to the photosystem II (PSII), while the excess energy is nonphotochemically quenched by interaction of the PBS with the orange carotenoid protein (OCP). Here, the molecular model of the PBS-PSII-OCP supercomplex was utilized to assess the resonance energy transfer from PBS to PSII and, using the excitonic theory, the transfer from PBS to OCP. Our estimates show that the effective energy migration from PBS to PSII is realized due to the existence of several transfer pathways from phycobilin chromophores of the PBS to the neighboring antennal chlorophyll molecules of the PSII. At the same time, the single binding site of photoactivated OCP and the PBS is sufficient to realize the quenching.

Site, trigger, quenching mechanism and recovery of non-photochemical quenching in cyanobacteria: recent updates

Cyanobacteria exhibit a novel form of non-photochemical quenching (NPQ) at the level of the phycobilisome. NPQ is a process that protects photosystem II (PSII) from possible highlight-induced photo-damage. Although significant advancement has been made in understanding the NPQ, there are still some missing details. This critical review focuses on how the orange carotenoid protein (OCP) and its partner fluorescence recovery protein (FRP) control the extent of quenching. What is and what is not known about the NPQ is discussed under four subtitles; where does exactly the site of quenching lie? (site), how is the quenching being triggered? (trigger), molecular mechanism of quenching (quenching) and recovery from quenching. Finally, a recent working model of NPQ, consistent with recent findings, is been described.

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.

The photocycle of orange carotenoid protein conceals distinct intermediates and asynchronous changes in the carotenoid and protein components

The 35-kDa Orange Carotenoid Protein (OCP) is responsible for photoprotection in cyanobacteria. It acts as a light intensity sensor and efficient quencher of phycobilisome excitation. Photoactivation triggers large-scale conformational rearrangements to convert OCP from the orange OCP^O state to the red active signaling state, OCP^R, as demonstrated by various structural methods. Such rearrangements imply a complete, yet reversible separation of structural domains and translocation of the carotenoid. Recently, dynamic crystallography of OCP^O suggested the existence of photocycle intermediates with small-scale rearrangements that may trigger further transitions. In this study, we took advantage of single 7 ns laser pulses to study carotenoid absorption transients in OCP on the time-scale from 100 ns to 10 s, which allowed us to detect a red intermediate state preceding the red signaling state, OCP^R. In addition, time-resolved fluorescence spectroscopy and the assignment of carotenoid-induced quenching of different tryptophan residues derived thereof revealed a novel orange intermediate state, which appears during the relaxation of photoactivated OCP^R to OCP^O. Our results show asynchronous changes between the carotenoid- and protein-associated kinetic components in a refined mechanistic model of the OCP photocycle, but also introduce new kinetic signatures for future studies of OCP photoactivity and photoprotection.

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.

Biophysical modeling of in vitro and in vivo processes underlying regulated photoprotective mechanism in cyanobacteria

Non-photochemical quenching (NPQ) is a mechanism responsible for high light tolerance in photosynthetic organisms. In cyanobacteria, NPQ is realized by the interplay between light-harvesting complexes, phycobilisomes (PBs), a light sensor and effector of NPQ, the photoactive orange carotenoid protein (OCP), and the fluorescence recovery protein (FRP). Here, we introduced a biophysical model, which takes into account the whole spectrum of interactions between PBs, OCP, and FRP and describes the experimental PBs fluorescence kinetics, unraveling interaction rate constants between the components involved and their relative concentrations in the cell. We took benefit from the possibility to reconstruct the photoprotection mechanism and its parts in vitro, where most of the parameters could be varied, to develop the model and then applied it to describe the NPQ kinetics in the Synechocystis sp. PCC 6803 mutant lacking photosystems. Our analyses revealed  that while an excess of the OCP over PBs is required to obtain substantial PBs fluorescence quenching in vitro, in vivo the OCP/PBs ratio is less than unity, due to higher local concentration of PBs, which was estimated as ~10^-5 M, compared to in vitro experiments. The analysis of PBs fluorescence recovery on the basis of the generalized model of enzymatic catalysis resulted in determination of the FRP concentration in vivo close to 10% of the OCP concentration. Finally, the possible role of the FRP oligomeric state alteration in the kinetics of PBs fluorescence was shown. This paper provides the most comprehensive model of the OCP-induced PBs fluorescence quenching to date and the results are important for better understanding of the regulatory molecular mechanisms underlying NPQ in cyanobacteria.

Structure, function and evolution of the cyanobacterial orange carotenoid protein and its homologs

Contents 937 I. 937 II. 938 III. 939 IV. 943 V. 947 VI. 948 948 References 949 SUMMARY: The orange carotenoid protein (OCP) is a water-soluble, photoactive protein involved in thermal dissipation of excess energy absorbed by the light-harvesting phycobilisomes (PBS) in cyanobacteria. The OCP is structurally and functionally modular, consisting of a sensor domain, an effector domain and a keto-carotenoid. On photoactivation, the OCP converts from a stable orange form, OCP^O , to a red form, OCP^R . Activation is accompanied by a translocation of the carotenoid deeper into the effector domain. The increasing availability of cyanobacterial genomes has enabled the identification of new OCP families (OCP1, OCP2, OCPX). The fluorescence recovery protein (FRP) detaches OCP1 from the PBS core, accelerating its back-conversion to OCP^O ; by contrast, other OCP families are not regulated by FRP. N-terminal domain homologs, the helical carotenoid proteins (HCPs), have been found among diverse cyanobacteria, occurring as multiple paralogous groups, with two representatives exhibiting strong singlet oxygen (¹ O₂ ) quenching (HCP2, HCP3) and another capable of dissipating PBS excitation (HCP4). Crystal structures are presently available for OCP1 and HCP1, and models of other HCP subtypes can be readily produced as a result of strong sequence conservation, providing new insights into the determinants of carotenoid binding and ¹ O₂ quenching.

Fluorescent Labeling Preserving OCP Photoactivity Reveals Its Reorganization during the Photocycle

Orange carotenoid protein (OCP), responsible for the photoprotection of the cyanobacterial photosynthetic apparatus under excessive light conditions, undergoes significant rearrangements upon photoconversion and transits from the stable orange to the signaling red state. This is thought to involve a 12-Å translocation of the carotenoid cofactor and separation of the N- and C-terminal protein domains. Despite clear recent progress, the detailed mechanism of the OCP photoconversion and associated photoprotection remains elusive. Here, we labeled the OCP of Synechocystis with tetramethylrhodamine-maleimide (TMR) and obtained a photoactive OCP-TMR complex, the fluorescence of which was highly sensitive to the protein state, showing unprecedented contrast between the orange and red states and reflecting changes in protein conformation and the distances from TMR to the carotenoid throughout the photocycle. The OCP-TMR complex was sensitive to the light intensity, temperature, and viscosity of the solvent. Based on the observed Förster resonance energy transfer, we determined that upon photoconversion, the distance between TMR (donor) bound to a cysteine in the C-terminal domain and the carotenoid (acceptor) increased by 18 Å, with simultaneous translocation of the carotenoid into the N-terminal domain. Time-resolved fluorescence anisotropy revealed a significant decrease of the OCP rotation rate in the red state, indicating that the light-triggered conversion of the protein is accompanied by an increase of its hydrodynamic radius. Thus, our results support the idea of significant structural rearrangements of OCP, providing, to our knowledge, new insights into the structural rearrangements of OCP throughout the photocycle and a completely novel approach to the study of its photocycle and non-photochemical quenching. We suggest that this approach can be generally applied to other photoactive proteins.

Cyanobacterial photoprotection by the orange carotenoid protein

In photosynthetic organisms, the production of dangerous oxygen species is stimulated under high irradiance. To cope with this stress, these organisms have evolved photoprotective mechanisms. One type of mechanism functions to decrease the energy arriving at the photochemical centres by increasing thermal dissipation at the level of antennae. In cyanobacteria, the trigger for this mechanism is the photoactivation of a soluble carotenoid protein, the orange carotenoid protein (OCP), which is a structurally and functionally modular protein. The inactive orange form (OCP^o) is compact and globular, with the carotenoid spanning the effector and the regulatory domains. In the active red form (OCP^r), the two domains are completely separated and the carotenoid has translocated entirely into the effector domain. The activated OCP^r interacts with the phycobilisome (PBS), the cyanobacterial antenna, and induces excitation-energy quenching. A second protein, the fluorescence recovery protein (FRP), dislodges the active OCP^r from the PBSs and accelerates its conversion to the inactive OCP.

A comparative study of three signaling forms of the orange carotenoid protein

Orange carotenoid protein (OCP) is a water-soluble photoactive protein responsible for a photoprotective mechanism of nonphotochemical quenching in cyanobacteria. Under blue-green illumination, OCP converts from the stable orange into the signaling red quenching form; however, the latter form could also be obtained by chemical activation with high concentrations of sodium thiocyanate (NaSCN) or point mutations. In this work, we show that a single replacement of tryptophan-288, normally involved in protein-chromophore interactions, by alanine, results in formation of a new protein form, hereinafter referred to as purple carotenoid protein (PCP). Comparison of resonance Raman spectra of the native photoactivated red form, chemically activated OCP, and PCP reveals that carotenoid conformation is sensitive to the structure of the C-domain, implicating that the chromophore retains some interactions with this part of the protein in the active red form. Combination of differential scanning fluorimetry and picosecond time-resolved fluorescence anisotropy measurements allowed us to compare the stability of different OCP forms and to estimate relative differences in protein rotation rates. These results were corroborated by hydrodynamic analysis of proteins by dynamic light scattering and analytical size-exclusion chromatography, indicating that the light-induced conversion of the protein is accompanied by a significant increase in its size. On the whole, our data support the idea that the red form of OCP is a molten globule-like protein in which, however, interactions between the carotenoid and the C-terminal domain are preserved.

Bright blue-shifted fluorescent proteins with Cys in the GAF domain engineered from bacterial phytochromes: fluorescence mechanisms and excited-state dynamics

Near-infrared fluorescent proteins (NIR FPs) engineered from bacterial phytochromes (BphPs) are of great interest for in vivo imaging. They utilize biliverdin (BV) as a chromophore, which is a heme degradation product, and therefore they are straightforward to use in mammalian tissues. Here, we report on fluorescence properties of NIR FPs with key alterations in their BV binding sites. BphP1-FP, iRFP670 and iRFP682 have Cys residues in both PAS and GAF domains, rather than in the PAS domain alone as in wild-type BphPs. We found that NIR FP variants with Cys in the GAF or with Cys in both PAS and GAF show blue-shifted emission with long fluorescence lifetimes. In contrast, mutants with Cys in the PAS only or no Cys residues at all exhibit red-shifted emission with shorter lifetimes. Combining these results with previous biochemical and BphP1-FP structural data, we conclude that BV adducts bound to Cys in the GAF are the origin of bright blue-shifted fluorescence. We propose that the long fluorescence lifetime follows from (i) a sterically more constrained thioether linkage, leaving less mobility for ring A than in canonical BphPs, and (ii) that π-electron conjugation does not extend on ring A, making excited-state deactivation less sensitive to ring A mobility.

The purple Trp288Ala mutant of Synechocystis OCP persistently quenches phycobilisome fluorescence and tightly interacts with FRP

In Cyanobacteria, the Orange Carotenoid Protein (OCP) and Fluorescence Recovery Protein (FRP) are central to the photoprotective mechanism consisting in regulated quenching of phycobilisome (PBs) fluorescence. Due to a transient and flexible nature of the light-activated red quenching form, OCP^R, which is obtained from the stable dark-adapted orange form, OCP^O, by photoconversion, the detailed mechanism of photoprotection remains unclear. Here we demonstrate that our recently described W288A mutant of the Synechocystis OCP (hereinafter called OCP^W288A) is a fully functional analogue of the OCP^R form which is capable of constitutive PBs fluorescence quenching in vitro with no need of photoactivation. This PBs quenching effect is abolished in the presence of FRP, which interacts with OCP^W288A with micromolar affinity and an apparent stoichiometry of 1:1, unexpectedly, implying dissociation of the FRP dimers. This establishes OCP^W288A as a robust model system providing novel insights into the interplay between OCP and FRP to regulate photoprotection in cyanobacteria.

The Signaling State of Orange Carotenoid Protein

Orange carotenoid protein (OCP) is the photoactive protein that is responsible for high light tolerance in cyanobacteria. We studied the kinetics of the OCP photocycle by monitoring changes in its absorption spectrum, intrinsic fluorescence, and fluorescence of the Nile red dye bound to OCP. It was demonstrated that all of these three methods provide the same kinetic parameters of the photocycle, namely, the kinetics of OCP relaxation in darkness was biexponential with a ratio of two components equal to 2:1 independently of temperature. Whereas the changes of the absorption spectrum of OCP characterize the geometry and environment of its chromophore, the intrinsic fluorescence of OCP reveals changes in its tertiary structure, and the fluorescence properties of Nile red indicate the exposure of hydrophobic surface areas of OCP to the solvent following the photocycle. The results of molecular-dynamics studies indicated the presence of two metastable conformations of 3'-hydroxyechinenone, which is consistent with characteristic changes in the Raman spectra. We conclude that rotation of the β-ionylidene ring in the C-terminal domain of OCP could be one of the first conformational rearrangements that occur during photoactivation. The obtained results suggest that the photoactivated form of OCP represents a molten globule-like state that is characterized by increased mobility of tertiary structure elements and solvent accessibility.

Energy transfer pathways among phycobilin chromophores and fluorescence emission spectra of the phycobilisome core at 293 and 77 K

Energy transfer pathways between phycobiliproteins chromophores in the phycobilisome (PBS) core of the cyanobacterium Synechocystis sp. PCC 6803 were investigated. The computer 3D model of the PBS core with determination of chromophore to chromophore distance was created. Our kinetic equations based on this model allowed us to describe the relative intensities of the fluorescence emission of the short(peaked at 665 nm) and long-wavelength (peaked at 680 nm) chromophores in the PBS core at low and room temperatures. The difference of emissions of the PBS core at 77 and 293 K are due to the back energy transfer, which is observed at room temperature and is negligible at 77 K.

The terminal phycobilisome emitter, LCM: A light-harvesting pigment with a phytochrome chromophore

Photosynthesis relies on energy transfer from light-harvesting complexes to reaction centers. Phycobilisomes, the light-harvesting antennas in cyanobacteria and red algae, attach to the membrane via the multidomain core-membrane linker, L(CM). The chromophore domain of L(CM) forms a bottleneck for funneling the harvested energy either productively to reaction centers or, in case of light overload, to quenchers like orange carotenoid protein (OCP) that prevent photodamage. The crystal structure of the solubly modified chromophore domain from Nostoc sp. PCC7120 was resolved at 2.2 Å. Although its protein fold is similar to the protein folds of phycobiliproteins, the phycocyanobilin (PCB) chromophore adopts ZZZssa geometry, which is unknown among phycobiliproteins but characteristic for sensory photoreceptors (phytochromes and cyanobacteriochromes). However, chromophore photoisomerization is inhibited in L(CM) by tight packing. The ZZZssa geometry of the chromophore and π-π stacking with a neighboring Trp account for the functionally relevant extreme spectral red shift of L(CM). Exciton coupling is excluded by the large distance between two PCBs in a homodimer and by preservation of the spectral features in monomers. The structure also indicates a distinct flexibility that could be involved in quenching. The conclusions from the crystal structure are supported by femtosecond transient absorption spectra in solution.

Modulating energy arriving at photochemical reaction centers: orange carotenoid protein-related photoprotection and state transitions

Photosynthetic organisms tightly regulate the energy arriving to the reaction centers in order to avoid photodamage or imbalance between the photosystems. To this purpose, cyanobacteria have developed mechanisms involving relatively rapid (seconds to minutes) changes in the photosynthetic apparatus. In this review, two of these processes will be described: orange carotenoid protein(OCP)-related photoprotection and state transitions which optimize energy distribution between the two photosystems. The photoactive OCP is a light intensity sensor and an energy dissipater. Photoactivation depends on light intensity and only the red-active OCP form, by interacting with phycobilisome cores, increases thermal energy dissipation at the level of the antenna. A second protein, the "fluorescence recovery protein", is needed to recover full antenna capacity under low light conditions. This protein accelerates OCP conversion to the inactive orange form and plays a role in dislodging the red OCP protein from the phycobilisome. The mechanism of state transitions is still controversial. Changes in the redox state of the plastoquinone pool induce movement of phycobilisomes and/or photosystems leading to redistribution of energy absorbed by phycobilisomes between PSII and PSI and/or to changes in excitation energy spillover between photosystems. The different steps going from the induction of redox changes to movement of phycobilisomes or photosystems remain to be elucidated.

Features of temporal behavior of fluorescence recovery in Synechocystis sp. PCC6803

Under high photon flux density of solar radiation, the photosynthetic apparatus can be damaged. To prevent this photodestruction, cyanobacteria developed special mechanisms of non-photochemical quenching (NPQ) of excitation energy in phycobilisomes. In Synechocystis, NPQ is triggered by the orange carotenoid protein (OCP), which is sensitive to blue-green illumination allowing it to bind to the phycobilisome reducing the flow of energy to the photosystems. Consequent decoupling of OCP and recovery of phycobilisome fluorescence in vivo is controlled by the so called fluorescence recovery protein (FRP). In this work, the role of the phycobilisome core components, apcD and apcF, in non-photochemical quenching and subsequent fluorescence recovery in the phycobilisomes of the cyanobacterium Synechocystis sp. PCC6803 has been investigated. Using a single photon counting technique, we have registered fluorescence decay spectra with picosecond time resolution during fluorescence recovery. In order to estimate the activation energy for the photocycle, spectroscopic studies in dependency on the temperature from 5 to 45 °C have been performed. It was found that fluorescence quenching and recovery were strongly temperature dependent for all strains exhibiting characteristic non-linear time courses. The rise of the fluorescence intensity during fluorescence recovery after NPQ can be completely described by the increase of the phycobilisome core fluorescence lifetime. It was shown that fluorescence recovery of apcD- and apcF-deficient mutants is characterized by a significantly lower activation energy barrier compared to wild type. This phenomenon indicates that apcD and apcF gene products may be required for proper interaction of FRP and OCP coupled to the phycobilisome core. In addition, we found that the rate of fluorescence recovery decreases with an increase of the non-photochemical quenching amplitude, probably due to depletion of substrate for the enzymatic reaction catalyzed by FRP.

A Practical Solution for 77 K Fluorescence Measurements Based on LED Excitation and CCD Array Detector

The fluorescence emission spectrum of photosynthetic microorganisms at liquid nitrogen temperature (77 K) provides important insights into the organization of the photosynthetic machinery of bacteria and eukaryotes, which cannot be observed at room temperature. Conventionally, to obtain such spectra, a large and costly table-top fluorometer is required. Recently portable, reliable, and largely maintenance-free instruments have become available that can be utilized to accomplish a wide variety of spectroscopy-based measurements in photosynthesis research. In this report, we show how to build such an instrument in order to record 77K fluorescence spectra. This instrument consists of a low power monochromatic light-emitting diode (LED), and a portable CCD array based spectrometer. The optical components are coupled together using a fiber optic cable, and a custom made housing that also supports a dewar flask. We demonstrate that this instrument facilitates the reliable determination of chlorophyll fluorescence emission spectra for the cyanobacterium Synechocystis sp. PCC 6803, and the green alga Chlamydomonas reinhardtii.

Electronic coupling of the phycobilisome with the orange carotenoid protein and fluorescence quenching

Using computational modeling and known 3D structure of proteins, we arrived at a rational spatial model of the orange carotenoid protein (OCP) and phycobilisome (PBS) interaction in the non-photochemical fluorescence quenching. The site of interaction is formed by the central cavity of the OCP monomer in the capacity of a keyhole to the characteristic external tip of the phycobilin-containing domain (PB) and folded loop of the core-membrane linker LCM within the PBS core. The same central protein cavity was shown to be also the site of the OCP and fluorescence recovery protein (FRP) interaction. The revealed geometry of the OCP to the PBLCM attachment is believed to be the most advantageous one as the LCM, being the major terminal PBS fluorescence emitter, gathers, before quenching by OCP, the energy from most other phycobilin chromophores of the PBS. The distance between centers of mass of the OCP carotenoid 3'-hydroxyechinenone (hECN) and the adjacent phycobilin chromophore of the PBLCM was determined to be 24.7 Å. Under the dipole-dipole approximation, from the point of view of the determined mutual orientation and the values of the transition dipole moments and spectral characteristics of interacting chromophores, the time of the direct energy transfer from the phycobilin of PBLCM to the S1 excited state of hECN was semiempirically calculated to be 36 ps, which corresponds to the known experimental data and implies the OCP is a very efficient energy quencher. The complete scheme of OCP and PBS interaction that includes participation of the FRP is proposed.

Molecular mechanism of photoactivation and structural location of the cyanobacterial orange carotenoid protein

The orange carotenoid protein (OCP) plays a photoprotective role in cyanobacterial photosynthesis similar to that of nonphotochemical quenching in higher plants. Under high-light conditions, the OCP binds to the phycobilisome (PBS) and reduces the extent of transfer of energy to the photosystems. The protective cycle starts from a light-induced activation of the OCP. Detailed information about the molecular mechanism of this process as well as the subsequent recruitment of the active OCP to the phycobilisome are not known. We report here our investigation on the OCP photoactivation from the cyanobacterium Synechocystis sp. PCC 6803 by using a combination of native electrospray mass spectrometry (MS) and protein cross-linking. We demonstrate that native MS can capture the OCP with its intact pigment and further reveal that the OCP undergoes a dimer-to-monomer transition upon light illumination. The reversion of the activated form of the OCP to the inactive, dark form was also observed by using native MS. Furthermore, in vitro reconstitution of the OCP and PBS allowed us to perform protein chemical cross-linking experiments. Liquid chromatography-MS/MS analysis identified cross-linking species between the OCP and the PBS core components. Our result indicates that the N-terminal domain of the OCP is closely involved in the association with a site formed by two allophycocyanin trimers in the basal cylinders of the phycobilisome core. This report improves our understanding of the activation mechanism of the OCP and the structural binding site of the OCP during the cyanobacterial nonphotochemical quenching process.