The orange carotenoid protein in photoprotection of photosystem II in cyanobacteria

AbstractPhotoprotective 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 (qEcya), 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

Comparison of state 1-state 2 transitions in the green alga Chlamydomonas reinhardtii and in the red alga Rhodella violacea: effect of kinase and phosphatase inhibitors

AbstractIn the green alga Chlamydomonas reinhardtii, state transitions occur upon phosphorylation of the light-harvesting complex. The protein kinase inhibitor staurosporine, and the phosphoprotein phosphatase inhibitor NaF, suppress state 2 and state 1 transitions, respectively. By contrast, in the red alga Rhodella violacea none of these inhibitors has any effect, suggesting that, in red algae, the mechanisms of redistribution of excitation energy are independent of protein phosphorylation

Interactions entre l Orange Carotenoid Protein et les phycobilisomes dans un mécanisme de photoprotection chez les cyanobactérie

Un excès d énergie lumineuse peut être délétère pour les organismesphotosynthétiques ; en effet, il en résulte la formation d espèces réactives de l oxygène ausein des centres réactionnels. Les cyanobactéries ont adopté divers mécanismes dephotoprotection afin de contrer ce phénomène. L un d eux repose sur l activité de l OrangeCarotenoid Protein (OCP), protéine soluble qui attache un kéto-caroténoïde (hydroxyechinenone).Subissant de fortes intensités de lumière bleu-verte, l OCP se convertit d uneforme inactive/orange vers sa forme active/rouge. L OCP ainsi photoactivée possède la facultéd interagir avec les phycobilisomes - principales antennes collectrices de lumière - induisantla dissipation de l énergie collectée par ces gigantesques complexes sous forme de chaleur. Lapression d excitation au niveau des centres réactionnels ainsi que la fluorescence du systèmedécroissent alors.L OCP photoactivée se fixe au coeur des phycobilisomes qui sont majoritairementconstitués de protéines chromophorylées de la famille des allophycocyanines (APC). J aiconstruit différentes souches mutantes de Synechocystis PCC 6803 en modifiant ousupprimant les sous-unités mineures d APC (ApcD, ApcF et ApcE). Ces sous-unités jouent lerôle essentiel d émetteurs terminaux des phycobilisomes, véhiculant l énergie qu ellesreçoivent à la Chlorophylle a. J ai aussi démontré que le mécanisme photoprotectif associé àl OCP chez ces mutants restait inchangé, aussi bien in vivo que in vitro. Ces résultatssuggèrent qu aucun émetteur terminal n est nécessairement requis pour l attachement del OCP aux phycobilisomes et sous-entendent que l OCP interagit probablement avec unesous-unité majeure d APC.Divers phycobilisomes, contenant 2, 3 ou 5 cylindres d APC dans leur coeur, ont étéisolés à partir de cyanobactéries variées. Les OCPs de Synechocytis et d Arthrospira ont étépurifiées à partir de souches mutantes de Synechocystis. J ai alors mené une étude in vitro desinteractions entre ces OCPs et les phycobilisomes. Le nombre de cylindres d APC présents ausein des phycobilisomes n affecte en rien la diminution de fluorescence. De plus, j ai constatéque l OCP de Synechocystis est spécifique pour ses propres phycobilisomes alors que l OCPd Arthrospira interagit avec tous les phycobilisomes employés ici. Des hypothèses, fondéessur les structures disponibles, ont été formulées pour élucider ces différences.Les domaines N- et C-terminaux de l OCP d Arthrospira ont été dissociés parprotéolyse. Le domaine N-terminal isolé conserve le caroténoïde attaché, ayant uneconformation similaire à celle observée lorsque l OCP est photoactivée. Ce domaine Nterminalest aussi capable d induire une importante diminution de la fluorescence desphycobilisomes. A l inverse, le domaine C-terminal isolé est incolore et n a aucun effet sur lafluorescence des phycobilisomes. Ces résultats suggèrent que seul le domaine N-terminal del OCP est impliqué dans l interaction avec les phycobilisomes. Le domaine C-terminal quantà lui module son activité.Too much light can be lethal for photosynthetic organisms. Under such conditionsharmful reactive oxygen species are generated at the reaction center level. Cyanobacteria havedeveloped photoprotective mechanisms to avoid this. One of them relies on the solubleOrange Carotenoid Protein (OCP) that binds a ketocarotenoid (hydroxyechinenone, hECN).Under strong blue-green illumination, OCP gets photoconverted from an orange inactive form(OCPo) to a red active one (OCPr). OCPr interacts with phycobilisomes, the majorcyanobacterial light harvesting antennae, and triggers heat dissipation of the excess lightenergy collected by these gigantic pigment-protein complexes. Consequently, excitationpressure on reaction centers and fluorescence emission decrease.OCPr binds to phycobilisome cores, containing mainly chromophorylated proteins ofthe allophycocyanin (APC) family. I constructed Synechocystis PCC 6803 mutants affected insome minor APC forms (ApcD, ApcF and ApcE). These special APCs play the role ofterminal emitters, i.e. funnel light energy to Chlorophyll a. Strong-blue green illuminationtriggered normal OCP-related fluorescence quenching in all mutant cells. The fluorescencedecrease induced by Synechocystis OCP in vitro was similar when using phycobilisomesisolated from wild-type or mutant cells. These results demonstrated that the terminal emittersare not needed for interaction with the OCP and they strongly suggested that OCPr interactswith one of the major APC forms of the phycobilisome core.Phycobilisomes containing 2, 3 or 5 APC cylinders per core were isolated fromdifferent cyanobacterial strains. Synechocystis and Arthrospira OCPs were purified from overexpressingSynechocystis mutant strains. I then performed in vitro OCP/phycobilisomeinteraction studies. The number of APC cylinders per core had no clear influence on theamount of fluorescence quenching. Both OCPs behaved very differently, one appearing muchmore species-specific than the other. Structure-based hypotheses were emitted to explain suchdissimilarity.Arthrospira OCP N-terminal and C-terminal domains were separated throughproteolysis. The isolated N-terminal domain retained a bound carotenoid, which displayedsimilar conformation than in OCPr. This isolated N-terminal domain triggered importantphycobilisome fluorescence quenching even under dark conditions. In contrast, the isolated Cterminaldomain attached no pigment and had no visible effect on phycobilisome emission. Itwas then proposed that only the N-terminal domain of OCP is implied in interactions withphycobilisomes. The C-terminal domain modulates its activity.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

Photosynthetic cytochrome c550

Cytochrome c550 (cyt c550) is a membrane component of the PSII complex in cyanobacteria and some eukaryotic algae, such as red and brown algae. Cyt c550 presents a bis-histidine heme coordination which is very unusual for monoheme c-type cytochromes. In PSII, the cyt c550 with the other extrinsic proteins stabilizes the binding of Cl− and Ca2 + ions to the oxygen evolving complex and protects the Mn4Ca cluster from attack by bulk reductants. The role (if there is one) of the heme of the cyt c550 is unknown. The low midpoint redox potential (Em) of the purified soluble form (from − 250 to − 314 mV) is incompatible with a redox function in PSII. However, more positive values for the Em have been obtained for the cyt c550 bound to the PSII. A very recent work has shown an Em value of + 200 mV. These data open the possibility of a redox function for this protein in electron transfer in PSII. Despite the long distance (22 Å) between cyt c550 and the nearest redox cofactor (Mn4Ca cluster), an electron transfer reaction between these components is possible. Some kind of protective cycle involving a soluble redox component in the lumen has also been proposed. The aim of this article is to review previous studies done on cyt c550 and to consider its function in the light of the new results obtained in recent years. The emphasis is on the physical properties of the heme and its redox properties. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.Ministerio de Ciencia e Innovación BFU2007-68107-C02-01Junta de Andalucía PADI CVI-26

Cytochrome c550 in the cyanobacterium Thermosynechococcus elongatus: Study of redox mutants

Cytochrome c550 is one of the extrinsic Photosystem II subunits in cyanobacteria and red algae. To study the possible role of the heme of the cytochrome c550 we constructed two mutants of Thermosynechococcus elongatus in which the residue His-92, the sixth ligand of the heme, was replaced by a Met or a Cys in order to modify the redox properties of the heme. The H92M and H92C mutations changed the midpoint redox potential of the heme in the isolated cytochrome by +125 mV and –30 mV, respectively, compared with the wild type. The binding-induced increase of the redox potential observed in the wild type and the H92C mutant was absent in the H92M mutant. Both modified cytochromes were more easily detachable from the Photosystem II compared with the wild type. The Photosystem II activity in cells was not modified by the mutations suggesting that the redox potential of the cytochrome c550 is not important for Photosystem II activity under normal growth conditions. A mutant lacking the cytochrome c550 was also constructed. It showed a lowered affinity for Cl– and Ca2+ as reported earlier for the cytochrome c550-less Synechocystis 6803 mutant, but it showed a shorter lived Formula state, rather than a stabilized S2 state and rapid deactivation of the enzyme in the dark, which were characteristic of the Synechocystis mutant. It is suggested that the latter effects may be caused by loss (or weaker binding) of the other extrinsic proteins rather than a direct effect of the absence of the cytochrome c55

Changes in photosynthetic electron transfer and state transitions in an herbicide-resistant D1 mutant from soybean cell cultures

The definitive version is available at: http://www.sciencedirect.com/science//journal/00052728Anomalies in photosynthetic activity of the soybean cell line STR7, carrying a single mutation (S268P) in the chloroplastic gene psbA that codes for the D1 protein of the photosystem II, have been examined using different spectroscopic techniques. Thermoluminescence emission experiments have shown important differences between STR7 mutant and wild type cells. The afterglow band induced by both white light flashes and far-red continuous illumination was downshifted by about 4 °C and the Q band was upshifted by 5 °C. High temperature thermoluminescence measurements suggested a higher level of lipid peroxidation in mutant thylakoid membranes. In addition, the reduction rate of P700+ was significantly accelerated in STR7 suggesting that the mutation led to an activation of the photosystem I cyclic electron flow. Modulated fluorescence measurements performed at room temperature as well as fluorescence emission spectra at 77 K revealed that the STR7 mutant is defective in state transitions. Here, we discuss the hypothesis that activation of the cyclic electron flow in STR7 cells may be a mechanism to compensate the reduced activity of photosystem II caused by the mutation. We also propose that the impaired state transitions in the STR7 cells may be due to alterations in thylakoid membrane properties induced by a low content of unsaturated lipids.This work was supported by grants from the Ministry of Education and Culture of Spain (BFU-BMC2004-04914-C02-01, BMC2002-00031 and BFU-BMC2005-07422-C02-01) and Andalusia Government (PAI CVI-261).Peer reviewe