De fortes illuminations peuvent être dommageables voire même létales pour les organismes photosynthétiques. Une des stratégies utilisées pour se protéger de tels effets délétères consiste à augmenter la dissipation thermique de l’énergie absorbée en excès au niveau des antennes. Chez les cyanobactéries une protéine photo-active, l’Orange Carotenoid Protein (OCP), contrôle ce processus. Une fois photo-activée l’OCP interagit avec le coeur des phycobilisomes (PBs, les antennes collectrices majoritaires chez les cyanobactéries) et déclenche le mécanisme, entrainant à la fois une baisse de l’énergie parvenant aux photosystèmes et une diminution de la fluorescence des PBs. L’énergie absorbée en excès est dissipée sous forme de chaleur. Pour que les PBs regagnent leur pleine capacité de transfert, une autre protéine nommée Fluorescence Recovery Protein (FRP) est requise. La FRP accélère la désactivation de l’OCP. Dans ce manuscrit, je vais présenter ma contribution à la compréhension du mécanisme de photo-protection lié à l’OCP.J’ai continué la caractérisation de la FRP chez Synechocystis PCC 6803, organisme modèle utilisé dans nos études. J’ai montré que la FRP de Synechocystis est plus courte que ce qui est indiqué dans Cyanobase, commençant en fait à la méthionine 26. Mes résultats ont aussi révélé que la photo-protection n’a lieu que lorsque le ratio OCP/FRP est élevé.Le plus grand aboutissement de ma thèse a été la reconstitution in vitro du mécanisme de photo-protection lié à l’OCP en utilisant de l’OCP, de la FRP et des PBs isolés. J’ai montré que la lumière est requise uniquement pour la photo-activation de l’OCP et que l’attachement de l’OCP au PB ne demande aucune illumination. Ce n’est qu’une fois photo-activée que l’OCP peut interagir avec le PB et entrainer la diminution de fluorescence (quenching). En se basant sur les résultats obtenus in vitro nous avons proposé un modèle moléculaire pour le mécanisme de photo-protection lié à l’OCP. Le système de reconstitution in vitro a été utilisé pour évaluer l’importance d’un pont salin conservé (Arg155-Glu244) entre les deux domaines de l’OCP et a révélé que celui-ci stabilise la forme inactive de l’OCP. La photo-activation entraine rupture du pont salin, l’Arg155 étant ensuite impliquée dans l’interaction entre OCP et PB. Le site d’attachement de l’OCP au coeur du PB a aussi été étudié en utilisant le système in vitro. Nos résultats ont montré que les émetteurs terminaux du PB ne sont pas requis et que le site primaire de quenching est un trimère d’allophycocyanine émettant à 660nm. Enfin nous avons étudié les propriétés des états excités du caroténoïde dans l’OCP photo-activée, montrant qu’un de ces états a un caractère de transfert de charge très prononcé et peut avoir un rôle principal dans la dissipation de l’énergie. Nos résultats suggèrent fortement que non seulement l’OCP induit dissipation de l’énergie absorbée sous forme de chaleur mais aussi que l’OCP agit directement comme dissipateur d’énergie. / Strong light can cause damage and be lethal for photosynthetic organisms. An increase of thermal dissipation of excess absorbed energy at the level of photosynthetic antenna is one of the processes protecting against deleterious effects of light. In cyanobacteria, a soluble photoactive carotenoid binding protein, Orange Carotenoid Protein (OCP) mediates this process. The photoactivated OCP by interacting with the core of phycobilisome (PB; the major photosynthetic antenna of cyanobacteria) triggers the photoprotective mechanism, which decreases the energy arriving at the reaction centres and PSII fluorescence. The excess energy is dissipated as harmless heat. To regain full PB capacity in low light intensities, theFluorescence Recovery Protein (FRP) is required. FRP accelerates the deactivation of OCP.In this work, I present my input in the understanding of the mechanism underlying the OCPrelated photoprotection. I further characterized the FRP of Synechocystis PCC6803, the model organism in our studies. I established that the Synechocystis FRP is shorter than what it was proposed in Cyanobase and it begins at Met26. Our results also revealed the great importance of a high OCP to FRP ratio for existence of photoprotection. The most remarkable achievement of this thesis is the in vitro reconstitution of the OCPrelated mechanism using isolated OCP, PB and FRP. I demonstrated that light is only needed for OCP photoactivation but OCP binding to PB is light independent. Only the photoactivated OCP is able to bind the PB and quench all its fluorescence. Based on our in vitro experiments we proposed a molecular model of OCP-related photoprotection. The in vitro reconstituted system was applied to examine the importance of a conserved salt bridge (Arg155-Glu244) between the two domains of OCP and showed that this salt bridge stabilises the inactive form of OCP. During photoactivation this salt bridge is broken and Arg155 is involved in the interaction between the OCP and the PB. The site of OCP binding in the core of a PB wasalso investigated with the in vitro reconstituted system. Our results demonstrated that the terminal energy emitters of the PB are not needed and that the first site of fluorescence quenching is an APC trimer emitting at 660 nm. Finally, we characterised the properties of excited states of the carotenoid in the photoactivated OCP showing that one of these states presents a very pronounced charge transfer character that likely has a principal role in energy dissipation. Our results strongly suggested that the OCP not only induces thermal energy dissipation but also acts as the energy dissipator.

Strong light can cause damage and be lethal for photosynthetic organisms. An increase of thermal dissipation of excess absorbed energy at the level of photosynthetic antenna is one of the processes protecting against deleterious effects of light. In cyanobacteria, a soluble photoactive carotenoid binding protein, Orange Carotenoid Protein (OCP) mediates this process. The photoactivated OCP by interacting with the core of phycobilisome (PB; the major photosynthetic antenna of cyanobacteria) triggers the photoprotective mechanism, which decreases the energy arriving at the reaction centres and PSII fluorescence. The excess energy is dissipated as harmless heat. To regain full PB capacity in low light intensities, theFluorescence Recovery Protein (FRP) is required. FRP accelerates the deactivation of OCP.In this work, I present my input in the understanding of the mechanism underlying the OCPrelated photoprotection. I further characterized the FRP of Synechocystis PCC6803, the model organism in our studies. I established that the Synechocystis FRP is shorter than what it was proposed in Cyanobase and it begins at Met26. Our results also revealed the great importance of a high OCP to FRP ratio for existence of photoprotection. The most remarkable achievement of this thesis is the in vitro reconstitution of the OCPrelated mechanism using isolated OCP, PB and FRP. I demonstrated that light is only needed for OCP photoactivation but OCP binding to PB is light independent. Only the photoactivated OCP is able to bind the PB and quench all its fluorescence. Based on our in vitro experiments we proposed a molecular model of OCP-related photoprotection. The in vitro reconstituted system was applied to examine the importance of a conserved salt bridge (Arg155-Glu244) between the two domains of OCP and showed that this salt bridge stabilises the inactive form of OCP. During photoactivation this salt bridge is broken and Arg155 is involved in the interaction between the OCP and the PB. The site of OCP binding in the core of a PB wasalso investigated with the in vitro reconstituted system. Our results demonstrated that the terminal energy emitters of the PB are not needed and that the first site of fluorescence quenching is an APC trimer emitting at 660 nm. Finally, we characterised the properties of excited states of the carotenoid in the photoactivated OCP showing that one of these states presents a very pronounced charge transfer character that likely has a principal role in energy dissipation. Our results strongly suggested that the OCP not only induces thermal energy dissipation but also acts as the energy dissipator.