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.
KeywordsPhotoprotective Mechanism Orange Carotenoid Protein Iron Starvation Anabaena Variabilis Synechococcus Elongatus
Many thanks to my students Adjélé Wilson, Clémence Boulay, and Claire Punginelli who largely contributed to the results described in this work. Thanks to AW Rutherford for helpful discussions and critical reading of the manuscript. Thanks to Cheryl Kerfeld for helpful discussions and collaboration. The research of DK and her group is supported by the “Commissariat à l’Energie Atomique,” the “Centre National de la Recherche Scientifique, and the “Agence Nationale de la Recherche” (project CAROPROTECT). The work was also partially supported by EU network INTRO2.
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