Photoprotection in Cyanobacteria: The Orange Carotenoid Protein and Energy Dissipation

Abstract

Exposure of photosynthetic organisms to high irradiance stimulates the production of singlet oxygen by Photosystem II and causes photo-oxidative stress. In order to avoid photodamage, plants, algae and cyanobacteria decrease the energy arriving from the antenna to the photosynthetic reaction centers by increasing heat dissipation. While the photoprotective mechanism existing in plants and algae has been well-studied, the equivalent process in cyanobacteria, utilizing a completely different molecular mechanism, was only recently discovered. In this review we describe the experiments and results that led to the discovery of this process to the more recent advances in the elucidation of the cyanobacterial photoprotective mechanism. In cyanobacteria, the increase of energy dissipation as heat involving the phycobilisome, the extramembranal cyanobacteria antenna, is induced by light activation of a soluble protein containing a single non-covalently bound carotenoid. The absorbance of blue-green light by this protein, the Orange-Carotenoid-Protein (OCP), induces structural changes in the carotenoid and the protein, converting its dark, stable orange form into a relatively unstable, active red form. The activated red form induces the decrease of the phycobilisome fluorescence emission and the energy arriving to the reaction centers. The OCP is the first photoactive protein containing a carotenoid as the chromophore. Moreover, it appears that its photocycle is completely different from those of other photoactive proteins.