Photoprotection in cyanobacteria: the orange carotenoid protein (OCP)-related non-photochemical-quenching mechanism.

Plants and algae have developed multiple protective mechanisms to survive under high light conditions. Thermal dissipation of excitation energy in the membrane-bound chlorophyll-antenna of photosystem II (PSII) decreases the energy arriving at the reaction center and thus reduces the generation of toxic photo-oxidative species. This process results in a decrease of PSII-related fluorescence emission, known as non-photochemical quenching (NPQ). It has always been assumed that cyanobacteria, the progenitor of the chloroplast, lacked an equivalent photoprotective mechanism. Recently, however, evidence has been presented for the existence of at least three distinct mechanisms for dissipating excess absorbed energy in cyanobacteria. One of these mechanisms, characterized by a blue-light-induced fluorescence quenching, is related to the phycobilisomes, the extramembranal antenna of cyanobacterial PSII. In this photoprotective mechanism the soluble carotenoid-binding protein (OCP) encoded by the slr1963 gene in Synechocystis sp. PCC 6803, of previously unknown function, plays an essential role. The amount of energy transferred from the phycobilisomes to the photosystems is reduced and the OCP acts as the photoreceptor and as the mediator of this antenna-related process. These are novel roles for a soluble carotenoid protein.