Energy dissipation: new role for a carotenoid protein in cyanobacteria.

Photosynthetic organisms have developed multiple mechanisms to protect the photosynthetic apparatus from high light stress. Plants and algae exhibit thermal dissipation of excitation energy in the membranebound light-harvesting complex of photosystem II (LHCII). In most species of cyanobacteria, which lack LHCII, light is captured by the phycobilisome, a membraneextrinsic complex attached to the outer surface of thylakoid membranes. Recent work has suggested that phycobilisomes display an alternate mechanism for dissipating excess absorbed energy. Wilson et al. (pages 992–1007) show that this photoprotective mechanism, characterized by blue light–induced fluorescence quenching, is indeed phycobilisome related and that a soluble carotenoid binding protein (OCP), encoded by the slr1963 gene in Synechocystis PCC 6803, plays an essential role in this process. The data suggest that OCP, which interacts with the thylakoids, acts as both the photoreceptor and as the mediator of the reduction of the amount of energy transferred from the phycobilisomes to the photosystems.