High light can be lethal for photosynthetic organisms. Similar to plants, most cyanobacteria protect themselves from high irradiance by increasing thermal dissipation of excess absorbed energy. The photoactive soluble orange carotenoid protein (OCP) is essential for the triggering of this photoprotective mechanism. Light induces structural changes in the carotenoid and the protein, leading to the formation of a red active form. Through targeted gene interruption we have now identified a protein that mediates the recovery of the full antenna capacity when irradiance decreases. In Synechocystis PCC 6803, this protein, which we called the fluorescence recovery protein (FRP), is encoded by the slr1964 gene. Homologues of this gene are present in all of the OCP-containing strains. The FRP is a 14-kDa protein, strongly attached to the membrane, which interacts with the active red form of the OCP. In vitro this interaction greatly accelerates the conversion of the red OCP form to the orange form. We propose that in vivo, FRP plays a key role in removing the red OCP from the phycobilisome and in the conversion of the free red OCP to the orange inactive form. The discovery of FRP and its characterization are essential elements in the understanding of the OCPrelated photoprotective mechanism in cyanobacteria.carotenoid | nonphotochemical quenching | phycobilisome | photoreceptor | Synechocystis B ecause too much light can be lethal for photosynthetic organisms, photoprotection against excess absorbed light energy is an essential and universal attribute of oxygenic photosynthetic organisms. Survival, productivity, and habitat preference are largely determined by development of photoprotective mechanisms. One of these mechanisms, which is induced by high irradiance, dissipates the excess harmful absorbed energy into heat at the levels of the antennae.In all photosynthetic organisms, the function of the light harvesting antenna is similar: to collect and concentrate light energy in the photochemical reaction centers in which light energy is converted into chemical energy. In plants and green algae, light is principally harvested by membrane-embedded complexes noncovalently binding chlorophyll and carotenoid molecules. These complexes show a large structural and functional flexibility. They are reversibly switched from a very efficient energy collecting state into a photoprotected state. This state allows the conversion of excess energy into heat, decreasing the energy arriving at the reaction centers under high light conditions (1-4). Cyanobacteria are oxygenic photosynthetic prokaryotes that play a key role in global carbon cycling. To harvest light. most cyanobacteria use a large, membrane-extrinsic complex called the phycobilisome (PB), which is composed of several types of chromophorylated phycobiliproteins and of linker peptides (for reviews, see refs. 5-7).It has been recently shown that, in cyanobacteria, like in plants, there exists a photoprotective process that decreases the energy transfer between the antenna and ...

In response to iron deficiency, cyanobacteria synthesize the iron stress–induced chlorophyll binding protein IsiA. This protein protects cyanobacterial cells against iron stress. It has been proposed that the protective role of IsiA is related to a blue light–induced nonphotochemical fluorescence quenching (NPQ) mechanism. In iron-replete cyanobacterial cell cultures, strong blue light is known to induce a mechanism that dissipates excess absorbed energy in the phycobilisome, the extramembranal antenna of cyanobacteria. In this photoprotective mechanism, the soluble Orange Carotenoid Protein (OCP) plays an essential role. Here, we demonstrate that in iron-starved cells, blue light is unable to quench fluorescence in the absence of the phycobilisomes or the OCP. By contrast, the absence of IsiA does not affect the induction of fluorescence quenching or its recovery. We conclude that in cyanobacteria grown under iron starvation conditions, the blue light–induced nonphotochemical quenching involves the phycobilisome OCP–related energy dissipation mechanism and not IsiA. IsiA, however, does seem to protect the cells from the stress generated by iron starvation, initially by increasing the size of the photosystem I antenna. Subsequently, the IsiA converts the excess energy absorbed by the phycobilisomes into heat through a mechanism different from the dynamic and reversible light-induced NPQ processes.

Cyanobacteria have developed a photoprotective mechanism that decreases the energy arriving at the photosynthetic reaction centers under high-light conditions. The photoactive orange carotenoid protein (OCP) is essential in this mechanism as a light sensor and energy quencher. When OCP is photoactivated by strong blue-green light, it is able to dissipate excess energy as heat by interacting with phycobilisomes. As a consequence, charge separation and recombination leading to the formation of singlet oxygen diminishes. Here, we demonstrate that OCP has another essential role. We observed that OCP also protects Synechocystis cells from strong orange-red light, a condition in which OCP is not photoactivated. We first showed that this photoprotection is related to a decrease of singlet oxygen concentration due to OCP action. Then, we demonstrated that, in vitro, OCP is a very good singlet oxygen quencher. By contrast, another carotenoid protein having a high similarity with the N-terminal domain of OCP is not more efficient as a singlet oxygen quencher than a protein without carotenoid. Although OCP is a soluble protein, it is able to quench the singlet oxygen generated in the thylakoid membranes. Thus, OCP has dual and complementary photoprotective functions as an energy quencher and a singlet oxygen quencher.