Pigment-protein and pigment-pigment interactions are of fundamental importance to the light-harvesting and photoprotective functions essential to oxygenic photosynthesis. The orange carotenoid protein (OCP) functions as both a sensor of light and effector of photoprotective energy dissipation in cyanobacteria. We report the atomic-resolution structure of an active form of the OCP consisting of the N-terminal domain and a single noncovalently bound carotenoid pigment. The crystal structure, combined with additional solution-state structural data, reveals that OCP photoactivation is accompanied by a 12 angstrom translocation of the pigment within the protein and a reconfiguration of carotenoid-protein interactions. Our results identify the origin of the photochromic changes in the OCP triggered by light and reveal the structural determinants required for interaction with the light-harvesting antenna during photoprotection.

The photoprotective processes of photosynthetic organisms involve the dissipation of excess absorbed light energy as heat. Photoprotection in cyanobacteria is mechanistically distinct from that in plants; it involves the orange carotenoid protein (OCP), a water-soluble protein containing a single carotenoid. The OCP is a new member of the family of blue light-photoactive proteins; blue-green light triggers the OCP-mediated photoprotective response. Here we report structural and functional characterization of the wild type and two mutant forms of the OCP, from the model organism Synechocystis PCC6803. The structural analysis provides high resolution detail of the carotenoid-protein interactions that underlie the optical properties of the OCP, unique among carotenoid-proteins in binding a single pigment per polypeptide chain. Collectively, these data implicate several key amino acids in the function of the OCP and reveal that the photoconversion and photoprotective responses of the OCP to blue-green light can be decoupled.The capture of light energy for oxygenic photosynthesis is arguably one of the most important metabolic processes on earth. It is also inherently risky; the absorbance of excess light energy beyond what can be used in photosynthesis can result in photooxidative damage to the organism. Consequently, photosynthetic organisms have evolved protective mechanisms to dissipate excess captured energy. In plants, one of these mechanisms involves the membrane-embedded chlorophyll-protein antenna of Photosystem II, the light-harvesting complex (for reviews, see Refs. 1-4). Under saturating light conditions, the decrease of the lumen pH activates the xanthophyll cycle (5, 6) and the protonation of PsbS, a Photosystem II subunit (7). Conformational changes in the light-harvesting complex, modifying the interaction between chlorophyll molecules and carotenoids and allowing thermal dissipation, are also involved in this mechanism (8 -10). Energy dissipation is accompanied by a diminution of Photosystem II-related fluorescence emission, also known as non-photochemical quenching (or NPQ; more specifically qE), which usually serves as a measure of the dissipation process.Although the photoprotective mechanism of plants is well studied, only recently have mechanisms for photoprotection in the cyanobacteria been discovered (11-17). One of these occurs at the water-soluble light-harvesting antenna (the phycobilisome) and involves a novel photosensory protein, the orange carotenoid protein (13, 18 -20). Most cyanobacterial species contain the OCP 5 (21,22), and in these organisms, it is constitutively expressed and is also up-regulated under extreme conditions, such as high light, iron starvation, and salt stress (18,(23)(24)(25). The OCP-mediated photoprotective mechanism is completely distinct from any known in plants and algae; it involves the absorption of blue-green light, which induces a shift in the absorbance properties of the OCP; visibly, the protein changes from orange to red (photoconversion). The structur...

SUMMARYCarotenoid cleavage, catalyzed by the 9-cis-epoxycarotenoid dioxygenase (NCED) constitutes a key step in the regulation of ABA biosynthesis. In Arabidopsis, this enzyme is encoded by five genes. NCED3 has been shown to play a major role in the regulation of ABA synthesis in response to water deficit, whereas NCED6 and NCED9 have been shown to be essential for the ABA production in the embryo and endosperm that imposes dormancy. Reporter gene analysis was carried out to determine the spatiotemporal pattern of NCED5 and NCED9 gene expression. GUS activity from the NCED5 promoter was detected in both the embryo and endosperm of developing seeds with maximal staining after mid-development. NCED9 expression was found at early stages in the testa outer integument layer 1, and after mid-development in epidermal cells of the embryo, but not in the endosperm. In accordance with its temporal-and tissue-specific expression, the phenotypic analysis of nced5 nced6 nced9 triple mutant showed the involvement of the NCED5 gene, together with NCED6 and NCED9, in the induction of seed dormancy. In contrast to nced6 and nced9, however, nced5 mutation did not affect the gibberellin required for germination. In vegetative tissues, combining nced5 and nced3 mutations reduced vegetative growth, increased water loss upon dehydration, and decreased ABA levels under both normal and stressed conditions, as compared with nced3. NCED5 thus contributes, together with NCED3, to ABA production affecting plant growth and water stress tolerance.