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...
The cyanobacterium Synechococcus PCC 7942 grown under iron starvation assembles a supercomplex consisting of a trimeric Photosystem I (PSI) complex encircled by a ring of 18 CP43' or IsiA light-harvesting complexes [Nature 412 (2001) 745]. Here we present a spectroscopic characterization by temperature-dependent absorption and fluorescence spectroscopy, site-selective fluorescence spectroscopy at 5 K, and circular dichroism of isolated PSI-IsiA, PSI and IsiA complexes from this cyanobacterium grown under iron starvation. The results suggest that the IsiA ring increases the absorption cross-section of PSI by about 100%. Each IsiA subunit binds about 16-17 chlorophyll a (Chl a) molecules and serves as an efficient antenna for PSI. Each of the monomers of the trimeric PSI complex contains two red chlorophylls, which presumably give rise to one exciton-coupled dimer and at 5 K absorb and fluoresce at 703 and 713 nm, respectively. The spectral properties of these C-703 chlorophylls are not affected by the presence of the IsiA antenna ring. The spectroscopic properties of the purified IsiA complexes are similar to those of the related CP43 complex from plants, except that the characteristic narrow absorption band of CP43 at 682.5 nm is missing in IsiA.
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