title: Carotenoid-triggered energy dissipation in phycobilisomes of Synechocystis sp. PCC 6803 diverts excitation away from reaction centers of both photosystems.
abstract: Cyanobacteria are capable of using dissipation of phycobilisome-absorbed energy into heat as part of their photoprotective strategy. Non-photochemical quenching in cyanobacteria cells is triggered by absorption of blue-green light by the carotenoid-binding protein, and involves quenching of phycobilisome fluorescence. In this study, we find direct evidence that the quenching is accompanied by a considerable reduction of energy flow to the photosystems. We present light saturation curves of photosystems' activity in quenched and non-quenched states in the cyanobacterium Synechocystis sp. PCC 6803. In the quenched state, the quantum efficiency of light absorbed by phycobilisomes drops by about 30-40% for both photoreactions-P700 photooxidation in the photosystem II-less strain and photosystem II fluorescence induction in the photosystem I-less strain of Synechocystis. A similar decrease of the excitation pressure on both photosystems leads us to believe that the core-membrane linker allophycocyanin APC-L(CM) is at or beyond the point of non-photochemical quenching. We analyze 77 K fluorescence spectra and suggest that the quenching center is formed at the level of the short-wavelength allophycocyanin trimers. It seems that both chlorophyll and APC-L(CM) may dissipate excess energy via uphill energy transfer at physiological temperatures, but neither of the two is at the heart of the carotenoid-binding protein-dependent non-photochemical quenching mechanism.
authors.affiliation: A.N. Bakh Institute of Biochemistry, Russian Academy of Sciences, 119071 Moscow, Russia. rakhimberd@inbi.ras.ru
authors.forename: Marina G
authors.initials: MG
authors.lastname: Rakhimberdieva
authors.forename: Irina V
authors.initials: IV
authors.lastname: Elanskaya
authors.forename: Wim F J
authors.initials: WF
authors.lastname: Vermaas
authors.forename: Navassard V
authors.initials: NV
authors.lastname: Karapetyan
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D002338:Carotenoids / Q000494:pharmacology*; D004735:Energy Transfer / Q000187:drug effects*; D005453:Fluorescence; D045331:Photosystem I Protein Complex / Q000235:genetics / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000235:genetics / Q000378:metabolism*; D045524:Phycobilisomes / Q000378:metabolism*; D046939:Synechocystis / Q000235:genetics / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D045524:Phycobilisomes; C109473:orange carotenoid protein, Synechocystis; D002338:Carotenoids