Trimeric organization of photosystem I is required to maintain the balanced photosynthetic electron flow in cyanobacterium Synechocystis sp. PCC 6803

Kłodawska, Kinga and Kovács, László and Vladkova, Radka and Rzaska, Agnieszka and Gombos, Zoltán and Laczkó-Dobos, Hajnalka and Malec, Przemysław (2020) Trimeric organization of photosystem I is required to maintain the balanced photosynthetic electron flow in cyanobacterium Synechocystis sp. PCC 6803. PHOTOSYNTHESIS RESEARCH, 143. pp. 251-262. ISSN 0166-8595 (print); 1573-5079 (online)

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Abstract

In Synechocystis sp. PCC 6803 and some other cyanobacteria photosystem I reaction centres exist predominantly as trimers, with minor contribution of monomeric form, when cultivated at standard optimized conditions. In contrast, in plant chloroplasts photosystem I complex is exclusively monomeric. The functional significance of trimeric organization of cyanobacterial photosystem I remains not fully understood. In this study, we compared the photosynthetic characteristics of PSI in wild type and psaL knockout mutant. The results show that relative to photosystem I trimer in wild-type cells, photosystem I monomer in psaL- mutant has a smaller P700+ pool size under low and moderate light, slower P700 oxidation upon dark-to-light transition, and slower P700+ reduction upon light-to-dark transition. The mutant also shows strongly diminished photosystem I donor side limitations [quantum yield Y(ND)] at low, moderate and high light, but enhanced photosystem I acceptor side limitations [quantum yield Y(NA)], especially at low light (22 µmol photons m-2 s-1). In line with these functional characteristics are the determined differences in the relative expression genes encoding of selected electron transporters. The psaL- mutant showed significant (ca. fivefold) upregulation of the photosystem I donor cytochrome c6, and downregulation of photosystem I acceptors (ferredoxin, flavodoxin) and proteins of alternative electron flows originating in photosystem I acceptor side. Taken together, our results suggest that photosystem I trimerization in wild-type Synechocystis cells plays a role in the protection of photosystem I from photoinhibition via maintaining enhanced donor side electron transport limitations and minimal acceptor side electron transport limitations at various light intensities.

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