Poster: Photosynthesis

Abs # 354: Interchange between PsaK1 and PsaK2 in Photosystem I is Involved in State Transition under High-Light Condition in Synechocystis sp. PCC 6803

Presenter:	Fujimori, Tamaki , kk16546@mail.ecc.u-tokyo.ac.jp
Authors	Fujimori, Tamaki  (A)   Sonoike, Kintake  (A)   Hihara, Yukako  (B)
Affiliations:	(A): Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo (B): Department of Molecular Biology, Faculty of Science, University of Saitama

In order to avoid photodamage, cyanobacteria regulates the distribution of light energy from phycobilisome either to photosystem II (PSII) or to photosystem I (PSI) under high-light condition. This rapid process is referred to as state transition. We examined the subunit composition of the purified PSI reaction center complexes and found that PsaK1, one of the reaction center subunit of PSI, was replaced with PsaK2 in acclimation of the cells to high light. We determined the photosynthetic parameters of wild type and two mutant strains, DpsaK1 and DpsaK2, under low- or high-light conditions using pulse amplitude modulation (PAM) fluorometer. In the cells of wild type and DpsaK1, non-photochemical quenching (qN), which reflects state transition in cyanobacteria, increased under high-light condition. On the other hand, qN was not affected by high-light shift in the cells of DpsaK2. This result indicates that PsaK2 is involved in the energy transfer from phycobilisome to PSI. The role of PsaK2 in state transition was also confirmed by fluorescence emission spectra determined at 77 K. We conclude that PsaK2, which is newly incorporated into PSI under high-light condition, regulates the energy transfer from phycobilisome to PSI. This PsaK2-dependent state transition may maintain a balanced photosynthetic electron transport between PSII and PSI under high-light condition.