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Identification of the special pair of photosystem II in a chlorophyll d-dominated cyanobacterium
- Tomo, Tatsuya, Okubo, Tatsunori, Akimoto, Seiji, Yokono, Makio, Miyashita, Hideaki, Tsuchiya, Tohru, Noguchi, Takumi, Mimuro, Mamoru
- Proceedings of the National Academy of Sciences of the United States of America 2007 v.104 no.17 pp. 7283-7288
- polypeptides, chlorophyll, D2 protein, electron transfer, D1 protein, photosystem II, Fourier transform infrared spectroscopy, cytochrome b, Cyanobacteria
- The composition of photosystem II (PSII) in the chlorophyll (Chl) d-dominated cyanobacterium Acaryochloris marina MBIC 11017 was investigated to enhance the general understanding of the energetics of the PSII reaction center. We first purified photochemically active complexes consisting of a 47-kDa Chl protein (CP47), CP43' (PcbC), D1, D2, cytochrome b₅₅₉, PsbI, and a small polypeptide. The pigment composition per two pheophytin (Phe) a molecules was 55 ± 7 Chl d, 3.0 ± 0.4 Chl a, 17 ± 3 α-carotene, and 1.4 ± 0.2 plastoquinone-9. The special pair was detected by a reversible absorption change at 713 nm (P713) together with a cation radical band at 842 nm. FTIR difference spectra of the specific bands of a 3-formyl group allowed assignment of the special pair. The combined results indicate that the special pair comprises a Chl d homodimer. The primary electron acceptor was shown by photoaccumulation to be Phe a, and its potential was shifted to a higher value than that in the Chl a/Phe a system. The overall energetics of PSII in the Chl d system are adjusted to changes in the redox potentials, with P713 as the special pair using a lower light energy at 713 nm. Taking into account the reported downward shift in the potential of the special pair of photosystem I (P740) in A. marina, our findings lend support to the idea that changes in photosynthetic pigments combine with a modification of the redox potentials of electron transfer components to give rise to an energetic adjustment of the total reaction system.