Isolation of the elusive supercomplex that drives cyclic electron flow in photosynthesis

Masakazu Iwai, Kenji Takizawa, Ryutaro Tokutsu, Akira Okamuro, Yuichiro Takahashi, Jun Minagawa

Research output: Contribution to journalArticlepeer-review

311 Citations (Scopus)


Photosynthetic light reactions establish electron flow in the chloroplasts thylakoid membranes, leading to the production of the ATP and NADPH that participate in carbon fixation. Two modes of electron flow existlinear electron flow (LEF) from water to NADP + via photosystem (PS) II and PSI in series and cyclic electron flow (CEF) around PSI (ref. 2). Although CEF is essential for satisfying the varying demand for ATP, the exact molecule(s) and operational site are as yet unclear. In the green alga Chlamydomonas reinhardtii, the electron flow shifts from LEF to CEF on preferential excitation of PSII (ref. 3), which is brought about by an energy balancing mechanism between PSII and PSI (state transitions). Here, we isolated a protein supercomplex composed of PSI with its own light-harvesting complex (LHCI), the PSII light-harvesting complex (LHCII), the cytochrome b6f complex (Cyt bf), ferredoxin (Fd)-NADPH oxidoreductase (FNR), and the integral membrane protein PGRL1 (ref. 5) from C. reinhardtii cells under PSII-favouring conditions. Spectroscopic analyses indicated that on illumination, reducing equivalents from downstream of PSI were transferred to Cyt bf, whereas oxidised PSI was re-reduced by reducing equivalents from Cyt bf, indicating that this supercomplex is engaged in CEF (Supplementary Fig. 1). Thus, formation and dissociation of the PSI-LHCI-LHCII-FNR-Cyt bf-PGRL1 supercomplex not only controlled the energy balance of the two photosystems, but also switched the mode of photosynthetic electron flow.

Original languageEnglish
Pages (from-to)1210-1213
Number of pages4
Issue number7292
Publication statusPublished - Apr 22 2010

ASJC Scopus subject areas

  • General


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