Abstract
Two modes of electron flow exist in the chloroplast-linear electron flow (LEF) from water to NADPH via PSII and PSI in series and cyclic electron flow (CEF) around PSI. Although CEF is essential for satisfying the varying demand for ATP, the exact molecule(s) and operational site were remained elusive. In the unicellular green alga Chlamydomonas reinhardtii, the electron flow shifts from LEF to CEF upon preferential excitation of PSII, which is brought about by an energy balancing mechanism between PSII and PSI (state transitions). We isolated a protein supercomplex composed of PSI with LHCI, LHCII, Cyt bf, FNR, and the integral membrane protein PGRL1 from the cells under PSII-favoring conditions. Spectroscopic analyses indicated that upon illumination, reducing equivalents from downstream of PSI were transferred to Cyt bf, while oxidized PSI was re-reduced by reducing equivalents from Cyt bf, indicating that this supercomplex is engaged in CEF. 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.
