PGRL1 Is the Elusive Ferredoxin-Plastoquinone Reductase in Photosynthetic Cyclic Electron Flow

SummaryDuring plant photosynthesis, photosystems I (PSI) and II (PSII), located in the thylakoid membranes of the chloroplast, use light energy to mobilize electron transport. Different modes of electron flow exist. Linear electron flow is driven by both photosystems and generates ATP and NADPH, whereas cyclic electron flow (CEF) is driven by PSI alone and generates ATP only. Two variants of CEF exist in flowering plants, of which one is sensitive to antimycin A (AA) and involves the two thylakoid proteins, PGR5 and PGRL1. However, neither the mechanism nor the site of reinjection of electrons from ferredoxin into the thylakoid electron transport chain during AA-sensitive CEF is known. Here, we show that PGRL1 accepts electrons from ferredoxin in a PGR5-dependent manner and reduces quinones in an AA-sensitive fashion. PGRL1 activity itself requires several redox-active cysteine residues and a Fe-containing cofactor. We therefore propose that PGRL1 is the elusive ferredoxin-plastoquinone reductase (FQR).

Graphical AbstractFigure optionsDownload full-size imageDownload high-quality image (155 K)Download as PowerPoint slideHighlights► PGRL1 is the elusive FQR which has been sought for almost three decades ► PGR5 is required for the transfer of electrons from ferredoxin to PGRL1 ► The six redox-active cysteine residues of PGRL1 can form intra- and intermolecular disulfide bridges ► Thioredoxins destabilize PGRL1 homodimers