Locating the Missing Chlorophylls f in Far-red Photosystem I

The discovery of chlorophyll f -containing oxygenic photosynthesis, with its long-wavelength photochemistry, represented a new low-energy paradigm. However, subsequent structural studies on chlorophyll f -containing Photosystem I (PSI) found five chlorophylls f but none among the photochemically active pigments and concluded that chlorophyll f plays no photochemical role. Here we report a cryo-EM structure (2.01 Å) of far-red PSI from Chroococcidiopsis thermalis PCC 7203, showing all eight chlorophylls f , including the redox active A _-1B . Simulations of absorption difference spectra induced by charge separation indicate that the A _-1B chlorophyll f absorbs at 755 nm. The chlorophyll f sites, some wavelength assignments, and conserved far-red-specific amino acids, provide functional insights, including redox tuning of chlorophyll f as the primary donor and far-red excitation energy-sharing over the PSI trimer.