Locating the missing chlorophylls f in far-red photosystem I

The discovery of chlorophyll f-containing photosystems, with their long-wavelength photochemistry, represented a distinct, low-energy paradigm for oxygenic photosynthesis. Structural studies on chlorophyll f-containing photosystem I could identify some chlorophyll f sites, but none among the photochemically active pigments, and thus concluded that chlorophyll f plays no photochemical role. Here, we report two cryo-electron microscopy structures of far-red photosystem I from Chroococcidiopsis thermalis PCC 7203, allowing the assignment of eight chlorophyll f molecules, including the redox active A_-1B. Simulations of absorption difference spectra induced by charge separation indicated that the experimental spectra can be reproduced only by considering the presence of a chlorophyll f at the A_-1B site. The chlorophyll f locations, wavelength assignments, and conserved far-red-specific residues provide functional insights for efficient use of long-wavelength photons.