Locating the Missing Chlorophyllsfin Far-red Photosystem I

Abstract

AbstractThe discovery of chlorophyllf-containing oxygenic photosynthesis, with its long-wavelength photochemistry, represented a new low-energy paradigm. However, subsequent structural studies on chlorophyllf-containing Photosystem I (PSI) found five chlorophyllsfbut none among the photochemically active pigments and concluded that chlorophyllfplays no photochemical role. Here we report a cryo-EM structure (2.01 Å) of far-red PSI fromChroococcidiopsis thermalisPCC 7203, showing all eight chlorophyllsf, including the redox active A-1B. Simulations of absorption difference spectra induced by charge separation indicate that the A-1Bchlorophyllfabsorbs at 755 nm. The chlorophyllfsites, some wavelength assignments, and conserved far-red-specific amino acids, provide functional insights, including redox tuning of chlorophyllfas the primary donor and far-red excitation energy-sharing over the PSI trimer.