Structure of a photosystem I-ferredoxin complex from a marine cyanobacterium provides insights into far-red light photoacclimation
Far-red light photoacclimation (FaRLiP) exhibited by some cyanobacteria allows these organisms to use light of the far-red region of the solar spectrum (700 to 800 nm) for photosynthesis. Part of this process includes the replacement of six photosystem I (PSI) subunits with isoforms that confer the binding of chlorophyll (Chl) f molecules that are synthesized by chlorophyll f synthase and that absorb far-red light. However, the exact sites at which Chl f molecules are bound are still challenging to determine. To aid in the identification of Chl fbinding sites, we solved the cryo-EM structure of PSI from far-red light-acclimated cells (FRLPSI) of the cyanobacterium Synechococcus sp. PCC 7335. We identified six sites that bind Chl f with high specificity, and three additional sites that are likely to bind Chl f at lower specificity. All of these binding sites are in the core antenna regions of PSI, and Chl f was not observed among the electron transport cofactors. This structural analysis also reveals both conserved and non-conserved Chl f-binding sites, the latter of which exemplify the diversity in FRL-PSI among species. We found that the FRL-PSI structure also contains a bound soluble ferredoxin, PetF1, at low occupancy, which suggests that ferredoxin binds less transiently than expected according to the canonical view of ferredoxin-binding to facilitate electron transfer. We suggest that this may result from structural changes in FRL-PSI that occur specifically during FaRLiP.