Structure of a photosystem I-ferredoxin complex from a marine cyanobacterium provides insights into far-red light photoacclimation

• Published in: The Journal of biological chemistry 2022-01, Vol.298 (1), p.101408-101408, Article 101408
• Main Authors: Gisriel, Christopher J., Flesher, David A., Shen, Gaozhong, Wang, Jimin, Ho, Ming-Yang, Brudvig, Gary W., Bryant, Donald A.
• Format: Article
• Language: English
• Subjects: BASIC BIOLOGICAL SCIENCES; Chlorophyll - metabolism; chlorophyll f; cyanobacteria; far-red light photoacclimation; ferredoxin; Ferredoxins - metabolism; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Light; Photosynthesis; photosystem I; Photosystem I Protein Complex - metabolism; PsaF; PsaJ; Synechococcus - metabolism
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/j.jbc.2021.101408

Far-red light photoacclimation exhibited by some cyanobacteria allows these organisms to use the far-red region of the solar spectrum (700–800 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 absorb far-red light (FRL). However, the exact sites at which Chl f molecules are bound are still challenging to determine. To aid in the identification of Chl f-binding sites, we solved the cryo-EM structure of PSI from far-red light-acclimated cells 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 transfer cofactors. This structural analysis also reveals both conserved and nonconserved 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 FRL photoacclimation.