Quantitative assessment of chlorophyll types in cryo-EM maps of photosystem I acclimated to far-red light

• Published in: BBA advances 2021-01, Vol.1 (C), p.100019-100019, Article 100019
• Main Authors: Gisriel, Christopher J., Huang, Hao-Li, Reiss, Krystle M., Flesher, David A., Batista, Victor S., Bryant, Donald A., Brudvig, Gary W., Wang, Jimin
• Format: Article
• Language: English
• Subjects: BASIC BIOLOGICAL SCIENCES; Chlorophyll; Cryo-EM; Far-red light; Photosynthesis; Photosystem I
• ISSN: 2667-1603; 2667-1603
• DOI: 10.1016/j.bbadva.2021.100019

•A method of cryo-EM map analysis to determine chlorophyll substituents is developed.•An improved molecular structure of far-red light photosystem I is presented.•Cryo-EM maps of far-red light photosystem I provide direct evidence for chlorophyll f.•Chlorophyll f-binding sites are predicted in far-red light-acclimated photosystem II. Chlorophyll cofactors are vital for the metabolism of photosynthetic organisms. Cryo-electron microscopy (cryo-EM) has been used to elucidate molecular structures of pigment-protein complexes, but the minor structural differences between multiple types of chlorophylls make them difficult to distinguish in cryo-EM maps. This is exemplified by inconsistencies in the assignments of chlorophyll f molecules in structures of photosystem I acclimated to far-red light (FRL-PSI). A quantitative assessment of chlorophyll substituents in cryo-EM maps was used to identify chlorophyll f-binding sites in structures of FRL-PSI from two cyanobacteria. The two cryo-EM maps provide direct evidence for chlorophyll f-binding at two and three binding sites, respectively, and three more sites in each structure exhibit strong indirect evidence for chlorophyll f-binding. Common themes in chlorophyll f-binding are described that clarify the current understanding of the molecular basis for FRL photoacclimation in photosystems.