Cryo-EM structure of a tetrameric photosystem I from Chroococcidiopsis TS-821, a thermophilic, unicellular, non-heterocyst-forming cyanobacterium

Dmitry A. Semchonok, Jyotirmoy Mondal, Connor J. Cooper, Katrina Schlum, Meng Li, Muhamed Amin, Carlos O. S. Sorzano, Erney Ramírez-Aportela, Panagiotis L. Kastritis, Egbert J. Boekema, Albert Guskov, Barry D. Bruce*

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

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Photosystem I (PSI) is one of two photosystems involved in oxygenic photosynthesis. PSI of cyanobacteria exists in monomeric, trimeric, and tetrameric forms, in contrast to the strictly monomeric form of PSI in plants and algae. The tetrameric organization raises questions about its structural, physiological, and evolutionary significance. Here we report the ∼3.72 Å resolution cryo-electron microscopy structure of tetrameric PSI from the thermophilic, unicellular cyanobacterium Chroococcidiopsis sp. TS-821. The structure resolves 44 subunits and 448 cofactor molecules. We conclude that the tetramer is arranged via two different interfaces resulting from a dimer-of-dimers organization. The localization of chlorophyll molecules permits an excitation energy pathway within and between adjacent monomers. Bioinformatics analysis reveals conserved regions in the PsaL subunit that correlate with the oligomeric state. Tetrameric PSI may function as a key evolutionary step between the trimeric and monomeric forms of PSI organization in photosynthetic organisms.
Original languageEnglish
Article number100248
Number of pages18
JournalPlant Communications
Issue number1
Early online date13-Oct-2021
Publication statusPublished - 10-Jan-2022


  • cryo-EM
  • photosystem I
  • non-heterocyst-forming cyanobacteria
  • Chroococcidiopsis
  • evolution of chloroplast
  • high light adaptation

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