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.
Organizational Affiliation:
Groningen Biomolecular Sciences & Biotechnology Institute, University of Groningen, Groningen, the Netherlands.
Biochemistry & Cellular and Molecular Biology Department, University of Tennessee, Knoxville, TN, USA.
Program in Genome Science and Technology, University of Tennessee, Knoxville, TN, USA.
Bredesen Center for Interdisciplinary Research & Education, University of Tennessee, Knoxville, TN, USA.
Department of Sciences, University College Groningen, Groningen, the Netherlands.
Biocomputing Unit, National Center for Biotechnology (CSIC), Darwin 3, Campus Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain.
Universidad CEU San Pablo, Campus Urb. Montepríncipe, Boadilla del Monte, 28668 Madrid, Spain.
Institute of Biochemistry and Biotechnology, Martin Luther University Halle-Wittenberg, Halle/Saale, Germany.
Microbiology Department, University of Tennessee, Knoxville, TN, USA.
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