9s6p
Local refinement of a PSI monomer of A. marina NIES-2412
Structural highlights
FunctionA0AAT9GV97_9CYAN PsaA and PsaB bind P700, the primary electron donor of photosystem I (PSI), as well as the electron acceptors A0, A1 and FX. PSI is a plastocyanin/cytochrome c6-ferredoxin oxidoreductase, converting photonic excitation into a charge separation, which transfers an electron from the donor P700 chlorophyll pair to the spectroscopically characterized acceptors A0, A1, FX, FA and FB in turn. Oxidized P700 is reduced on the lumenal side of the thylakoid membrane by plastocyanin or cytochrome c6.[ARBA:ARBA00002612][HAMAP-Rule:MF_00482] PsaA and psaB bind P700, the primary electron donor of photosystem I (PSI), as well as the electron acceptors A0, A1 and FX.[RuleBase:RU003775] Publication Abstract from PubMedOxygenic photosynthesis is usually limited to visible light, but the marine cyanobacterium Acaryochloris marina pushes this boundary by harvesting far-red photons with chlorophyll d. The best-studied strain, MBIC11017, unexpectedly lacks low-energy chlorophylls ("red forms") in photosystem I, limiting absorption beyond 740 nanometers. Here, we show that another strain, A. marina NIES-2412, has evolved a strategy to absorb far-red photons up to 760 nanometers. Combining time-resolved fluorescence spectroscopy with cryo-electron microscopy at 2.64-angstrom resolution, we identify two distinct classes of chlorophyll d red forms in its photosystem I. One class originates from classical charge-transfer-exciton mixing, while the other arises purely from excitonic interactions. Mapping all 96 chlorophylls d reveals the precise pigments responsible for these far-red states. We also uncover a previously unreported subunit, PsaX2, which stabilizes the photosystem I complex and shapes pigment geometry and energetics to enable the formation of red forms. Last, we show that the protein modifications responsible for binding and tuning these red forms are widespread across the Acaryochloris genus but not within the model MBIC11017 strain. Far-red photons lie close to the energetic limit of oxygenic photosynthesis; their efficient use therefore requires fine-tuning of the photosynthetic machinery. To our knowledge, our findings provide the structural and mechanistic basis of one of the most red-shifted photosystem I complexes identified to date, highlighting a distinct adaptive strategy in far-red light environments and offering design principles for extending photosynthesis in crops into the infrared. Far-red chlorophyll d clusters extend photosystem I absorption toward the red limit.,Oliver TJ, Elias E, Consoli G, Leong HF, Cordon-Preciado V, Fantuzzi A, Cardona T, Rutherford AW, Croce R Sci Adv. 2026 Jun 12;12(24):eaed7355. doi: 10.1126/sciadv.aed7355. Epub 2026 Jun , 10. PMID:42268959[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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