Cryo-EM structure of Chlamydomonas reinhardtii Photosystem I complexed with cytochrome c

Photosynthetic electron transfer relies on small soluble carriers that shuttle electrons between the cytochrome b₆f complex and Photosystem I (PSI). While copper-containing plastocyanin (Pc) serves this role in plants, the heme protein cytochrome c₆ (Cyt c₆) is also employed in algae and cyanobacteria. Here, we present a cryo-electron microscopy structure of a Cyt c₆:PSI complex from Chlamydomonas reinhardtii. We observe that the heme group of Cyt c₆ is positioned ~11 Å away from P700, stabilized by extensive contacts involving a N-terminal helix-loop-helix motif of PSAF, characteristic of eukaryotic PSI. Notably, the algal Cyt c₆ also retains an arginine residue (R66) which is crucial for cyanobacterial donor:PSI reactions. Our structure reveals the previously uncharacterized interactions involving this residue; it can form a putative electrostatic contact with PsaB-D623 while also contributing to a tri-planar π(cation)-π interactions with adjacent residues. Our findings provide a structural framework for understanding the mechanism and evolution of donor:PSI interactions.