Cryo-EM structure of Chlamydomonas Photosystem I complexed with the alternative electron donor cytochrome c ₆

Photosynthetic electron transfer relies on small soluble carriers that shuttle electrons between the cytochrome b ₆ f complex and Photosystem I (PSI). While copper-containing plastocyanin (Pc) serves this role in plants, algae and cyanobacteria employ the heme protein cytochrome c ₆ (Cyt c ₆) as well. Here we present a cryo–electron microscopy structure of a Cyt c ₆:PSI complex from Chlamydomonas reinhardtii . The Cyt c ₆ heme is positioned ∼11 Å from P700, stabilized by extensive contacts involving the N-terminal domain of PSAF. Importantly, R66 in Cyt c ₆, a key residue in ancestral donors, forms a putative electrostatic contact with PsaB-D623 and participates in a tri-planar π-stacking interaction with nearby aromatic residues. Our findings provide a structural framework for ancestral PSI interactions and illuminate the evolutionary diversification of electron transfer pathways.