Unique organization of photosystem I–light-harvesting supercomplex revealed by cryo-EM from a red alga

Abstract

Significance Photosystem I (PSI) is one of the most efficient nanophotochemical machines in nature. To adapt to various environments, photosynthetic organisms developed different PSI structure during evolution from prokaryotic cyanobacteria to higher plants. Red algae are one of the most primitive eukaryotic algae, and their photosynthetic apparatus represents a transitional state between cyanobacteria and eukaryotes. We determined two forms of the PSI-LHCR structure from a red alga by cryo-EM. Our results revealed unique features and energy transfer pathways in the red algal PSI supercomplex with LHCI (light-harvesting complex I), as well as its remarkable differences with those of cyanobacterial PSI and higher plant PSI-LHCI. These results provide important information for delineating the function and evolution of PSI from prokaryotic to eukaryotic photosynthetic organisms.