Structure and evolution of Photosystem I in the early-branching cyanobacterium .

Thylakoid-free cyanobacteria are thought to preserve ancestral traits of early-evolving organisms capable of oxygenic photosynthesis. However, and until recently, photosynthesis studies in thylakoid-free cyanobacteria were only possible in the model strain . Here, we report the isolation, biochemical characterization, cryo-EM structure, and phylogenetic analysis of photosystem I from a newly-discovered thylakoid-free cyanobacterium, , a distant relative of the genus . We find that photosystem I exhibits a distinct carotenoid composition and has one conserved low-energy chlorophyll site, which was lost in . These features explain the capacity of to grow under high light intensity, unlike other Gloeobacteria. Furthermore, we find that, while at the sequence level photosystem I in thylakoid-free cyanobacteria has changed to a degree comparable to that of other strains, its subunit composition and oligomeric form might be identical to that of the most recent common ancestor of cyanobacteria.