show Abstracthide AbstractChroococcidiopsis sp. TS-821 has both tetrameric and trimeric PSI. To analyze the biochemical differences between those two different PSI oligomer, the genome sequence is needed for proteomic analysis. This helps elucidating how PSI evolved from trimeric to tetrameric and eventually monomeric. Understanding the PSI oligomer evolution may shed light on the evolution of plastid from cyanobacteria.