Distribution of the extrinsic proteins as a potential marker for the evolution of photosynthetic oxygen‐evolving photosystem II

Distribution of photosystem II (PSII) extrinsic proteins was examined using antibodies raised against various extrinsic proteins from different sources. The results showed that a glaucophyte (Cyanophora paradoxa) having the most primitive plastids contained the cyanobacterial‐type extrinsic proteins (PsbO, PsbV, PsbU), and the primitive red algae (Cyanidium caldarium) contained the red algal‐type extrinsic proteins (PsO, PsbQ′, PsbV, PsbU), whereas a prasinophyte (Pyraminonas parkeae), which is one of the most primitive green algae, contained the green algal‐type ones (PsbO, PsbP, PsbQ). These suggest that the extrinsic proteins had been diverged into cyanobacterial‐, red algal‐ and green algal‐types during early phases of evolution after a primary endosymbiosis. This study also showed that a haptophyte, diatoms and brown algae, which resulted from red algal secondary endosymbiosis, contained the red algal‐type, whereas Euglena gracilis resulted from green algal secondary endosymbiosis contained the green algal‐type extrinsic proteins, suggesting that the red algal‐ and green algal‐type extrinsic proteins have been retained unchanged in the different lines of organisms following the secondary endosymbiosis. Based on these immunological analyses, together with the current genome data, the evolution of photosynthetic oxygen‐evolving PSII was discussed from a view of distribution of the extrinsic proteins, and a new model for the evolution of the PSII extrinsic proteins was proposed.