Dimerization of the Free and Photosystem II-Associated PsbO Protein upon Irradiation with UV Light

The main function of the PsbO protein in photosystem II is to stabilize the water-splitting Mn 4 CaO 5 cluster. The present study shows for the first time that short-term exposure of both isolated and membrane-bound PsbO to UV light led to protein dimerization. A non-covalent dimer of the PsbO protein was detected by gel filtration. Separation of proteins of UV-irradiated PSII membrane fragments by polyacrylamide gel electrophoresis was indicative of the formation of covalent PsbO dimers, as with the isolated PsbO protein. The formation of covalent PsbO dimers following exposure to UV light supports the hypothesis that non-covalent PsbO dimers exist in vivo and may play a regulatory role in PSII.