Abstract
Cyanidium caldarium is a unicellular primitive red alga. Photosystem II complex (PS II) isolated from C. caldarium contained four extrinsic proteins, PsbO, PsbV, PsbU and PsbQ'. PsbQ' subunit was also found in other alga, such as diatom and other species of red algae, however, the localization of PsbQ' subunit is not clear at present. To reveal the localization of PsbQ' subunit, we crosslinked C. caldarium PSII with a water-soluble carbodiimide, 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide. We obtained several crosslinked products, and similar results were obtained with PSII from Cyanidioschyzon merolae. Additionally, we introduced a spin-label at the cysteine residue of the PsbQ' subunit, and measured the pulse ESR spectra. Electron electron double resonance for the radical pair of spin-labeled PsbQ' subunit and YD were observed after illumination of the sample. The results revealed that the average distance between the two radicals was 34-35 angstrom. From these results, we discuss the localization of the PsbQ' subunit in the red algal PSII.
