Location of PsbY in oxygen-evolving photosystem II revealed by mutagenesis and X-ray crystallography

PsbY is one of the low molecular mass subunits of oxygen-evolving photosystem II (PSII). Its location, however, has not been identified in the current crystal structure of PSII. We constructed a PsbY-deletion mutant of Thermosynechococcus elongatus, crystallized, and analyzed the crystal structure of the mutant PSII dimer. The results obtained showed that PsbY is located in the periphery of PSII close to the α- and β-subunits of cytochrome b559, which corresponded to an unassigned helix in the 3.7 Å structure of T. vulcanus or helix X2 in the 3.0 Å structure of T. elongatus. Our results also indicated that the C-terminal loop of PsbY is protruded toward the stromal side, instead of the lumenal side predicted previously.