Abstract
PsbY is one of the low molecular mass subunits of oxygen-evolving photosystem II (PSII), with a molecular mass of 4.7 kDa and single trans-membrane helix. Its location, however, has not been identified in the current crystal structure of PSII. Here, we constructed a PsbY-deletion mutant of Thermosynechococcus elongatus, crystallized, and analyzed the crystal structure of the PsbY-deleted PSII dimeric complex. From the difference-Fourier map of wild type-minus-mutant PSII, two strong positive regions were observed in the periphery of PSII dimer which corresponded to an unassigned trans-membrane helix located adjacent to the α- and β-subunits of cytochrome b559 in the 3.7 Å structure reported previously (Kamiya and Shen, 2003, PNAS, 100, 98-103). This helix was also found in the 3.0 Å structure and has been designated helix X2 (Loll et al., 2005, Nature, 438, 1040-1044), but was not present in the 3.5 Å structure (Ferreira et al., 2004, Science, 303, 1831-1838).
