Investigation of the structure of spinach photosystem II reaction center complex

The photosystem II (PSII) reaction center (RC) complex was isolated from sp inach and characterized by gel electrophoresis, gel filtration and analytic al ultracentrifugation. The purified complex contained the PsbA, PsbD, PsbE , PsbF and PsbI subunits. Gel filtration and analytical ultracentrifugation indicated the presence of a homogeneous complex. The mass of the RC comple xes was found to be 107 kDa by analytical ultracentrifugation and 132 kDa b y scanning transmission electron microscopy (STEM). The mass obtained showe d the isolated complex to exist as a monomer and only one cytochrome b(559) (cYt b(559)) to be associated with the RC complex. Digital images of negat ively stained RC complexes were recorded by STEM and analyzed by single-par ticle averaging. The complex was 9 nm long and 5 nm wide, and exhibited a p ronounced quasi-twofold symmetry. This supports the symmetric organization of the PSII complex, with the PsbA and the PsbD proteins in the center and symmetrically arranged PsbB and PsbC proteins at the periphery of the monom eric complex.