PHOTOSYSTEM-II 3-D STRUCTURE AND THE ROLE OF THE EXTRINSIC SUBUNITS IN PHOTOSYNTHETIC OXYGEN EVOLUTION

Two-dimensional (2-D) crystals of photosystem II (PSII) from spinach ( Spinacia oleracea) have been subjected to Triswashing in order to sele ctively remove the three oxygen-evolution enhancing (OEE) subunits. El ectron microscopy of the 2-D crystals of Tris-treated PSII (tPSII) con firmed the previously reported unit cell parameters alpha = 17.7 mn, b = 20.1 and gamma = 91 degrees (cf. Holzenburg ct al., 1994) and, in c onjunction with Fourier reconstruction techniques, was used to calcula te the three-dimensional (3-D) structure of tPSII at 1.8 nm resolution which is in good agreement with our earlier tPSII data in projection. A comparison between the 3-D structures of tPSII and untreated PSII ( Holzenburg et al., 1993) by Fourier difference analysis revealed direc tly and unambiguously the existence of a micro-cavity as part of the c omplex. We demonstrate that this intramolecular microenvironment is fo rmed by the OEE subunits and discuss its importance for oxygen evoluti on.