2-DIMENSIONAL STRUCTURE OF PLANT PHOTOSYSTEM-II AT 8-ANGSTROM RESOLUTION

The photosystem II complex, which is the most abundant membrane protei n in chloroplasts, comprises the light-harvesting complex II and a rea ction-centre core. The reaction centre uses the solar energy collected by the light-harvesting complex II to withdraw electrons from water, releasing oxygen into the atmosphere, It thus generates an electrochem ical potential, providing the energy for carbon dioxide fixation and t he synthesis of organic molecules, which make up the hulk of the biosp here(1). The structure of the light-harvesting complex II has been det ermined at 3.4-Angstrom resolution by electron crystallography(2), but the high-resolution structure of the photosystem II reaction centre a nd other core components remained unknown. We have grown well-ordered two-dimensional crystals of a sub-core complex containing the reaction centre from spinach thylakoid membranes and used electron crystallogr aphy to obtain a projection map of its structure at 8-Angstrom resolut ion. The features reveal the likely location of the key components tha t are active in electron transport, and suggest a structural homology and evolutionary links, not only with the purple bacterial reaction ce ntre but also with the reaction centre of photosystem I.