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Photosynthesis uses light energy to drive the oxidation of water at an oxygen-evolving catalytic site within photosystem II (PSII). We report the structure of PSII of the cyanobacterium Thermosynechococcus elongatus at 3.5 angstrom resolution. We have assigned most of the amino acid residues of this 650-kilodalton dimeric multisubunit complex and refined the structure to reveal its molecular architecture. Consequently, we are able to describe details of the binding sites for cofactors and propose a structure of the oxygen-evolving center (OEC). The data strongly suggest that the OEC contains a cubane-like [Mn.sub.3]Ca[O.sub.4] cluster linked to a fourth Mn by a mono-μ-oxo bridge. The details of the surrounding coordination sphere of the metal cluster and the implications for a possible oxygen-evolving mechanism are discussed., Photosynthesis uses light energy to couple the formation of molecular dioxygen to the fixation of C[O.sub.2]. This process simultaneously generates an aerobic atmosphere and produces a readily usable carbon source, [...]