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The supramolecular structure of the exoplasmic freeze-fracture particles of thylakoids of the thermophilic cyanobacterium Synechococcus sp. is compared with that of isolated photosystem-II complexes. The in-situ EF particles are scattered on the thylakoids or organized in rows of variable length; the latter aligned particles measure 10 nmx20 nm and are separated perpendicular to their long axis into two parts. We propose that they represent dimers composed of two monomeric 10-nm EF particles side by side. Isolated photosystem (PS)II particles correspond in size to the monomeric 10-nm EF particles as analysed by negative contrast and freeze-fracture electron microscopy. Dimeric PSII particles, very similar to the in-situ 10 nmx20 nm EF particles, are obtained after incorporation of purified PSII complexes into liposomes made from phospholipid and cholesterol. Each monomeric complex consists of the reaction center, the water-splitting system, the chlorophyll antennae and phycobilisome-binding polypeptides. We propose that the dimeric complexes bind one hemidiscoidal phycobilisome at their domains exposed to the external side of the thylakoids. The implications of this arrangement of the PSII-phycobilisome complexes within the thylakoids upon excitation-energy distribution are discussed. |
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