PHOTOSYSTEM-I AT 4-ANGSTROM RESOLUTION REPRESENTS THE FIRST STRUCTURAL MODEL OF A JOINT PHOTOSYNTHETIC REACTION-CENTER AND CORE ANTENNA, SYSTEM

The 4 Angstrom X-ray structure model of trimeric photosystem I of the cyanobacterium Synechococcus elongatus reveals 31 transmembrane, nine surface and three stromal alpha-helices per monomer, assigned to the 1 1 protein subunits: PsaA and PsaB are related by a pseudo two-fold axi s normal to the membrane plane, along which the electron transfer pigm ents are arranged, 65 antenna chlorophyll a (Chi a) molecules separate d by less than or equal to 16 Angstrom form an oval, clustered net, co ntinuous with the electron transfer chain through the second and third Chi a pairs of the electron transfer system, This suggests a dual rol e for these Chi a both in excitation energy and electron transfer, The architecture of the protein core indicates quinone and iron-sulphur t ype reaction centres to have a common ancestor.