PHOTOSYSTEM-I OF SYNECHOCOCCUS-ELONGATUS AT 4 ANGSTROM RESOLUTION - COMPREHENSIVE STRUCTURE-ANALYSIS

An improved structural model of the photosystem I complex from the the rmophilic cyanobacterium Synechococcus elongatus is described at 4 Ang strom resolution. This represents the most complete model of a photosy stem presently available, uniting both a photosynthetic reaction centr e domain and a core antenna system. Most constituent elements of the e lectron transfer system have been located and their relative centre-to -centre distances determined at an accuracy of similar to 1 Angstrom. These include three pseudosymmetric pairs of Chin and three iron-sulph ur centres, F-X, F-A and F-B. The first pair, a Chin dimer, has been a ssigned to the primary electron donor P700. One or both Chla of the se cond pair, eC(2) and eC(2)', presumably functionally link P700 to the corresponding Chla of the third pair, eC(3) and eC'(3), which is assum ed to constitute the spectroscopically-identified primary electron acc eptor(s), A(0), of PSI. A likely location of the subsequent phylloquin one electron acceptor, Q(K), in relation to the properties of the spec troscopically identified electron acceptor A(1) is discussed. The posi tions of a total of 89 Chin, 83 of which constitute the core antenna s ystem, are presented, The maximal centre-to-centre distance between an tenna Chla is less than or equal to 16 Angstrom; 81 Chin are grouped i nto four clusters comprising 21, 23, 17 and 20 Chin, respectively. Two ''connecting'' Chla are positioned to structurally (and possibly func tionally) link the Chla of the core antenna to those of the electron t ransfer system. Thus the second and third Chla pairs of the electron t ransfer system may have a dual function both in energy transfer and el ectron transport. A total of 34 transmembrane and nine surface alpha-h elices have been identified and assigned to the 11 subunits of the PSI complex. The connectivity of the nine C-terminal (seven transmembrane , two ''surface'') alpha-helices of each of the large core subunits Ps aA and PsaB is described. The assignment of the amino acid sequence to the transmembrane cl-helices is proposed and likely residues involved in co-ordinating the Chla of the electron transfer system discussed. (C) 1997 Academic Press Limited.