THE STRUCTURAL ORGANIZATION OF THE PSAC PROTEIN IN PHOTOSYSTEM-I FROMSINGLE-CRYSTAL EPR AND X-RAY CRYSTALLOGRAPHIC STUDIES

In Photosystem I(PS I) the terminal electron accepters, F-A and F-B, a re iron-sulfur (4Fe-4S) centers, which are bound to the stromal subuni t PsaC. The orientation of PsaC is determined relative to the whole PS I complex (see Schubert, W.-D. et al. (1995) in From Light to Biosphe re (Mathis, P. ed.), Vol. II, pp. 3-10, Kluwer) from which a molecular model for the structure of PsaC within PS I is derived. Two strategie s are followed: (i) PS I single crystal EPR data on the orientation of the g tensors of both F-A(-) and F-B(-) relative to each other and re lative to the crystal axes (see preceding paper) an used in conjuction with the central structural part of the bacterial 2[Fe4S4] ferredoxin s, the cysteine binding motifs of which are known to be homologous to those of PsaC; (ii) the same core structure is fitted into the interme diate resolution electron density map of PS I. The PsaC orientation ob tained both ways agree well. The local twofold symmetry axis inherent to the ferredoxin model leaves a twofold ambiguity in the structural c onclusion. Deviations from this C-2-symmetry in the amino acid sequenc e of PsaC are analyzed with respect to observable properties which wou ld resolve the remaining structural ambiguity. Arguments both for and against F-A being the distal iron-sulfur center (to F-X) are discussed .