Low temperature EPR on photosystem I single crystals: Orientation of the iron-sulfur centers F(A) and F(B)

Low temperature EPR results from Photosystem I (PS I) single crystals of Synechococcus elongatus are presented. Illumination at 150 K and pH 6.4 is used to photoreduce the two terminal 4Fe-4S centers to the noninteracting state (F(A)^-) + (F(B)^-). From the EPR data and the analysis of the rotation pattern for both F(A)^- and F(B)^- the following information is obtained: (i) the principal values of the g tensors of F(A)^- and F(B)^-, (ii) the orientations of their principal g tensor axes with respect to the crystal axes for each of the six PS I centers per unit cell, and (iii) their orientation with respect to one another. In addition, significant differences between F(A)^- and F(B)^- are noted with respect to the orientational dependence of the linewidth and the saturation behavior of their EPR signals. In principle, six relative arrangements of the g tensors of F(A)^- and F(B)^- are consistent with the EPR data. Only two out of these six are compatible with the known structure of the bacterial fenedoxin from P. aerogenes (PaFd) (Adman, E.T., Siecker, L.C, and Jensen, L.H. (1976) J. Biol. Chem. 251, 3801) which has been used as a model for the core of the PsaC protein carrying F(A) and F(B) in PS I. It is concluded that the PaFd and the PsaC protein are analogous with respect to the central part of their structures. The results and conclusions are compared to those obtained from studies on oriented membranes.