LOW-TEMPERATURE EPR ON PHOTOSYSTEM-I SINGLE-CRYSTALS - ORIENTATION OFTHE IRON-SULFUR CENTERS F-A AND F-B

Low temperature EPR results from Photosystem I(PS I) single crystals o f Sqnechococcus elongatus are presented. Illumination at 150 K and pH 6.4 is used to photoreduce the two terminal 4Fe-3S centers to the noni nteracting state (F-A(-))+(F-B(-)). From the EPR data and the analysis of the rotation pattern for both F-A(-) and F, the following informat ion is obtained: (i) the principal values of the g tensors of F-A(-) a nd 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 addi tion, significant differences between F-A(-) and F-B(-) are noted with respect to the orientational dependence of the linewidth and the satu ration behavior of their EPR signals. In principle, six relative arran gements of the g tensors of F-A(-) and F-B(-) an consistent with the E PR data. Only two out of these six are compatible with the known struc ture of the bacterial ferredoxin from P. aerogenes (PaFd) (Adman, E.T. , Siecker, L.C. and Jensen, L.H. (1976) J. Biol. Chem. 251, 3801) whic h 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. T he results and conclusions are compared to those obtained from studies on oriented membranes.