Orientations of Iron-Sulfur Clusters F_A and F_B in the Homodimeric Type-I Photosynthetic Reaction Center of Heliobacterium modesticaldum

Orientations of the F_A and F_B iron-sulfur (FeS) clusters in a structure-unknown type-I homodimeric heriobacterial reaction center (hRC) were studied in oriented membranes of the thermophilic anaerobic photosynthetic bacterium Heliobacterium modesticaldum by electron paramagnetic resonance (EPR), and compared with those in heterodimeric photosystem I (PS I). The Rieske-type FeS center in the cytochrome b/c complex showed a well-oriented EPR signal. Illumination at 14 K induced an F_B^- signal with g-axes of g_z = 2.066, g_y = 1.937, and g_x = 1.890, tilted at angles of 60°, 60°, and 45°, respectively, with respect to the membrane normal. Chemical reduction with dithionite produced an additional signal of F_A^-, which magnetically interacted with F_B^-, with g_z = 2.046, g_y = 1.942, and g_x = 1.911 at 30°, 60°, and 90°, respectively. The angles and redox properties of F_A^- and F_B^- in hRC resemble those of F_B^- and F_A^-, respectively, in PS I. Therefore, F_A and F_B in hRC, named after their g-value similarities, seem to be located like F_B and F_A, not like F_A and F_B, respectively, in PS I. The reducing side of hRC could resemble those in PS I, if the names of F_A and F_B are interchanged with each other.