Orientations of Iron-Sulfur Clusters FA and FB 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 degrees, 60 degrees, and 45 degrees, 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 degrees, 60 degrees, and 90 degrees, 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.