| Abstract
| - Type I homodimeric reaction centers, particularly the class present in heliobacteria, are notwell understood. Even though the primary amino acid sequence of PshA in Heliobacillus mobilis hasbeen shown to contain an FX binding site, a functional Fe−S cluster has not been detected by EPRspectroscopy. Recently, we reported that PshB, which contains FA- and FB-like Fe−S clusters, could beremoved from the Heliobacterium modesticaldum reaction center (HbRC), resulting in 15 ms lifetimecharge recombination between P798+ and an unidentified electron acceptor [Heinnickel, M., Shen, G.,Agalarov, R., and Golbeck, J. H. (2005) Biochemistry 44, 9950−9960]. We report here that when a HbRCcore is incubated with sodium dithionite in the presence of light, the 15 ms charge recombination isreplaced with a kinetic transient in the sub-microsecond time domain, consistent with the reduction ofthis electron acceptor. Concomitantly, a broad and intense EPR signal arises around g = 5 along with aminor set of resonances around g = 2 similar to the spectrum of the [4Fe-4S]+ cluster in the Fe proteinof Azotobacter vinelandii nitrogenase, which exists in two conformations having S = 3/2 and S = 1/2ground spin states. The Mössbauer spectrum in the as-isolated HbRC core shows that all of the Fe ispresent in the form of a [4Fe-4S]2+ cluster. After reduction with sodium dithionite in the presence oflight, approximately 65% of the Fe appears in the form of a [4Fe-4S]+ cluster; the remainder is in the[4Fe-4S]2+ state. Analysis of the non-heme iron content of HbRC cores indicates an antenna size of 21.6± 1.1 BChl g molecules/P798. The evidence indicates that the HbRC contains a [4Fe-4S] cluster identifiedas FX that is coordinated between the PshA homodimer; in contrast to FX in other type I reaction centers,this [4Fe-4S] cluster exhibits an S = 3/2 ground spin state.
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