The chlorosome envelope of Chlorobaculum tepidum contains 10 polypeptides, three of which, CsmI, CsmJ, and CsmX, have an adrenodoxin-like domain harboring a single [2Fe-2S] cluster. Mutants that produced chlorosomes containing two, one, or none of these Fe-S proteins were constructed [Li, H., et al. (2013) Biochemistry 52, preceding paper in this issue (DOI: 10.1021/bi301454g)]. The electron paramagnetic resonance (EPR) spectra, g values, and line widths of the Fe-S clusters in individual CsmI, CsmJ, and CsmX proteins were obtained from studies with isolated chlorosomes. The Fe-S clusters in these proteins were characterized by EPR and could be differentiated on the basis of their g values and line widths. The EPR spectrum of wild-type chlorosomes could be simulated by a 1:1 admixture of the CsmI and CsmJ spectra. No contribution of CsmX to the EPR spectrum of chlorosomes was observed because of its low abundance. In chlorosomes that contained only CsmI or CsmJ, the midpoint potential of the [2Fe-2S] clusters was -205 or 8 mV, respectively; the midpoint potential of the [2Fe-2S] cluster in CsmX was estimated to be more oxidizing than -180 mV. In wild-type chlorosomes, the midpoint potentials of the [2Fe-2S] clusters were -348 mV for CsmI and 92 mV for CsmJ. The lower potential for CsmI in the presence of CsmJ, and the higher potential for CsmJ in the presence of CsmI, were attributed to interactions that occur when these proteins form complexes in the chlorosome envelope. The redox properties of CsmI and CsmJ are consistent with their proposed participation in the transfer of electrons to and from quenchers of energy transfer in chlorosomes.
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