EVIDENCE FOR A RIESKE-TYPE FES CENTER IN THE THERMOACIDOPHILIC ARCHAEBACTERIUM SULFOLOBUS-ACIDOCALDARIUS

A high-potential iron-sulfur cluster with characteristics similar to a Rieske-type center was detected in the plasma membrane of Sulfolobus acidocaldarius by EPR spectroscopy. In the reduced form this center has g-values of g(z) = 2.031, g(y) = 1.890 and g(z) = 1.725 (g(av) = 1.88, rhombicity = 0.37) and its reduction potential at pH 7.4 was determined to be +325 +/-10 mV. The archaebacterial cluster exhibits some unique properties. in comparison to eubacterial and eukaryotic Rieske-type centers. First, the reduction potential is pH-dependent in the range from pH 6.7 to 8.2. Second, the typical inhibitor of Rieske FeS centers, DBMIB, had no effect on the g-values of this cluster. The center is reducible by both NADH and succinate in the presence of cyanide, an inhibitor of the terminal oxidases. The possible role of a Rieske-type center in an organism lacking any c-type cytochromes is discussed.