Ubiquinol-cytochrome c oxidoreductase (cytochrome bej) complexes were demonstrated to be present in the membranes of the alkaliphilic and halophilic purple sulfur bacteria Ectothiorhodospira halophila, Ectothiorhodospira mobilis, and Ectothiorhodospira shaposhnikovii by protoheme extraction, immunoblotting, and electron paramagnetic resonance spectroscopy. The gy values of the Rieske [2Fe-2S] clusters observed in membranes of E. mobifis and E. halophila were 1.895 and 1.910, respectively. In E. mobilis membranes, the cytochrome be1 complex was present in a stoichiometry of approximately 0.2 per reaction center. This complex was isolated and characterized. It contained four prosthetic groups: low-potential cytochrome b (cytochrome bL; Em = -142 mV), high-potential cytochrome b (cytochrome bH; E. = 116 mV), cytochrome cl (Em = 341 mV), and a Rieske iron-sulfur cluster. The absorbance spectrum of cytochrome bL displayed an asymmetric a-band with a maximum at 564 nm and a shoulder at 559 nm. The a bands of cytochrome bH and cytochrome cl peaked at 559.5 and 553 nm, respectively. These prosthetic groups were associated with three different polypeptides: cytochrome b, cytochrome cl, and the Rieske iron-sulfur protein, with apparent molecular masses of 43, 30, and 21 kDa, respectively. No evidence for the presence of a fourth subunit was obtained.Maximal ubiquinol-cytochrome c oxidoreductase activity of the purified complex was observed at pH 8; the turnover rate was 57 mol of cytochrome c reduced (mol of cytochrome cj)-1 s-1. The complex showed a strikingly low sensitivity towards typical inhibitors of cytochrome bc complexes.