Abundance, Subunit Composition, RedoxProperties, andCatalytic Activity oftheCytochrome be1Complex fromAlkaliphilic and Halophilic, Photosynthetic Membersofthe Family Ectothiorhodospiraceae

shaposhnikovii byprotoheme extraction, immunoblotting, and electron paramagnetic resonance spectroscopy. Thegyvalues oftheRieske [2Fe-2S] clusters observed in membranes ofE.mobifis andE.halophila were1.895 and1.910, respectively. InE.mobilis membranes, the cytochrome be1complex waspresent inastoichiometry ofapproximately 0.2perreaction center. Thiscomplex wasisolated andcharacterized. Itcontained fourprosthetic groups: low-potential cytochrome b(cytochrome bL;Em= -142mV),high-potential cytochrome b(cytochrome bH;E.= 116mV),cytochrome cl(Em= 341 mV),andaRieske iron-sulfur cluster. Theabsorbance spectrum ofcytochrome bLdisplayed anasymmetric a-band withamaximumat564nmandashoulder at559nm.Theabandsofcytochrome bHandcytochrome clpeaked at559.5 and553nm,respectively. Theseprosthetic groups wereassociated withthree different polypeptides: cytochrome b,cytochrome cl,andtheRieske iron-sulfur protein, withapparent molecular masses of43,30,and21kDa,respectively. Noevidence forthepresence ofafourth subunit wasobtained. Maximalubiquinol-cytochrome coxidoreductase activity ofthepurified complex wasobserved atpH8;the turnover ratewas57molofcytochrome creduced(molofcytochrome cj)-1 s-1. Thecomplex showed astrikingly lowsensitivity towards typical inhibitors ofcytochrome bccomplexes. TheEctothiorhodospiraceae family consists ofasingle