THE ELECTRON-TRANSPORT SYSTEM OF THE FACULTATIVE PHOTOTROPH RHODOFERAX-FERMENTANS .1. A FUNCTIONAL, THERMODYNAMIC AND SPECTROSCOPIC STUDY OF THE RESPIRATORY-CHAIN OF DARK-GROWN AND LIGHT-GROWN CELLS

Membranes isolated from light- and dark-grown cells of the recently es tablished new taxon of the purple nonsulfur bacteria, Rhodoferax ferme ntans, gen, nov., sp, nov., have been examined. The results have been interpreted to show that the oxidative electron transport chain is bra nched at the ubiquinone level and does not involve rhodoquinone. Dark- grown membranes contain four b-type and three c-type membrane-bound cy tochromes with E(m7.0) Of +371, +315, +76 and -18 mV and +298, +201 an d +44 mV, respectively. No significant amount of soluble c was found i n aerobic cells. Conversely, photosynthetically grown cells contain a soluble c-type haem (alpha(max) at 551 nm, E(m7.0) = +287 mV), four me mbrane-bound c-type haems with E(m7.0) of +358, +296, +78 and -1 mV an d three cytochromes b with E(m7.0) of +320, +30 and -50 mV. Notably, t he absence of cyt b-371 from light-grown membranes parallels the very low rate of cyt c oxidase activity catalyzed by this type of membrane. Oxide-reduction kinetics demonstrated that most of the c-type haems d etected in light-grown membranes are not involved in respiration. Thes e data suggest that the facultative phototroph Rf. fermentans is endow ed with an electron transport system different from that of typical fa cultative phototrophs, e.g., Rhodobacter and Rhodospirillum species, b ut similar to that of green- and purple-nonsulfur genera such as Chlor oflexus, Rhodocyclus and Rhodopseudomonas.