The electron transport system of the facultative phototroph Rhodoferax fermentans. I. A functional, thermodynamic and spectroscopic study of the respiratory chain of dark- and light-grown cells

Abstract

Membranes isolated from light- and dark-grown cells of the recently established new taxon of the purple nonsulfur bacteria, Rhodoferax fermentans, gen. nov., sp. nov., have been examined. The results have been interpreted to show that the oxidative electron transport chain is branched at the ubiquinone level and does not involve rhodoquinone. Dark-grown membranes contain four b-type and three c-type membrane-bound cytochromes with E m7.0 of +371, +315, +76 and −18 mV and +298, +201 and +44 mV, respectively. No significant amount of soluble c was found in aerobic cells. Conversely, photosynthetically grown cells contain a soluble c-type haem ( α max at 551 nm, E m7.0 = +287 mV), four membrane-bound c-type haems with E m7.0 of +358, +296, +78 and −1 mV and three cytochromes b with E m7.0 of +320, +30 and −50 mV. Notably, the 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. Oxido-reduction kinetics demonstrated that most of the c-type haems detected in light-grown membranes are not involved in respiration. These data suggest that the facultative phototroph Rf. fermentans is endowed with an electron transport system different rrom that of typical facultative phototrophs, e.g., Rhodobacter and Rhodospirillum species, but similar to that of green- and purple-nonsulfur genera such as Chloroflexus, Rhodocyclus and Rhodopseudomonas.