Energy transduction in biological systems involves the translocation of protons across a membrane to generate a proton motive force that is used for the synthesis of ATP. How protons are moved across this membrane remains a central problem in bioenergetics. The cytochrome b-c complex couples electron transport from reduced quinones to high potential electron acceptors to proton translocation in mitochondrial, bacterial and chloroplasts membranes. In complexes from all these membranes, two b-cytochromes, one ccytochrome and a Rieskatype iron- sulfur protein are present. An associated quinone is usually also found in these complexes. This project focuses on the role of the Rieske ironsulfur protein in the function of the cytochrome b-c complex with particular emphasis on how this protein interacts with quinone molecules. Factors which are involved in quinone and Rieske protein binding will be considered using the biochemical techniques of resolution and reconstitution. The mode of interaction of inhibitors which are quinone analogs will also be examined. The role of lipids in these interactions will receive particular attention. The mechanism of electron transfer and associated proton translocation will be examined from a kinetic standpoint using liposome incorporated cytochrome complex in conjunction with homologous photosystem complexes for light activation. This allows for control of the electon flow into and out of the complex. In addition to these biochemical approaches, studies of specifically altered cytochrome complexes, either generated by the isolation of inhibitorinsensitive mutants or produced using sitedirected mutagenesis of isolated genes, will be done in an attempt to define specific regions of individual proteins, such as the Rieske protein, that are involved in specified functions of the complex. These studies are aimed at elucidating structural and organizational features which influence the function of a bound iron-sulfur protein that is involved in a basic cellular process, the generation of the energy required for the synthesis of ATP.