MEMBRANE-ANCHORED CYTOCHROME C(Y) MEDIATED MICROSECOND TIME RANGE ELECTRON-TRANSFER FROM THE CYTOCHROME BC(1) COMPLEX TO THE REACTION-CENTER IN RHODOBACTER-CAPSULATUS

In Rhodobacter capsulatus, the soluble cytochrome (cyt) c(2) and membr ane-associated cyt c(y) are the only electron carriers which operate b etween the photochemical reaction center (RC) and the cyt bc(1) comple x. In this work, cyt c(y) mediated microsecond time range electron tra nsfer kinetics were studied by light-activated time-resolved absorptio n spectroscopy using a mutant strain lacking cyt c(2). In intact cells and in isolated chromatophores of this mutant, only similar to 30% of the RCs had their photooxidized primary donor rapidly rereduced by cy t c(y). Of these 30%, about half were reduced with a half-time of simi lar to 5 mu s attributed to preformed complexes, and the other half wi th a half-time of similar to 40 mu s attributed to cyt c(y) having to move from another site. This slower phase was affected by addition of glycerol, indicating its dependence on the viscosity of the medium. Cy t c(y), despite its rereduction by ubihydroquinone oxidation in the mi llisecond time range, remained virtually unable to deliver electrons t o other RCs which stayed photooxidized for several seconds. Furthermor e, using two flashes separated by a variable time interval, it was sho wn that the fast electron donating complex was reformed in about 60 mu s, a time span probably reflecting electron transfer from cyt c(1) to cyt c(y). In the absence of the cyt bc(1) complex, the steady-state l evel of cyt c(y) in xthe chromatophore membranes obtained using cells grown in minimal medium was decreased to similar to 50%. The remaining cyt c(y), however, was able to form the fast electron donating comple x with the RC (half-time of similar to 5 mu s), whereas the slower pha se with a half-time of similar to 40 mu s was strongly decelerated. Th is finding suggests a role for the cyt bc(1) complex in stabilizing cy t c(y) and providing its ''other'' site, possibly via a close associat ion between these components. Taken together, it is concluded that alt hough cyt c(y) is present in substoichiometric amount compared to the RCs, it supports efficiently photosynthetic growth of R. capsulatus in the absence of cyt c(2) because it can mediate fast electron transfer from the cyt bc(1) complex to the RC during multiple turnovers of the cyclic electron flow.