ROLES OF THE SOLUBLE CYTOCHROME C(2) AND MEMBRANE-ASSOCIATED CYTOCHROME C(Y) OF RHODOBACTER-CAPSULATUS IN PHOTOSYNTHETIC ELECTRON-TRANSFER

Genetic evidence indicates that Rhodobacter capsulatus has two different pathways for reduction of the photooxidized reaction center (RC) [Jenney, F. E., and Daldal, F. (1993) EMBO J. 12, 1283-1292]. One pathway is via the water soluble cytochrome (cyt) C-2, and the other is via a novel, membrane-associated c-type cytochrome, cyt c(y), now believed to be identical to the cyt c(x) of Jones et al. [Jones, M. R., et al. (1990) Biochim. Biophys. Acta 975, 59-66] and C-354 Of Zannoni et al. [Zannoni, D., et al. (1992) Arch, Microbiol. 157, 367-374]. Mutants lacking either cyt c(2), cyt c(y), or the bc(1) complex, as well as various combinations, were utilized to probe the functional role of these cytochromes in electron transfer. Data obtained by monitoring flash induced electron transfer kinetics in the RC, cyt c pool, cyt b, and the carotenoid band shift indicate that there are two pathways for electron transfer from the bc(1) complex to the RC in R. capsulatus, one via cyt c(2) and the other through cyt c(y). The two pathways show strikingly different kinetics for RC reduction and cyt c oxidation, and both are present in the wild-type strain MT-1131. After genetic inactivation of both cyt c(2) and cyt c(y) there remains no flash oxidizible c-type cytochrome, and inactivation of cyt c(y) rather than cyt c(2) has a more pronounced effect on the extent of the light-induced membrane potential under the conditions tested. Finally, heme-stained SDS-PAGE and flash spectroscopy experiments indicate that cyt c(y) is detectable in strains lacking the bc(1) complex when grown on minimal growth medium but not on rich medium. These findings complement the earlier genetic data and further establish that cyt c(y) is the electron carrier permitting soluble cyt c(2)-independent photosynthetic growth in R. capsulatus.