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 differe nt pathways for reduction of the photooxidized reaction center (RC) [J enney, F. E., and Daldal, F. (1993) EMBO J. 12, 1283-1292]. One pathwa y 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 l acking either cyt c(2), cyt c(y), or the bc(1) complex, as well as var ious combinations, were utilized to probe the functional role of these cytochromes in electron transfer. Data obtained by monitoring flash i nduced electron transfer kinetics in the RC, cyt c pool, cyt b, and th e carotenoid band shift indicate that there are two pathways for elect ron transfer from the bc(1) complex to the RC in R. capsulatus, one vi a cyt c(2) and the other through cyt c(y). The two pathways show strik ingly different kinetics for RC reduction and cyt c oxidation, and bot h are present in the wild-type strain MT-1131. After genetic inactivat ion 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 detectabl e in strains lacking the bc(1) complex when grown on minimal growth me dium but not on rich medium. These findings complement the earlier gen etic data and further establish that cyt c(y) is the electron carrier permitting soluble cyt c(2)-independent photosynthetic growth in R. ca psulatus.