Cytochrome c4 Can Be Involved in the Photosynthetic Electron Transfer System in the Purple Bacterium Rubrivivax gelatinosus

Three periplasmic electron carriers, HiPIP and two cytochromes c(8) with low- and high-midpoint potentials, are present in the purple photosynthetic bacterium Rubrivivax gelatinosus. Comparison of the growth rates of mutants lacking one, two, or all three electron carrier proteins showed that HiPIP is the main electron donor to the photochemical reaction center and that high-potential cytochrome c(8) plays it subsidiary role in the electron donation in photosynthetically growing cells. However, the triple deletion mutant was still capable of photosynthetic growth, indicating that another electron donor could be present. A new soluble cytochrome c, which can reduce the photooxidized reaction center in vitro, was purified. Based on amino acid sequence comparisons to known cytochromes, this cytochrome was identified as a diheme cytochrome c of the family,of cytochromes c(4). The quadruple mutant lacking this cytochrome and three other electron carriers showed about three times slower growth than the triple mutant under photosynthetic growth conditions. In conclusion. cytochrome c(4) can function as a physiological electron carrier in the photosynthetic electron transport chain in R. gelatinosus.