Cytochromes c(550), c(552), and c(1) in the electron transport network of Paracoccus denitrificans: Redundant or subtly different in function?

Paracoccus denitrificans strains with mutations in the genes encoding the c ytochrome c(550), c(552), or c(1) and in combinations of these genes were c onstructed, and their growth characteristics, were determined. Each mutant was able to grow heterotrophically with succinate as the carbon and free-en ergy source, although their specific growth rates and maximum cell numbers fell variably behind those of the wild type. Maximum cell numbers and rates of growth were also reduced when these strains were grown with methylamine as the sole free-energy source, with the triple cytochrome c mutant failin g to grow on this substrate. Under anaerobic conditions in the presence of nitrate, none of the mutant strains lacking the cytochrome be, complex redu ced nitrite, which is cytotoxic and accumulated in the medium. The cytochro me c(550)-deficient mutant did denitrify provided copper was present. The c ytochrome c(552) mutation had no apparent effect on the denitrifying potent ial of the mutant cells. The studies show that the cytochromes c have multi ple tasks in electron transfer. The cytochrome be, complex is the electron acceptor of the Q-pool and of amicyanin. It is also the electron donor to c ytochromes C-550 and c(552) and to the cbb(3)-type oxidase. Cytochrome c(55 2) is an electron acceptor both of the cytochrome be, complex and of amicya nin, as well as a dedicated electron donor to the cbb(3)-type oxidase. Cyto chrome esso can accept electrons from the cytochrome be, complex and from a micyanin, whereas it is also the electron donor to both cytochrome c oxidas es and to at least the nitrite reductase during denitrification. Deletion o f the c-type cytochromes also affected the concentrations of remaining cyto chromes c, suggesting that the organism is plastic in that it adjusts its i nfrastructure in response to signals derived from changed electron transfer routes.