Isolation and characterization of Rhodobacter capsulatus mutants affected in cytochrome cbb3 oxidase activity

The facultative phototrophic bacterium Rhodobacter capsulatus contains only one form of cytochrome (cyt) c oxidase, which has recently been identified as a cbb3-type cyt c oxidase. This is unlike other related species, such as Rhodobacter sphaeroides and Paracoccus denitrificans, which contain an additional mitochondrial-like aa3-type cyt c oxidase. An extensive search for mutants affected in cyt c oxidase activity in R. capsulatus led to the isolation of at least five classes of mutants. Plasmids complementing them to a wild-type phenotype were obtained for all but one of these classes from a chromosomal DNA library. The first class of mutants contained mutations within the structural genes (ccoNOQP) of the cyt cbb3 oxidase. Sequence analysis of these mutants and of the plasmids complementing them revealed that ccoNOQP in R. capsulatus is not flanked by the oxygen response regulator fnr, which is located upstream of these genes in other species. Genetic and biochemical characterizations of mutants belonging to this group indicated that the subunits CcoN, CcoO, and CcoP are required for the presence of an active cyt cbb3 oxidase, and unlike in Bradyrhizobium japonicum, no active CcoN-CcoO subcomplex was found in R. capsulatus. In addition, mutagenesis experiments indicated that the highly conserved open reading frame 277 located adjacent to ccoNOQP is required neither for cyt cbb3 oxidase activity or assembly nor for respiratory or photosynthetic energy transduction in R. capsulatus. The remaining cyt c oxidase-minus mutants mapped outside of ccoNOQP and formed four additional groups. In one of these groups, a fully assembled but inactive cyt cbb3 oxidase was found, while another group had only extremely small amounts of it. The next group was characterized by a pleiotropic effect on all membrane-bound c-type cytochromes, and the remaining mutants not complemented by the plasmids complementing the first four groups formed at least one additional group affecting the biogenesis of the cyt cbb3 oxidase of R. capsulatus.