Differential levels of specific cytochrome c biogenesis proteins in response to oxygen: Analysis of the ccl operon in Rhodobacter capsulatus

The photosynthetic bacterium Rhodobacter capsulatus synthesizes c-type cytochromes under a variety of growth conditions. For example, under aerobic growth, c-type cytochromes are synthesized as part of an electron transport pathway, using oxygen as the terminal electron acceptor. Anaerobically in the light, R. capsulatus requires cytochrome bc(i) and other c-type cytochromes for the photosynthetic electron transport pathway. It is shown here that the ccl1 and ccl2 genes of R. capsulatus are required for the synthesis of all c- type cytochromes, including the cytochrome c' protein of unknown function but of structural similarity to cytochrome b₅₆₂. Polar and nonpolar mutations constructed in each gene demonstrated that the ccl12 genes form an operon. Expression of the ccl12 genes was examined by using lacZ and phoA fusions as translational reporters. Primer extension analysis was used to determine transcriptional control and the start site of the ccl12 promoter. Finally, antiserum to the Cc12 protein was used to quantitate levels of Cc12 under six different growth conditions. The Cc12 protein is present at 20-fold-higher levels under conditions where oxygen is present. In contrast, other cytochromes c biogenesis proteins, HelA and HelX, previously shown to be part of an helABCDX operon, are at relatively similar levels under these six growth conditions. This discovery is discussed in terms of the physiology and evolution of cytochromes c biogenesis, with particular attention to oxidative environments.