The ScoI homologue SenC is a copper binding protein that interacts directly with the cbb₃-type cytochrome oxidase in Rhodobacter capsulatus.

Sco proteins are widespread assembly factors for the Cu(A) centre of aa₃-type cytochrome oxidases in eukaryotic and prokaryotic organisms. However, Sco homologues are also found in bacteria like Rhodobacter capsulatus which lack aa₃-type cytochrome oxidases and instead use a cbb₃-type cytochrome oxidase (cbb₃ Cox) without a Cu(A) centre as a terminal oxidase. In the current study, we have analyzed the role of Sco (SenC) during cbb₃ Cox assembly in R. capsulatus. In agreement with earlier works, we found a strong cbb₃ Cox defect in the absence of SenC that impairs the steady-state stability of the CcoN, CcoO and CcoP core subunits, without the accumulation of detectable assembly intermediates. In vivo cross-linking results demonstrate that SenC is in close proximity to the CcoP and CcoH subunits of cbb₃ Cox, suggesting that SenC interacts directly with cbb₃ Cox during its assembly. SenC binds copper and the cbb₃ Cox assembly defect in the absence of SenC can be rescued by the addition of least 0.5μM Cu. Neither copper nor SenC influenced the transcription of the ccoNOQP operon encoding for cbb₃ Cox. Transcription of senC itself was also not influenced by Cu unless the putative Cu-export ATPase CcoI was absent. As CcoI is specifically required for the cbb₃ Cox assembly, these data provide a direct link between Cu delivery to cbb₃ Cox and SenC function.