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 CuA 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 CuA 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 0.5 μM 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.

► SenC is required for cbb3 Cox assembly at low Cu2+ concentrations/high growth rates.
► SenC interacts directly with the CcoP and CcoH subunits of R. capsulatus cbb3 Cox.
► SenC is regulated by Cu availability in the absence of the Cu export ATPase CcoI.