Intracytoplasmic Copper Homeostasis Controls Cytochrome c Oxidase Production

Copper is an essential micronutrient used as a metal cofactor by a variety of enzymes, including cytochrome c oxidase (Cox). In all organisms from bacteria to humans, cellular availability and insertion of copper into target proteins are tightly controlled due to its toxicity. The major subunit of Cox contains a copper atom that is required for its catalytic activity. Previously, we identified CcoA (a member of major facilitator superfamily transporters) as a component required for cbb(3)-type Cox production in the Gram-negative, facultative phototroph Rhodobacter capsulatus. Here, first we demonstrate that CcoA is a cytoplasmic copper importer. Second, we show that bypass suppressors of a ccoA deletion mutant suppress cbb(3)-Cox deficiency by increasing cellular copper content and sensitivity. Third, we establish that these suppressors are single-base-pair insertion/deletions located in copA, encoding the major P1B-type ATP-dependent copper exporter (CopA) responsible for copper detoxification. A copA deletion alone has no effect on cbb(3)-Cox biogenesis in an otherwise wild-type background, even though it rescues the cbb(3)-Cox defect in the absence of CcoA and renders cells sensitive to copper. We conclude that a hitherto unknown functional interplay between the copper importer CcoA and the copper exporter CopA controls intracellular copper homeostasis required for cbb(3)-Cox production in bacteria like R. capsulatus.