In vivo assembly of the cytochrome d terminal oxidase complex of Escherichia coli from genes encoding the two subunits expressed on separate plasmids

The cytochrome d terminal oxidase complex is a heterodimer located in the cytoplasmic membrane of Escherichia coli. Subunit II of the cytochrome d terminal oxidase complex was expressed independently of subunit I of the complex. It was found that the polypeptide is produced and is associated with the cytoplasmic membrane in the absence of subunit I, and is not associated with any of the three cytochrome components of the complex. Oxidase activity and heme binding are restored when the subunit I is expressed in the same cells using a second compatible plasmid. It has been previously demonstrated that subunit I, expressed in the absence of subunit II, contains cytochrome b-558, one of the three heme prosthetic groups found in the oxidase. Association of the two other heme moieties, cytochromes b-595 and d, apparently requires the association of the two subunits, and must be a late step in the assembly of the membrane-bound protein. It was also shown that under heme-deficient conditions, the two polypeptide subunits are expressed and are associated with the cytoplasmic membrane.