Cytochrome caa(3) from Bacillus subtilis is a member of the heme-copper oxidase family of integral membrane enzymes that includes mitochondrial cytochrome c oxidase. Subunit 11 of cytochrome caa3 has an extra 100 amino acids at its C-terminus, relative to its mitochondrial counterpart, and this extension encodes a heme C binding domain. Cytochrome caa3 has many of the properties of the complex formed between mitochondrial cytochrome c and mitochondrial cytochrome c oxidase. To examine more closely the interaction between cytochrome c and the oxidase we have cloned and expressed the Cu-A-cytochrome c portion of subunit 11 from the cytochrome caa3 complex of B. subtilis. We are able to express about 2000 nmol, equivalent to 65 mg, of the Cu-A-cytochrome c protein per litre of Escherichia coli culture. About 500 nmol is correctly targeted to the periplasmic space and we purify 50% of that by a combination of affinity chromatography and ammonium sulfate fractionation. The cytochrome c containing sub-domain is well-folded with a stable environment around the heme C center, as its mid-point potential and rates of reduction are indistinguishable from values for the cytochrome c domain of the holo-enzyme. However, the Cu-A site lacks copper leading to an inherent instability in this sub-domain. Expression of B. subtilis cytochrome c, as exemplified by the Cu-A-cytochrome c protein, can be achieved in E. coli, and we conclude that the cytochrome c and Cu-A sub-domains behave independently despite their close physical and functional association. (c) 2004 Elsevier Inc. All rights reserved.