Might be part of cytochrome bd-II oxidase (appB and appC). Able to restore reductant resistance to a cydX deletion mutant upon overexpression. CydX and this protein may have some functional overlap. Able to interact with CydA and CydB upon overexpression. In sucrose cushions fractionates with the outer membrane, in sarcosyl fractionation with the inner membrane. May have its C-terminus in the cytoplasm (PubMed:21778229). Expressed during stationary phase (PubMed:19121005) and in minimal glycerol medium, at 45 degrees Celsius, strongly (30-fold) induced in low oxygen (PubMed:19734316) (at protein level). Not essential, no change in sensitivity to reductant (beta-mercaptoethanol). Belongs to the cytochrome ubiquinol oxidase subunit X family.