Required for correct functioning of cytochrome bd-I oxidase. This protein and AppX may have some functional overlap. 2 a ubiquinol + 4 H(+)(in) + O2(in) = 2 a ubiquinone + 4 H(+)(out) + 2 H2O(in) Energy metabolism; oxidative phosphorylation. May be a subunit of cytochrome bd-I ubiquinol oxidase. Probably interacts with CydA and CydB. In sucrose cushions fractionates with both the inner and outer membranes. Constitutively expressed during aerobic growth (PubMed:19121005), induced in low oxygen, and maybe by SDS/EDTA (envelope stress) (PubMed:19734316) (at protein level). Slow growth in aerobic liquid culture, forms mixed colonies on aerobic solid media, increased sensitivity to reductant (beta-mercapoethanol). Reduced cytochrome bd-I oxidase activity. Belongs to the cytochrome ubiquinol oxidase subunit X family.