Required for correct functioning of cytochrome bd oxidase. 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 ubiquinol oxidase. Expressed in stationary phase. Part of the cydB-cydX operon. Highly attenuated in macrophage-like line J774 and female BALB/c ByJ mouse infections. Loss of viability in stationary phase growth in culture. Increased sensitivity to H(2)O(2), acid pH, highly sensitive to the combination of NaN(3) plus NiSO(4). Belongs to the cytochrome ubiquinol oxidase subunit X family. Extended N-terminus.