The Small Protein CydX Is Required for Cytochrome Quinol Oxidase Stability and Function in Salmonella enterica Serovar Typhimurium: a Phenotypic Study.

Cytochrome quinol oxidases, which have a greater affinity for oxygen than heme-copper cytochrome oxidases (HCOs), promote bacterial respiration and fitness in low-oxygen environments, such as host tissues. Here, we show that, in addition to the CydA and CydB subunits, the small protein CydX is required for the assembly and function of the cytochrome complex in the enteric pathogen serovar Typhimurium. Mutant Typhimurium lacking CydX showed a loss of proper heme arrangement and impaired oxidase activity comparable to that of a Δ mutant lacking all cytochrome subunits.