Specific expression and function of the A-type cytochrome c oxidase under starvation conditions in Pseudomonas aeruginosa

Pseudomonas aeruginosa has one A-type (caa₃) and multiple C-type (cbb₃) cytochrome c oxidases as well as two quinol oxidases for aerobic respiration. The caa₃ oxidase is highly efficient in creating a proton gradient across the cell membrane, but it is not expressed under normal growth conditions and its physiological role has not been investigated. In the present study, a mutant strain deficient in the coxBA-PA0107-coxC genes encoding caa₃ exhibited normal growth under any test conditions, but it had low relative fitness under carbon starvation conditions, indicating that the expression of caa₃ is advantageous under starvation conditions. A mutant that lacked four terminal oxidase gene clusters except for the cox genes was unable to grow aerobically because of low expression level of caa₃. However, suppressor mutants that grew aerobically using caa₃ as the only terminal oxidase emerged after aerobic subculturing. Analyses of the suppressor mutants revealed that a mutation of roxS encoding a sensor kinase of a two-component regulator RoxSR was necessary for the aerobic growth in synthetic medium. Two additional mutations in the 5′-flanking region of coxB were necessary for the aerobic growth in LB medium. Although the expression level of caa₃ was higher in the suppressor mutants, their growth rates were lower than when the other terminal oxidases were utilized, suggesting that caa₃ was not suited for utilization as the only terminal oxidase. Overexpression of the cox genes also inhibited the aerobic growth of the wild-type strain. These results indicate that caa₃ is tightly regulated to be expressed only under starvation conditions at low level and it functions in cooperation with other terminal oxidases to facilitate survival in nutrient starvation conditions.