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

Pseudomonas aeruginosa has one A-type (caa(3)) and multiple C-type (cbb(3)) cytochrome c oxidases as well as two quinol oxidases for aerobic respiration. The caa(3) 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 caa(3) exhibited normal growth under any test conditions, but it had low relative fitness under carbon starvation conditions, indicating that the expression of caa(3) 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 caa(3). However, suppressor mutants that grew aerobically using caa(3) 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 caa(3) was higher in the suppressor mutants, their growth rates were lower than when the other terminal oxidases were utilized, suggesting that caa(3) 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 caa(3) 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.