Abstract
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.
Highlights
The bacterial A-type cytochrome c oxidase is a member of the heme-copper oxidase superfamily and is closely related to the mitochondrial terminal oxidase [1]
In our previous study [14], we found that a quadruple mutant strain designated as QXAa, which was deficient in the four terminal oxidase gene clusters encoding two C-type cytochrome c oxidases and two quinol oxidases, was unable to grow under aerobic conditions, thereby suggesting that the cox genes were tightly repressed or the gene product was not functional in the mutant
The aerobic growth profile of SAa did not differ from that of the wildtype stain (WT) in nutrient-rich LB medium or synthetic medium supplemented with carbon sources, i.e., glucose, glutamate, or succinate
Summary
The bacterial A-type (aa3-type) cytochrome c oxidase is a member of the heme-copper oxidase superfamily and is closely related to the mitochondrial terminal oxidase [1]. This enzyme comprises of three core subunits and it catalyzes four electron reduction of oxygen to water at the end of the respiratory chain. The main catalytic subunit (subunit I) has a binuclear catalytic center, which comprises a high spin heme a3 and CuB. There was no additional external funding received for this study. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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