Differences in two Pseudomonas aeruginosa cbb3 cytochrome oxidases

Bacterial cytochrome cbb3 oxidases are members of the haeme‐copper oxidase superfamily that are important for energy conservation by a variety of proteobacteria under oxygen‐limiting conditions. The opportunistic pathogen Pseudomonas aeruginosa is unusual in possessing two operons that each potentially encode a cbb3 oxidase (cbb3‐1 or cbb3‐2). Our results demonstrate that, unlike typical enzymes of this class, the cbb3‐1 oxidase has an important metabolic function at high oxygen tensions. In highly aerated cultures, cbb3‐1 abundance and expression were greater than that of cbb3‐2, and only loss of cbb3‐1 influenced growth. Also, the activity of cbb3‐1, not cbb3‐2, inhibited expression of the alternative oxidase CioAB and thus influenced a signal transduction pathway much like that found in the α‐proteobacterium Rhodobacter sphaeroides. Cbb3‐2 appeared to play a more significant role under oxygen limitation by nature of its increased abundance and expression compared to highly aerated cultures, and the regulation of the cbb3‐2 operon by the putative iron‐sulphur protein Anr. These results indicate that each of the two P. aeruginosa cbb3 isoforms have assumed specialized energetic and regulatory roles.