Regulation of expression of the cyanide-insensitive terminal oxidase in Pseudomonas aeruginosa

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Abstract
The regulation of the cyanide-insensitive oxidase (CIO) inPseudomonas aeruginosa, a bacterium that can synthesize HCN, is reported. The expression of acioA–lacZtranscriptional fusion, CioA protein levels and CIO activity were low in exponential phase but induced about fivefold upon entry into stationary phase. Varying the O2transfer coefficient from 11·5 h−1to 87·4 h−1had no effect on CIO expression and no correlation was observed between CIO induction and the dissolved O2levels in the growth medium. However, a mutant deleted for the O2-sensitive transcriptional regulator ANR derepressed CIO expression in an O2-sensitive manner, with the highest induction occurring under low-O2conditions. Therefore, CIO expression can respond to a signal generated by low O2levels, but this response is normally kept in check by ANR repression. ANR may play an important role in preventing overexpression of the CIO in relation to other terminal oxidases. A component present in spent culture medium was able to induce CIO expression. However, experiments with purifiedN-butanoyl-l-homoserine lactone orN-(3-oxododecanoyl)homoserine lactone ruled out a role for these quorum-sensing molecules in the control of CIO expression. Cyanide was a potent inducer of the CIO at physiologically relevant concentrations and experiments using spent culture medium from a ΔhcnBmutant, which is unable to synthesize cyanide, showed that cyanide was the inducing factor present inP. aeruginosaspent culture medium. However, the finding that in a ΔhcnBmutantcioA–lacZexpression was induced normally upon entry into stationary phase indicated that cyanide was not the endogenous inducer of the terminal oxidase. The authors suggest that the failure of O2to have an effect on CIO expression in the wild-type can be explained either by the requirement for an additional, stationary-phase-specific inducing signal or by the loss of an exponential-phase-specific repressing signal.