Fine-tuned regulation of the dissimilatory nitrite reductase gene by oxygen and nitric oxide in Pseudomonas aeruginosa

Nitrite reductase (NIR) catalyses the reduction of nitrite to nitric oxide (NO) in the denitrification pathway. In Pseudomonas aeruginosa, expression of the gene encoding NIR (nirS) is induced by NO and is under control of the NO-sensing regulator DNR (dissimilatory nitrate respiration regulator). Because DNR is under control of the oxygen-sensing regulator ANR (anaerobic regulator of arginine deiminase and nitrate reductase), nirS is expressed only under low oxygen and anaerobic conditions. Both ANR and DNR are FNR (fumarate and nitrate reductase regulator)-type regulators and recognize the consensus FNR-binding motif. The motif of the nirS promoter is thought to be recognized only by DNR, and not by ANR. Here, mutant strains expressing either ANR or DNR were constructed and used to analyse the role of ANR and DNR in the activation of nirS expression. Analysis of transcriptional activity by microarray and quantitative reverse transcription polymerase chain reaction revealed that nirS is transcribed under low oxygen conditions in an ANR-dependent manner, although the expression level was 10-fold lower than that of the DNR-dependent expression. An artificial promoter containing the FNR-binding motif of the nirS promoter was also twofold upregulated by ANR. These results indicate that low-level expression of NIR in the presence of nitrite may provide NO as a trigger for the full expression of denitrification genes when oxygen is depleted.