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After shifting an oxygen-respiring culture ofPseudomonas stutzeri to nitrate or nitrite respiration, we directly monitored the expression of thenirS gene by mRNA analysis.nirS encodes the 62-kDa subunit of the homodimeric cytochromecd 1 nitrite reductase involved in denitrification. Information was sought about the requirements for gene activation, potential regulators of such activation, and signal transduction pathways triggered by the alternative respiratory substrates. We found thatnirS , together withnirT andnirB (which encode tetra- and diheme cytochromes, respectively), is part of a 3.4-kb operon. In addition, we found a 2-kb monocistronic transcript. The half-life of each of these messages was approximately 13 min in denitrifying cells with a doubling time of around 2.5 h. When the culture was subjected to a low oxygen tension, we observed a transient expression ofnirS lasting for about 30 min. The continued transcription of thenirS operon required the presence of nitrate or nitrite. This anaerobically manifestedN -oxide response was maintained in nitrate sensor (NarX) and response regulator (NarL) knockout strains. Similar mRNA stability and transition kinetics were observed for thenorCB operon, encoding the NO reductase complex, and thenosZ gene, encoding nitrous oxide reductase. Our results suggest that a nitrate- and nitrite-responsive regulatory circuit independent of NarXL is necessary for the activation of denitrification genes.

journal of bacteriology

Kinetics of nirS Expression (Cytochrome cd 1 Nitrite Reductase) in Pseudomonas stutzeri during the Transition from Aerobic Respiration to Denitrification: Evidence for a Denitrification-Specific Nitrate- and Nitrite-Responsive Regulatory System