After shifting an
oxygen-respiring
culture of Pseudomonas stutzeri to nitrate or nitrite
respiration,
we directly monitored the expression of the nirS gene by mRNA
analysis.
nirS
encodes the 62-kDa subunit of the homodimeric cytochrome cd1
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 that nirS, together with nirT and nirB
(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 of nirS
lasting for about 30 min. The continued transcription of the nirS
operon required the presence of nitrate or nitrite. This anaerobically
manifested N-oxide response was maintained in nitrate sensor
(NarX)
and response regulator (NarL) knockout strains. Similar mRNA stability
and transition kinetics were observed for the norCB operon,
encoding
the NO reductase complex, and the nosZ 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.