Bacterial denitrification is
expressed
in response to the concurrent exogenous signals of low-oxygen tension
and
nitrate or one of its reduction products. The mechanism by which
nitrate-dependent
gene activation is effected was investigated in the denitrifying
bacterium
Pseudomonas stutzeri ATCC 14405. We have identified and isolated
from this organism the chromosomal region encoding the two-component
sensor-regulator
pair NarXL and found that it is linked with the narG operon for
respiratory nitrate reductase. The same region encodes two putative
nitrate
or nitrite translocases, NarK and NarC (the latter shows the highest
similarity
to yeast [Pichia] and plant [Nicotiana] nitrate
transporters),
and the nitrate-regulated transcription factor, DnrE, of the FNR
family.
The roles of NarX and NarL in nitrate respiration were studied with
deletion
mutants. NarL activated the transcription of narG, narK,
and dnrE but did not affect the denitrification regulons for
the
respiratory substrates nitrite, nitric oxide, and nitrous oxide. The
promoters
of narG, narK, and dnrE carry sequence motifs,
TACYYMT,
which correspond to the NarL recognition sequence established for Escherichia
coli. The cellular response toward nitrate and nitrite was mediated
by the sensor protein NarX, which discriminated weakly between these
oxyanions.
Our data show that the NarXL two-component regulatory system has been
incorporated
into the bacterial denitrification process of P. stutzeri for
selective
regulation of nitrate respiration.