Bacterial denitrification transforms nitrate to dinitrogen. The
process
is expressed facultatively in response to environmental conditions.
Around
50 components make up the denitrification apparatus and its assembly
pathways.
We are beginning to understand how exogenous signals provided by oxygen
and N oxides are processed for activating the underlying gene programs.
Key signals are provided by nitrate, nitric oxide, and a low oxygen
tension.
In the genus Pseudomonas the nitrate signal is processed by a
two
component regulatory system which activates the nar operon encoding
respiratory
nitrate reductase. Nitric oxide is not only an essential respiratory
substrate
of the denitrifying cell but constitutes in nanomolar concentrations
also
a key signal for the expression of nitrite reductase and NO reductase
which
control cellular NO homeostasis. The signal pathway in the genera Pseudomonas,
Paracoccus and Rhodobacter involves regulators of the
FNR
family of transcription factors, which cluster phylogenetically in a
separate
subgroup. In contrast, Ralstonia eutropha requires a sigma-54
dependent
regulator of the NtrC family for the expression of its quinol-dependent
NO reductase. Important questions are directed currently at the
mechanism(s)
of activation of these transcription factors by NO, and avoidance of
crosstalk
with FNR factors at target promoters operating with identical
recognition
motifs.