Nitric oxide signaling and NO dependent transcriptional control in bacterial denitrification by members of the FNR-CRP regulator family.

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.