Regulation of the nitric oxide reduction operon (norRVW) in Escherichia coli -: Role of norR and σ54 in the nitric oxide stress response

Nitric oxide (NO) induces NO-detoxifying enzymes in Escherichia coli suggesting sensitive mechanisms for coordinate control of NO defense genes in response to NO stress. Exposure of E. coli to sub-micromolar NO levels under anaerobic conditions rapidly induced transcription of the NO reductase (NOR) structural genes, norV and norW, as monitored by lac gene fusions. Disruption of rpoN (sigma(54)) impaired the NO-mediated induction of norV and norW transcription and NOR expression, whereas disruption of the upstream regulatory gene, norR, completely ablated NOR induction. NOR inducibility was restored to NorR null mutants by expressing NorR in trans. Furthermore, an internal deletion of the N-terminal domain of NorR activated NOR expression independent of NO exposure. Neither NorR nor sigma(54) was essential for NO-mediated induction of the NO dioxygenase (flavohemoglobin) encoded by hmp. However, elevated NOR activity inhibited NO dioxygenase induction, and, in the presence of dioxygen, NO dioxygenase inhibited norV induction by NO. The results demonstrate the role of NorR as a sigma(54)-dependent regulator of norVW expression. A role for the NorR N-terminal domain as a transducer or sensor for NO is suggested.