Regulation of the Nitric Oxide Reduction Operon (norRVW) inEscherichia coli: ROLE OF NorR AND ς54IN THE NITRIC OXIDE STRESS RESPONSE

Abstract 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 ofrpoN (ς54) 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 intrans. Furthermore, an internal deletion of the N-terminal domain of NorR activated NOR expression independent of NO exposure. Neither NorR nor ς54 was essential for NO-mediated induction of the NO dioxygenase (flavohemoglobin) encoded byhmp. 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 ς54-dependent regulator ofnorVW expression. A role for the NorR N-terminal domain as a transducer or sensor for NO is suggested.