NO sensing by FNR: regulation of the Escherichia coli NO-detoxifying flavohaemoglobin, Hmp

Hugo Cruz-Ramos, Jason Crack, Guanghui Wu, Martin N. Hughes, Colin Scott, Andrew J. Thomson, Jeffrey Green, Robert K. Poole

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249 Citations (Scopus)


Nitric oxide (NO) is a signalling and defence molecule of major importance in biology. The flavohaemoglobin Hmp of Escherichia coli is involved in protective responses to NO. Because hmp gene transcription is repressed by the O2-responsive regulator FNR, we investigated whether FNR also senses NO. The [4Fe-4S]2+ cluster of FNR is oxygen labile and controls protein dimerization and site-specific DNA binding. NO reacts anaerobically with the Fe-S cluster of purified FNR, generating spectral changes consistent with formation of a dinitrosyl-iron-cysteine complex. NO-inactivated FNR can be reconstituted, suggesting physiological relevance. FNR binds at an FNR box within the hmp promoter (Phmp). FNR samples inactivated by either 0y2 or NO bind specifically to Phmp, but with lower affinity. Dose-dependent up-regulation of Phmp in vivo by NO concentrations of pathophysiological relevance is abolished by fnr mutation, and NO also modulates expression from model FNR-regulated promoters. Thus, FNR can respond to not only O2, but also NO, with major implications for global gene regulation in bacteria. We propose an NO-mediated mechanism of hmp regulation by which E. coli responds to NO challenge.

Original languageEnglish
Pages (from-to)3235-3244
Number of pages10
JournalEMBO Journal
Issue number13
Publication statusPublished - 1 Jul 2002
Externally publishedYes


  • flavohaemoglobin
  • FNR
  • gene regulation
  • iron-sulfur clusters
  • nitric oxide sensing


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