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  • The EMBO Journal (2002) 21, 3235 - 3244
  • doi:10.1093/emboj/cdf339

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

Hugo Cruz-Ramos1, Jason Crack2, Guanghui Wu1, Martin N. Hughes3, Colin Scott1, Andrew J. Thomson2, Jeffrey Green1 and Robert K. Poole1

  1. The Krebs Institute for Biomolecular Research, Department of Molecular Biology and Biotechnology, The University of Sheffield, Sheffield S10 2TN, UK
  2. Centre for Metalloprotein Spectroscopy and Biology, School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, UK
  3. Chemistry Department, King's College London, Strand, London WC2R 2LS, UK

Correspondence to:

Robert K. Poole, E-mail: r.poole@sheffield.ac.uk

Received 1 February 2002; Accepted 8 May 2002; Revised 8 May 2002

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 O2 or NO bind specifically to Phmp, but with lower affinity. Dose-dependent up-regulation of Phmpin 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.

  • Keywords:

    • flavohaemoglobin,
    • FNR,
    • gene regulation,
    • iron–sulfur clusters,
    • nitric oxide sensing