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Infection and Immunity, February 2004, p. 896-907, Vol. 72, No. 2
0019-9567/04/$08.00+0     DOI: 10.1128/IAI.72.2.896-907.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

RhuR, an Extracytoplasmic Function Sigma Factor Activator, Is Essential for Heme-Dependent Expression of the Outer Membrane Heme and Hemoprotein Receptor of Bordetella avium

The Witebsky Center for Microbial Pathogenesis and Immunology, Department of Microbiology and Immunology, The University of Buffalo, The State University of New York, Buffalo, New York 14214

Received 20 August 2003/ Returned for modification 20 September 2003/ Accepted 23 October 2003

Genes involved in iron (Fe) acquisition often are regulated in response to the local availability of Fe. In many bacteria, Fe-dependent responsiveness is mediated by Fur, a global Fe-dependent transcriptional repressor. Tighter regulatory control of Fur-responsive genes is afforded by incorporating additional regulators into Fur-dependent regulatory cascades. RhuI, a Fur-dependent extracytoplasmic function sigma factor of Bordetella avium, in response to the dual stimulation of Fe starvation and the presence of heme (or hemoproteins), regulates P_bhuR, a heme-responsive promoter which directs expression of the bhuRSTUV heme utilization operon. While BhuR, the outer membrane heme receptor, and RhuI have been shown to be indispensable for heme-dependent activation of P_bhuR, collateral components of the regulatory cascade have not been described. In this investigation, RhuR, an integral cytoplasmic membrane protein with homology to anti-sigma factors, is shown to be an essential activator of P_bhuR expression. The functional domain of RhuR required for heme-dependent activation of P_bhuR expression was mapped to the N-terminal 97 amino acids of the protein by use of a chimeric RhuR-BlaM fusion. Expression of the chimera in a rhuR mutant rendered P_bhuR constitutive, thereby decoupling the promoter from heme dependency. Growth studies confirmed that B. avium requires RhuR for optimal utilization of hemoglobin, but not hemin, as a sole source of nutrient Fe. These data imply that B. avium expresses, in addition to the BhuR heme/hemoprotein utilization system, an alternative RhuR-independent heme utilization mechanism. A model is proposed in which RhuR is the functional bridge between BhuR and RhuI in a heme-dependent regulatory cascade.

Editor: J. T. Barbieri

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