title: RhuR, an extracytoplasmic function sigma factor activator, is essential for heme-dependent expression of the outer membrane heme and hemoprotein receptor of Bordetella avium.
abstract: 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.
authors.affiliation: 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, USA.
authors.forename: Amy E
authors.initials: AE
authors.lastname: Kirby
authors.forename: Natalie D
authors.initials: ND
authors.lastname: King
authors.forename: Terry D
authors.initials: TD
authors.lastname: Connell
mesh_terms: D000595:Amino Acid Sequence; D001425:Bacterial Outer Membrane Proteins / Q000378:metabolism*; D001426:Bacterial Proteins / Q000502:physiology*; D001884:Bordetella / Q000254:growth & development / Q000378:metabolism*; D006418:Heme / Q000378:metabolism*; D006420:Hemeproteins / Q000378:metabolism*; D007501:Iron / Q000378:metabolism; D008969:Molecular Sequence Data; D012097:Repressor Proteins / Q000502:physiology; D012808:Sigma Factor / Q000737:chemistry / Q000502:physiology*
chemical_list: D001425:Bacterial Outer Membrane Proteins; D001426:Bacterial Proteins; D006420:Hemeproteins; D012097:Repressor Proteins; C439648:RhuI protein, Bordetella avium; C093265:SigX protein, Bacillus subtilis; D012808:Sigma Factor; C068477:ferric uptake regulating proteins, bacterial; D006418:Heme; D007501:Iron