Molecular characterization of the ecfIR-bfrH locus of Bordetella bronchiseptica : Role of an ECF sigma factor
Burgos, Jonathan M.
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Iron (Fe) in soluble elemental form is found in the tissues and fluids of animals at concentrations insufficient for sustaining growth of bacteria. Consequently, to promote colonization and persistence, pathogenic bacteria evolved a myriad of scavenging mechanisms to acquire Fe from the host. Bordetella bronchiseptica , the etiologic agent of upper respiratory infections in a wide range of mammalian hosts, expresses a number of proteins for acquisition of Fe. Using proteomic and genomic approaches, three Fe-regulated genes in the Bordetellae were identified: bfrH , a gene encoding a putative siderophore receptor, ecfI , a gene encoding a putative extracellular function (ECF) sigma factor, and ecfR , a gene encoding a putative EcfI modulator. All three genes are highly conserved in B. pertussis, B. parapertussis , and B. avium . Genetic analysis revealed that transcription of bfrH was co-regulated by EcfI and EcfR, and by fur1 , one of two fur homologues encoded by B. bronchiseptica . Overexpression of ecfI decoupled bfrH from Fe-dependent regulation. In contrast, expression of bfrH was significantly reduced in an ecfI deletion mutant. Deletion of ecfR , however, was correlated with a significant increase in expression of bfrH due, in part, to a cis -acting nucleotide (nt) sequence located within ecfR which likely reduces the frequency of read-through transcription of bfrH from the ecfIR promoter. Using a murine competition infection model, bfrH was shown to be required for optimal virulence of B. bronchiseptica . The role of BfrH as a siderophore receptor was also evaluated utilizing a newly developed plate assay. Results, however, provided inconclusive data regarding the function of BfrH as a siderophore receptor. This study demonstrated ecfIR-bfrH to be another member of the growing family of Fe and ECF sigma factor-modulated regulons.