FIG. 7.
Genetic and expression model of the ecfIR-bfrH locus. Under conditions of Fe sufficiency, Fur1 represses transcription of the ecfIR-bfrH locus. Upon encountering Fe-limiting conditions, Fur1 dissociates from PecfIR and possibly also from PbfrH, thus derepressing expression of ecfI and ecfR and enabling readthrough transcription from PecfIR into bfrH. At the genetic level, a cis-acting element (solid black line) in ecfR limits readthrough transcription of bfrH from PecfIR. A transduction signal initiated by BfrH after binding of the extracellular inducing ligand (an unknown siderophore) is transmitted through EcfR and EcfI to highly upregulate expression of bfrH from PbfrH. The extracellular inducing ligand for the ecfIR-bfrH locus is believed to be an unknown xenosiderophore. OM, outer membrane; IM, cytoplasmic membrane; RNAP, RNA core polymerase.