Iron regulates transcription of the Escherichia coli ferric citrate transport genes directly and through the transcription initiation proteins.

Ferric citrate induces transcription of the ferric citrate transport genes fecABCDE in Escherichia coli by binding to the outer-membrane receptor protein FecA without entering the cell. Replete iron concentrations inhibit transcription of the fec transport system via the iron-loaded Fur repressor. Here we show that the Fur repressor activated by Mn2+ (used instead of Fe2+) binds to the promoter of the regulatory genes fecIR and to the promoter of fecABCDE. DNase I footprint analysis revealed that Mn2+-Fur (50 nM) protected 30 nucleotides of the coding strand and 24 nucleotides of the noncoding strand of the fecIR promoter. Higher amounts of Mn2+-Fur (100 nM) covered 41 nucleotides of the coding strand of the fecIR promoter and 38 nucleotides of the coding strand of the fecA promoter. The corresponding region of the noncoding strand of the fecA promoter was hypersensitive to DNase I. The results of a deletion analysis of the fecA promoter supported the previously assigned -35 and -10 regions and nucleotide position +11 for FecI-RNA polymerase interaction. Induction of fecIR transcription by iron limitation increased fecB-lacZ transcription 3.5-fold, whereas under constitutive fecIR transcription, iron limitation increased fecB-lacZ transcription twofold. The two iron-regulated sites of fec transport gene transcription suggest a fast response to sufficient intracellular iron concentrations by repression of fecABCDE transcription and a slower adaptation as the result of fecIR transcription inhibition.