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The mechanism involved in transcriptional repression of thefepA-fes divergent promoters ofEscherichia coli by the Fur (ferric uptake regulation) protein has been examined in vitro. This DNA region includes a suboptimal and single Fur-binding site with two divergent and overlapped −35/−10 hexamers. Comparison of transcription patterns generated with runoff experiments in either the presence or the absence of heparin showed that access of the RNA polymerase to the principal −35/−10 hexamers was fully prevented by Fur-Mn2+ bound to its target site within the divergent promoter region. Neither RNA polymerase bound to thefes andfepA promoters could be displaced by Fur-Mn2+ , nor could the bound repressor be outcompeted by an excess of the enzyme. However, the repressor blocked reinitiation as soon as the polymerase moved away from thefes promoter during transcription. The spatial distribution of regulatory elements within the DNA region allowed the simultaneous binding of the RNA polymerase to thefes andfepA promoters and their coordinate regulation regardless of their different transcriptional activities. Comparisons with other iron-regulated systems support a general mechanism for Fur-controlled promoters that implies a direct competition between the polymerase and the regulator for overlapping target sites in the DNA.

Coordinated Repression In Vitro of the Divergent fepA-fes Promoters of Escherichia coli by the Iron Uptake Regulation (Fur) Protein