COORDINATED REPRESSION IN-VITRO OF THE DIVERGENT FEPA-FES PROMOTERS OF ESCHERICHIA-COLI BY THE IRON UPTAKE REGULATION (FUR) PROTEIN

The mechanism involved in transcriptional repression of the fepA-fes d ivergent promoters of Escherichia coli by the Fur (ferric uptake regul ation) protein has been examined in vitro. This DNA region includes a suboptimal and single Fur-binding site: with two divergent and overlap ped -35/-10 hexamers. Comparison of transcription patterns generated w ith runoff experiments in either the presence or the absence of hepari n showed that access of the RNA polymerase to the principal -35/-10 he xamers was fully prevented by Fur-Mn2+ bound to its target site within the divergent promoter region. Neither RNA polymerase bound to the fe s and fepA promoters could be displaced by Fur-Mn2+, nor could the bou nd repressor be outcompeted by an excess of the enzyme. However, the r epressor blocked reinitiation as soon as the polymerase moved away fro m the fes promoter during transcription. The spatial distribution of r egulatory elements within the DNA region allowed the simultaneous bind ing of the RNA polymerase to the fes and fepA promoters and their coor dinate regulation regardless of their different transcriptional activi ties. Comparisons with other iron-regulated systems support a general mechanism for Fur-controlled promoters that implies a direct competiti on between the polymerase and the regulator for overlapping target sit es in the DNA.