Evidence of an unusually long operator for the fur repressor in the aerobactin promoter of Escherichia coli

Production of the siderophore aerobactin in Escherichia coli is transcripti onally metalloregulated through the iron-dependent binding of the Fur (ferr ic uptake regulator) to a large region (>100 base pairs) within the cognate promoter in the pColV-K30 plasmid. We show in this article that such an un usually long operator results from the specific addition of degenerate repe ats 5'-NAT(APT)AT-3' and not from a fortuitous occupation of the DNA adjace nt to the primary binding sites by an excess of the repressor. Furthermore, the protection pattern revealed by DNase I and hydroxyl radical footprinti ng reflected a side-by-side oligomerization of the protein along an extende d DNA stretch. This type of DNA-protein interactions is more like those obs erved in some eukaryotic factors and nucleoid-associated proteins than typi cal of specific prokaryotic regulators.