Architecture of a Fur binding site: a comparative analysis

Fur is an iron-binding transcriptional repressor that recognizes a 19-bp consensus site of the sequence 5'-GATAATGATAATCATTATC-3'. This site can be defined as three adjacent hexamers of the sequence 5'-GATAAT-3', with the third being slightly imperfect (an F-F-F configuration), or as two hexamers in the forward orientation separated by one base pair from a third hexamer in the reverse orientation (an F-F-x-R configuration). Although Fur can bind synthetic DNA sequences containing the F-F-F arrangement, most natural binding sites are variations of the F-F-x-R arrangement. The studies presented here compared the ability of Fur to recognize synthetic DNA sequences containing two to four adjacent hexamers with binding to sequences containing variations of the F-F-x-R arrangement (including natural operator sequences from the entS and fepB promoter regions of Escherichia coli). Gel retardation assays showed that the F-F-x-R architecture was necessary for high-affinity Fur-DNA interactions and that contiguous hexamers were not recognized as effectively. In addition, the stoichiometry of Fur at each binding site was determined, showing that Fur interacted with its minimal 19-bp binding site as two overlapping dimers. These data confirm the proposed overlapping-dimer binding model, where the unit of interaction with a single Fur dimer is two inverted hexamers separated by a C:G base pair, with two overlapping units comprising the 19-bp consensus binding site required for the high-affinity interaction with two Fur dimers.