A map of the biotin repressor-biotin operator interface: binding of a winged helix-turn-helix protein dimer to a forty base-pair site.

The Escherichia coli biotin repressor is a member of the "winged helix-turn-helix" class of site-specific DNA binding proteins. The protein binds as a dimer to the 40 bp biotin operator sequence. Although the structure of the aporepressor has been solved by X-ray crystallographic techniques, no structure of the holorepressor-DNA complex is yet available. In order to characterize the structural features of the biotin repressor-biotin operator interface we have applied a number of solution techniques including DNase I, hydroxyl radical and dimethyl sulfate footprinting and the circular permutation or "bending" assay. Results of these combined studies indicate that each repressor monomer forms a bipartite interface with each half-site of the biotin operator sequence. The results imply that, in addition to the helix-turn-helix module of each monomer, a second structural element participates in the protein-DNA interface. The two bipartite protein-DNA interfaces appear, moreover, to primarily involve the two 12 bp termini of the operator site. Results of combined DNase I footprinting and circular permutation analysis indicate, furthermore, that the central 16 bp region that links the two termini becomes distorted concomitant with binding of holoBirA.