Dimerization of the Escherichia coli biotin repressor: Corepressor function in protein assembly

The repressor of biotin biosynthesis binds to the biotin operator sequence to repress transcription initiation at the biotin biosynthetic operon. Site specific binding of; BirA to the biotin operator is allosterically regulat ed by binding of the small molecule, biotinyl-5'-adenylate (bio5'-AMP). The operator is a 40 base pair imperfect inverted palindrome and two holorepre ssor monomers bind cooperatively to the two operator half-sites. Results of previous detailed analyses of binding of holoBirA to bioO indicate that si te-specific DNA binding and protein dimerization are obligatorily linked in the system. In the present work equilibrium sedimentation measurements hav e been used to examine the assembly properties of the aporepressor and its complexes with small ligands biotin and bio-5'-AMP. Results of these measur ements indicate that while the free protein and the biotin complex exhibit no tendency to self-associate, the adenylate-bound protein assembles into d imers with an equilibrium constant of 11 mu M. The results suggest that one mechanism by which the adenylate promotes binding of BirA to the biotin op erator is by promoting repressor dimerization.