Evidence for interdomain interaction in the Escherichia coli repressor of biotin biosynthesis from studies of an N-terminal domain deletion mutant

The Escherichia coli repressor of biotin biosynthesis (BirA) is an allosteric site-specific DNA binding protein. The protein is composed of three structural domains. Contact with the biotin operator (bioO) in the transcriptional repression complex is made by the N-terminal domain which contains a helix-turn-helix structural module. The central domain is required for the catalytic functions of BirA including synthesis of biotinyl-5'-AMP from substrates ATP and transfer of biotin from the adenylate to a lysine residue of the biotin carboxyl carrier protein (BCCP) of acetyl CoA carboxylase. The adenylate serves not only as the activated intermediate in the biotin transfer reaction but also as the positive allosteric effector for site-specific DNA binding. Little interaction between the N-terminal and central domains is observed in the ape-repressor structure (Wilson et al., 1992). In this work, we have engineered an N-terminal deletion mutant of BirA, BirA65-321. Biochemical analysis of the purified truncated repressor indicates that, as expected, it does not bind to biotin operator DNA. BirA65-321 is, moreover, identical to intact BirA in catalysis of synthesis of bio-5'-AMP and in transfer of biotin from the adenylate to BCCP. Deletion of the DNA binding domain severely compromises the ability of BirA to bind to biotin or bio-5'-AMP. The affinity of BirA65-321 for biotin is decreased 100-fold while that for bio-5'-AMP is decreased 1000-fold, relative to intact BirA. The significant functional role of the DNA binding domain in tight binding of the two ligands to the central domain may be indicative of formation of extensive interdomain contacts in the holorepressor structure.