Transcriptional analysis of the Escherichia coli bio operon

Biotin Sensing: Universal Influence of Biotin Status on Transcription

Although the role of biotin in metabolic reactions has long been recognized, its influence on transcription has only recently been discovered. A key protein in biotin-mediated transcription regulation is the biotin protein ligase, the enzyme responsible for catalyzing covalent linkage of the vitamin to biotin-dependent carboxylases. In the biotin regulatory system of Escherichia coli, the best characterized of the biotin-sensing systems, the biotin protein ligase functions both as the biotinylating enzyme and as a transcription repressor. Detailed mechanistic studies of this system are reviewed. In addition, recent studies have revealed other biotin-sensing systems in organisms ranging from bacteria to humans. These systems and the central role of the biotin protein ligase in each are also reviewed.

Coupling of site-specific DNA binding to protein dimerization in assembly of the biotin repressor-biotin operator complex

The Escherichia coli repressor of biotin biosynthesis, BirA, binds site-specifically to the biotin operator, a 40 base pair imperfect inverted palindrome. Two repressor monomers have been shown to bind to the two operator half-sites. Analysis of results of quantitative DNase I footprint titrations performed on the wild-type biotin operator template indicate that binding is well described by a cooperative mechanism. The data obtained from these studies were, however, insufficient to independently resolve all of the energetic parameters associated with cooperative binding of the two repressor monomers to the operator site. In this work, to further dissect the energetics of assembly of the biotin repressor-biotin operator complex, measurements of binding of BirA to four bioO variants designed to reduce the valency of repressor binding from 2 to 1 have been performed. Results of these measurements indicate, as was found with the wild-type biotin operator template, that two repressor monomers bind simultaneously to the two half-sites of all variant operators. Protein dimerization and DNA binding are thus obligatorily coupled in the biotin repressor system. Furthermore, the results suggest that, in the context of a cooperative binding mechanism, the cooperative free energy associated with the biotin repressor-biotin operator interaction is significantly more favorable than the previously estimated -2 kcal/mol.

Construction and characterization of a biotin-regulated gene expression system in Escherichia coli

An autoregulated gene expression system in Escherichia coli was designed such that the cloned genes on the vector were not expressed until biotin was depleted during cell growth. The expression vectors were constructed by assembling the DNA fragments containing the regulatory region of the E. coli biotin operon (bio operon), the universal ribosome-binding site (RBS) and the strong transcription terminator rrnBT1T2. The promoter region was further modified by site-directed mutagenesis to create promoters of varied strength. The feasibility of this system was examined in E. coli strain R901 (with bio operon deleted) using various marker genes, including the E. coli birA gene, T7 RNA polymerase gene and yellowfin-porgy growth-hormone gene. The results demonstrated that the induction of marker-gene expression can be triggered as the biotin concentration drops to a threshold value of approximately 2 ng/mL by metabolic utilization.

Isolation and characterization of the Erwinia herbicola bio operon and the sequences of the bioA and bioB genes

The biotin operon of Erwinia herbicola was cloned and characterized. The operon consists of five genes arranged in the order, bioABFCD. The operon is negatively regulated via the interaction of a proposed biotin repressor with an operator sequence that lies between the bioA and bioB genes. The nucleotide sequences of bioA (7,8-diaminopelargonic acid transferase), bioB (biotin synthetase) and the regulatory region were determined and analyzed. The deduced amino acid sequences of bioA and bioB are also aligned with currently available homologs to obtain the UPGMA (unweighted pair group method with arithmetic mean) evolutionary tree.