In Vitro Transcriptional Studies of thebkd Operon of Pseudomonas putida:l-Branched-Chain Amino Acids and d-Leucine Are the Inducers
ABSTRACT BkdR is the transcriptional activator of the bkdoperon, which encodes the four proteins of the branched-chain keto acid dehydrogenase multienzyme complex of Pseudomonas putida. In this study, hydroxyl radical footprinting revealed that BkdR bound to only one face of DNA over the same region identified in DNase I protection assays. Deletions of even a few bases in the 5′ region of the BkdR-binding site greatly reduced transcription, confirming that the entire protected region is necessary for transcription. In vitro transcription of the bkd operon was obtained by using a vector containing the bkdR-bkdA1 intergenic region plus the putative ρ-independent terminator of the bkdoperon. Substrate DNA, BkdR, and any of thel-branched-chain amino acids or d-leucine was required for transcription. Branched-chain keto acids,d-valine, and d-isoleucine did not promote transcription. Therefore, the l-branched-chain amino acids and d-leucine are the inducers of the bkdoperon. The concentration of l-valine required for half-maximal transcription was 2.8 mM, which is similar to that needed to cause half-maximal proteolysis due to a conformational change in BkdR. A model for transcriptional activation of the bkdoperon by BkdR during enzyme induction which incorporates these results is presented.