Binding of L-branched-chain amino acids causes a conformational change in BkdR.

BkdR is the positive transcriptional activator of the inducible bkd operon of Pseudomonas putida. Evidence is accumulating that L-branched-chain amino acids are the inducers of the operon, and the data obtained in this study show that they induce a conformational change in BkdR. Addition of L-branched-chain amino acids increased the susceptibility of BkdR to trypsin with the cleavage between Arg-51 and Gln-52 on the C-terminal side of the DNA-binding domain. L-Valine also caused an increased fluorescence emission intensity and produced significant changes in the circular dichroism spectrum of BkdR. Analytical ultracentrifugation confirmed earlier data obtained from gel filtration that BkdR was a tetramer with a Stokes radius of 32 +/- 3 A and an axial ratio of 2:1.