A temperature dependent transition in the Pribnow box of the trp promoter

Proton NMR spectra of the trp operator-promoter (sequence CGTACTAGTT.AACTAGTACG) show selective changes in chemical shift and relaxation rates over the range of temperature 0-45 degrees C for the non-exchangeable protons of A11 and A12 only. These bases are in the centre of the Pribnow box. The changes imply that at least three conformational states become significantly populated in this range of temperature, and probably involve a change in the propellor twists of A11 and A12 for one transition, and changes in the helical twist and local pitch for the other. As (1) mutations in the Pribnow box that destroy the TAA sequence impair promoter activity, and (2) the abortive initiation assay for RNA polymerase shows a transition near 20 degrees C, we propose that the observed conformational transitions in the trp promoter are an essential feature of good promoters.