Spectroscopic Investigations of Conformational Changes Induced in the Tet Repressor Protein Upon Effector Binding

Abstract

The Tn 10 encoded tetracycline resistance of gram negative bacteria is a mechanism by which active tetracycline efflux is mediated by a membrane standing protein, Tet Antiporter. Since this process causes a severe loss of membrane potential it is strictly regulated on the transcriptional level. In the presence of tetracycline in the cytoplasm, two tetracycline molecules are forming a complex with two M²⁺ ions and the TetR^B protein, thus changing the protein structure and concomitantly the binding constant to DNA (TetOl and TetO2). TetR^B is active as a dimer having a total number of 414 residues. The α-helices 2 and 3 provide the helix turn helix DNA binding motif, whereas the other helices form the core of the protein with the two tetracycline binding sites. It was suggested that α-helix 9 and the preceeding loop are involved in the structural changes induced by the effector binding. Therefore, site directed mutagenesis was used to generate triple mutants which lack the naturally occurring tryptophanes (trp) W43 and W75 and possess instead one trp in position 164-171 as fluorescence label. (Y43F75W164–171). The interference of the substituted trp with the binding pocket and, if present, the chosen effector anhydrotetracycline (ATC) depends sensitively on the actual position and can be monitored conveniently by the fluorescence of trp (and /or ATC).