CHARACTERIZATION OF NONINDUCIBLE TET REPRESSOR MUTANTS SUGGESTS CONFORMATIONAL-CHANGES NECESSARY FOR INDUCTION

Non-inducible tetracycline repressor (TetR) mutants were grouped in th ree structurally distinct classes. We quantitated in vivo operator bin ding, inducibility, and in vitro tetracycline binding of mutants from each class. Mutation of residues close to tetracycline (class 1) leads to reduced affinity for the drug. Mutation of residues located at the connection of the DNA-reading head with the protein core (class 2) an d at the dimerization interface (class 3) bind inducer with the same a ffinity as wild-type TetR. These mutations interfere with the induced, but not the operator-binding conformation of TetR. The affinity of so me class 1 mutants for tetracycline is less affected than their induci bility, suggesting that the mutated residues are important for trigger ing those conformational changes necessary for induction.