Mutational studies with the trp repressor of Escherichia coli support the helix-turn-helix model of repressor recognition of operator DNA.

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Abstract
Several classes of trp repressor mutants were selected and analyzed in vivo. Mutants that produced repressors with either enhanced or reduced activity were obtained. One class of mutants produced inactive or slightly active repressors that were trans-dominant to the wild-type repressor. The amino acid substitutions in many of these repressors were clustered in a segment of the polypeptide that is homologous to the DNA recognition domain of the lambda cro repressor. A second functionally important region of the trp repressor was identified; this region could participate in L-tryptophan binding. Observations with trpR nonsense mutants suggest that the first 67 residues of the repressor polypeptide are sufficient for subunit association.