Even if tryptophan was abundant, transcription of the operon would occur. By removing the operator, any impediment to transcriptional control is removed, and the operon is expressed under all conditions.
A nucleotide change in the sequence of the operator would result from a mutation in the trp operon's operator region.The trp operon responds to a repressor protein that binds to two tryptophan molecules. This repressor-tryptophan complex binds to the trp operator when tryptophan is abundant. Because this binding prevents RNA polymerase from binding, the operon is not transcribed. When there is little tryptophan in the cell, the trp repressor is inactive (because there is no tryptophan to bind to and activate).
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