Which mechanism is responsible for the negative regulation of tryptophan expression?

The correct response highlights that the Trp repressor binding to the DNA is the mechanism responsible for the negative regulation of tryptophan expression. In the presence of sufficient tryptophan in the cell, tryptophan molecules bind to the Trp repressor protein, causing a conformational change in the repressor. This active form of the repressor then attaches to a specific region of the DNA, typically located at the operator site of the trp operon.

When the Trp repressor is bound to the operator, it inhibits the binding of RNA polymerase (RNAP) to the promoter region, preventing the transcription of genes involved in tryptophan biosynthesis. This regulation ensures that the synthesis of tryptophan is curtailed when the amino acid is present in adequate amounts, conserving resources for the cell.

Understanding this mechanism illustrates how genes can be turned off in response to the presence of specific metabolites, which is a fundamental concept in gene regulation and metabolic control in organisms.