Final answer:
The trp repressor protein has two binding sites for tryptophan, which, when bound, change the repressor's shape, allowing it to bind to the DNA operator sequence and inhibit the transcription of the trp operon genes.
Step-by-step explanation:
Binding Sites on the trp Repressor Protein
The trp repressor protein plays a vital role in regulating the expression of the trp operon, which is responsible for the synthesis of the amino acid tryptophan in bacteria such as E. coli. The trp repressor has specific binding sites critical for its function. It contains two tryptophan binding sites. These sites are where tryptophan molecules bind when they are present in the cell. This binding changes the shape and conformation of the trp repressor, allowing it to attach to another crucial site: the operator sequence on the DNA.
When tryptophan is not present, the repressor does not bind to the operator. Consequently, the operon becomes active, and the genes for synthesizing tryptophan are transcribed. However, when the tryptophan levels are high, it functions as a co-repressor: two molecules bind to the repressor and alter its conformation. This tryptophan-repressor complex can then bind to the operator sequence, preventing the RNA polymerase from transcribing the downstream genes needed for the synthesis of tryptophan, thus turning off the trp operon. The interaction between tryptophan, the repressor, and the operator is an elegant example of a negative feedback mechanism critical for cellular homeostasis.
This regulation mechanism is an example of a negatively regulated system, making the trp operon a classic model in molecular biology for understanding gene regulation.