The structure that controls access to the structural genes in the TRP operon is called what?

In the context of the TRP operon, the structure that controls access to the structural genes is the operator. The operator acts as a regulatory sequence that serves as a binding site for the repressor protein. When the repressor is bound to the operator, it prevents RNA polymerase from transcribing the downstream structural genes. This mechanism is crucial for the operon's function in gene regulation, allowing the cell to efficiently control the synthesis of tryptophan based on its availability.

This regulation is vital for conserving cellular resources, as transcription of the genes responsible for tryptophan synthesis only occurs when tryptophan levels are low. In summary, the operator's role in mediating the interaction between the regulatory proteins and the structural genes is fundamental to the operon model, making it the correct answer to the question.