The trp operon is repressible. This means it is usually __________ and is directly controlled by a(n) ___________ .
a. active / inducer
b. active / repressor
c. inactive / inducer
d. inactive /repressor

Verified step by step guidance

1. Understand the concept of a repressible operon: A repressible operon is typically active (turned on) under normal conditions and can be turned off (repressed) when a specific molecule is present.
2. Recall that the trp operon controls the synthesis of tryptophan, an amino acid, and it is usually active to produce enzymes for tryptophan synthesis when tryptophan levels are low.
3. Identify the regulatory molecule involved: In a repressible operon like the trp operon, the regulatory protein is a repressor that is inactive by itself but becomes active when bound to a corepressor (tryptophan in this case).
4. Recognize that the repressor protein directly controls the operon by binding to the operator region to block transcription when activated by the corepressor.
5. Conclude that the trp operon is usually active and is directly controlled by a repressor protein, matching option (b) active / repressor.

Key Concepts

Repressible Operon

A repressible operon is typically active, allowing gene expression under normal conditions. It can be turned off (repressed) when a specific molecule, often the end product of the pathway, binds to a regulatory protein, signaling that the gene products are no longer needed.

Role of the Repressor Protein

The repressor is a regulatory protein that binds to the operator region of the operon to block transcription. In repressible operons like the trp operon, the repressor is inactive by default and becomes active only when it binds to a corepressor molecule.

Corepressor Molecule

A corepressor is a small molecule, often the end product of the operon's biosynthetic pathway, that binds to the repressor protein. This binding activates the repressor, enabling it to attach to the operator and inhibit gene transcription.