Aporepressor is inactive repressor in Tryptophan repressible operon system. It is

Options

A. always protein
B. may be protein or a nucleic acid
C. a metabolite
D. a substrate

Answer

### Step-by-Step Solution:

1. **Understanding the Tryptophan Repressible Operon System**:
- The tryptophan operon is a set of genes responsible for the synthesis of tryptophan in bacteria. It includes five structural genes (A, B, C, D, and E) that code for enzymes involved in tryptophan production.

2. **Components of the Operon**:
- The operon consists of structural genes, a promoter, and an operator. The promoter is where RNA polymerase binds to initiate transcription, while the operator is a regulatory sequence where the repressor can bind.

3. **Role of the Repressor**:
- The repressor is a protein that can inhibit the transcription of the operon. It is inactive when not bound to tryptophan, which is why it is referred to as an "aporepressor."

4. **Activation of the Repressor**:
- When tryptophan is present in the environment, it binds to the aporepressor, converting it into an active repressor. This active repressor then binds to the operator, blocking RNA polymerase from transcribing the genes necessary for tryptophan synthesis.

5. **Function in Absence of Tryptophan**:
- If tryptophan is not present, the aporepressor remains inactive and does not bind to the operator. This allows RNA polymerase to transcribe the genes, leading to the synthesis of tryptophan.

6. **Conclusion**:
- The aporepressor is an inactive form of the repressor protein that requires tryptophan (the corepressor) to become active. Therefore, the answer to the question is that the aporepressor is a **protein**.