Tryptophan operon is
(a) Catabolic system
(b) Repressible system
(c) Inducible
(d) Having three structural genes

Options

A. Catabolic system
B. Repressible system
C. Inducible
D. Having three structural genes

Answer

To solve the question regarding the Tryptophan operon, let's analyze each option step by step:

### Step 1: Understanding the Tryptophan Operon
The Tryptophan operon, also known as the tryp operon, is a cluster of genes involved in the biosynthesis of the amino acid tryptophan in bacteria.

### Step 2: Evaluating the Options
1. **Option (a) Catabolic system**:
- The Tryptophan operon is not primarily involved in breaking down substances (catabolism); instead, it is involved in the synthesis of tryptophan. Therefore, this option is incorrect.

2. **Option (b) Repressible system**:
- The Tryptophan operon is indeed a repressible operon. It is turned off (repressed) when tryptophan is present in sufficient quantities, as the presence of tryptophan inhibits the transcription of the operon. This option is correct.

3. **Option (c) Inducible**:
- An inducible operon is one that is turned on in response to a specific substance. The lac operon is an example of this. The Tryptophan operon does not function this way; hence, this option is incorrect.

4. **Option (d) Having three structural genes**:
- The Tryptophan operon actually contains five structural genes, not three. Therefore, this option is also incorrect.

### Conclusion
Based on the analysis, the correct answer is **(b) Repressible system**.