In lac-operon , which protein is not regulated by the repressor?

Options

A. Galactosidase
B. Lactose permease
C. Tryptophan synthetase
D. Transacetylase

Answer

To solve the question regarding which protein is not regulated by the repressor in the lac operon, we will follow these steps:

### Step 1: Understand the lac operon
The lac operon is a set of genes in bacteria that are involved in the metabolism of lactose. It consists of three structural genes: lacZ, lacY, and lacA, which code for the enzymes β-galactosidase, lactose permease, and transacetylase, respectively. The operon is regulated by the presence or absence of lactose and the repressor protein.

**Hint:** Remember that the lac operon is specifically involved in lactose metabolism.

### Step 2: Identify the role of the repressor
In the lac operon, the repressor protein binds to the operator region and prevents transcription of the structural genes when lactose is absent. When lactose is present, it binds to the repressor, causing a conformational change that releases the repressor from the operator, allowing transcription to occur.

**Hint:** Focus on how the presence of lactose affects the repressor's function.

### Step 3: Analyze the options
- **A. Galactosidase (β-galactosidase)**: This enzyme is produced in the presence of lactose and is regulated by the lac operon.
- **B. Lactose permease**: This enzyme facilitates the entry of lactose into the cell and is also regulated by the lac operon.
- **C. Tryptophan synthetase**: This enzyme is involved in the synthesis of tryptophan and is regulated by the TRP operon, not the lac operon.
- **D. Transacetylase**: This enzyme is also produced as part of the lac operon and is involved in lactose metabolism.

**Hint:** Identify which enzyme is associated with a different operon.

### Step 4: Conclusion
From the analysis, it is clear that tryptophan synthetase (Option C) is not regulated by the lac operon repressor, as it is part of the TRP operon instead.

**Final Answer:** The protein that is not regulated by the repressor in the lac operon is **C. Tryptophan synthetase**.