A polycistronic structural gene of lac operon is regulated by

Options

A. A common promoter and operator genes
B. Promotor gene only
C. A common promoter and regulatory genes
D. Regulatory gene only

Answer

To answer the question about what regulates the polycistronic structural gene of the lac operon, we can break down the solution into the following steps:

### Step-by-Step Solution:

1. **Understanding the lac operon**:
- The lac operon is a well-studied example of gene regulation in prokaryotes, specifically in E. coli. It consists of structural genes that encode proteins necessary for the metabolism of lactose.

2. **Identifying the components of the lac operon**:
- The lac operon includes three structural genes:
- **lacZ**: encodes beta-galactosidase, which breaks down lactose into glucose and galactose.
- **lacY**: encodes permease, which facilitates the entry of lactose into the cell.
- **lacA**: encodes transacetylase, which is involved in the metabolism of lactose.

3. **Role of the promoter and operator**:
- The operon is regulated by a common promoter and an operator region. The promoter is where RNA polymerase binds to initiate transcription of the structural genes, while the operator is a segment of DNA that can bind a repressor protein.

4. **Regulatory genes**:
- The lac operon is regulated by the **lacI** gene, which encodes the lac repressor protein. This protein can bind to the operator region and inhibit transcription when lactose is not present.

5. **Polycistronic nature**:
- The term "polycistronic" refers to the fact that the lac operon can produce a single mRNA molecule that contains coding sequences for all three structural genes. This allows for coordinated expression of the genes involved in lactose metabolism.

6. **Conclusion**:
- The regulation of the polycistronic structural genes of the lac operon is primarily controlled by the interaction between the promoter, operator, and the lac repressor protein produced by the regulatory gene lacI.

### Final Answer:
The polycistronic structural gene of the lac operon is regulated by the common promoter and the regulatory gene (lacI) that produces the repressor protein.

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