The promoter gene in lac operon :

Options

A. inhibits m-RNA transcription
B. inhibits enzyme synthesis
C. inhibits t-RNA translation
D. both (a) and (b)

Answer

### Step-by-Step Solution:

1. **Understanding the Lac Operon**: The lac operon is a set of genes in bacteria that are involved in the metabolism of lactose. It consists of structural genes, a promoter, an operator, and regulatory genes.

2. **Identifying the Promoter**: The promoter in the lac operon is a specific DNA sequence where RNA polymerase binds to initiate transcription of the downstream genes.

3. **Function of the Promoter**: The primary function of the promoter is to serve as a binding site for RNA polymerase. When RNA polymerase binds to the promoter, it begins the process of transcription, converting DNA into mRNA.

4. **Role of the Lac Repressor**: In the lac operon, the lac repressor protein can bind to the operator region, which overlaps with the promoter. When the repressor is bound to the operator, it physically blocks RNA polymerase from binding to the promoter.

5. **Consequences of Repressor Binding**: When the repressor is bound to the operator, RNA polymerase cannot access the promoter. This results in the inhibition of transcription, meaning that mRNA is not synthesized, and consequently, the enzymes needed for lactose metabolism are not produced.

6. **Conclusion**: The promoter gene in the lac operon plays a crucial role in regulating gene expression. It inhibits mRNA transcription when the lac repressor is bound to the operator, which also prevents the synthesis of enzymes necessary for lactose metabolism.

7. **Correct Answer**: The promoter gene in the lac operon inhibits mRNA transcription and enzyme synthesis. Therefore, the correct answer is option D: both A and B.