Bt-gene occurs in:

Options

A. Bacillus thuringiensis
B. Escherichia coli
C. Agrobacterium tumefaciens
D. Rhizobium leguminosarum

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the occurrence of the Bt-gene, which is related to a specific bacterium.

2. **Expanding the Term "Bt"**: The term "Bt" stands for Bacillus thuringiensis. This is crucial as it directly points to the bacterium in question.

3. **Identifying the Properties of Bacillus thuringiensis**: Bacillus thuringiensis is known for producing insecticidal proteins that are effective against various insect pests, such as caterpillars and mosquitoes.

4. **Mechanism of Action**: The proteins produced by Bacillus thuringiensis are inactive in bacteria but become active when ingested by insects. This leads to the formation of pores in the insect's gut, ultimately causing death.

5. **Gene Responsible for Insecticidal Proteins**: The gene that codes for these insecticidal proteins is known as the Cry gene. This gene can be isolated and introduced into crops to provide them with pest resistance.

6. **Conclusion**: Based on the above information, the Bt-gene occurs specifically in **Bacillus thuringiensis**.

**Final Answer**: The Bt-gene occurs in Bacillus thuringiensis.

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