Bacillus thuringiensis forms protein crystals which contain insecticidal protein. This protein:

Options

A. does not kill the carrier bacterium
B. binds with epithelial cells of midgut of the insect pest ultimately killing it
C. is coded by several genes including the gene cry
D. is activated by acid pH of the foregut of the insect pest

Answer

### Step-by-Step Solution:

1. **Identify the Source of the Protein**:
- The protein crystals are formed by the bacterium Bacillus thuringiensis (Bt). This bacterium is known for producing insecticidal proteins that are effective against certain pests.

2. **Understand the Function of the Protein**:
- The protein produced by Bacillus thuringiensis is a toxin specifically designed to target insects. It is harmful to them and can cause death or severe allergic reactions.

3. **Gene Involvement**:
- The specific gene responsible for producing this insecticidal protein is known as the cry gene. This gene encodes the protein that becomes toxic to insects.

4. **Application in Agriculture**:
- Scientists have utilized the cry gene by inserting it into the DNA of cotton plants. This genetic modification allows the cotton plants to produce the insecticidal protein themselves.

5. **Activation of the Protein**:
- The protein remains inactive while in the plant. When an insect, such as a bollworm, consumes the cotton, the protein enters the insect's digestive system.

6. **Mechanism of Action**:
- The protein is activated in the alkaline environment of the insect's small intestine. The high pH level in the gut triggers the protein to become toxic.

7. **Effect on Insects**:
- Once activated, the protein binds to the epithelial cells of the insect's mid-gut. This binding disrupts the gut cells, leading to the insect's death.

8. **Conclusion**:
- Therefore, the insecticidal protein from Bacillus thuringiensis binds with the epithelial cells of the mid-gut of the insect pest, ultimately killing it.