The first discovered restriction endonuclease enzyme was :-

Answer

**Step-by-Step Solution:**

1. **Understanding Restriction Endonucleases**:
- Restriction endonucleases are enzymes that cut DNA at specific sequences of nucleotides. They play a crucial role in molecular biology, particularly in genetic engineering and cloning.

2. **Historical Context**:
- The discovery of restriction endonucleases was a significant milestone in biotechnology. It allowed scientists to manipulate DNA more effectively.

3. **Key Scientists**:
- The first restriction endonuclease was discovered by three scientists: Werner Arber, Hamilton Smith, and Daniel Nathans. They were awarded the Nobel Prize in Physiology or Medicine in 1978 for their contributions to the field.

4. **Identification of the First Restriction Endonuclease**:
- The first restriction endonuclease enzyme identified was HIND-2. This enzyme is derived from the bacterium Haemophilus influenzae, specifically from the Rd strain 2.

5. **Abbreviation**:
- HIND-2 is often abbreviated as "Hind II," which is a common nomenclature in molecular biology for restriction enzymes.

**Final Answer**: The first discovered restriction endonuclease enzyme was HIND-2, derived from Haemophilus influenzae Rd strain 2.

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