65. The T-DNA of Agrobacterium must be cut out from its circular plasmid for its transferinto plant
cells. Which one of the following Vir proteins are involved in this process?
(a) Vir A / Vir C
(b) Vir B6 / Vir B7
(c) Vir D1 / Vir D2
(d) Vir E2 / Vir G

---

Vir D1 and D2: Key Proteins in T-DNA Excision from Agrobacterium Plasmid

The genetic transformation of plants using Agrobacterium tumefaciens hinges on the T-DNA transfer mechanism, where a segment of DNA from the bacterium’s Ti plasmid is inserted into the plant genome. Before this can happen, the T-DNA must be precisely excised from the plasmid — a critical step mediated by specific proteins.

Correct Answer:

(c) Vir D1 / Vir D2

---

How Vir D1 and Vir D2 Work

• Vir D1: Acts as a topoisomerase-like protein, helping in the recognition of the T-DNA border sequences.

• Vir D2: Functions as a nuclease, making a single-stranded nick at the right border of the T-DNA. It covalently binds to the 5′ end of the nicked strand, forming the T-strand, which is the DNA segment to be transferred.

Together, these proteins form a complex that initiates the T-DNA transfer process from the circular Ti plasmid into the plant cell.

---

Why T-DNA Cutting Is Essential

For successful genetic transformation:

• The T-DNA must be in linear, single-stranded form (T-strand).

• This strand must be processed and protected during transfer into the plant cell nucleus.

• Vir D2 plays a protective and guiding role even after the excision.

---

Explanation of Other Options:

• (a) Vir A / Vir C: Involved in signal sensing and T-DNA border enhancement, not cutting.

• (b) Vir B6 / B7: Components of the Type IV secretion system for T-DNA transport, not processing.

• (d) Vir E2 / Vir G: E2 coats the T-strand during transfer; Vir G is a regulatory protein.

---

Conclusion

The Vir D1 and D2 proteins are essential for the precise excision of T-DNA from Agrobacterium tumefaciens‘ plasmid, laying the foundation for its transfer into plant cells. Their roles highlight the intricate molecular machinery behind plant genetic engineering.