T-DNA nicking endonuclease Upon perception of plant signals, Agrobacterium produces an endonuclease that nicks the T-DNA at its borders to initiate transfer. Which vir gene encodes this activity?

Introduction / Context:
Processing of T-DNA is a critical early step in Agrobacterium-mediated transformation. The endonucleolytic nick at the right border generates the T-strand, which is then coated and delivered into plant cells. Identifying the vir operon responsible for nicking clarifies how the transfer intermediate is formed.

Given Data / Assumptions:

• Plant wound signals induce vir gene expression via VirA/VirG.
• Nicking occurs at conserved T-DNA borders.
• Multiple vir operons participate in processing and export.

Concept / Approach:
The virD operon encodes the endonuclease (VirD1/VirD2 complex) that introduces site-specific nicks at T-DNA borders. VirD2 covalently attaches to the 5′ end of the T-strand and, together with other vir proteins, guides the single-stranded DNA through the type IV secretion system. virB forms the secretion channel; virE encodes a single-stranded DNA-binding protein; virA/virG are regulatory.

Step-by-Step Solution:

Verification / Alternative check:
Mutations in virD abolish border nicking and T-strand formation, blocking transformation even when other vir genes are intact, confirming its essential role.

Why Other Options Are Wrong:

• virA/virG: regulators, not nucleases.
• virB: secretion pore components.
• virC: assists right-border utilization but is not the nuclease.
• virE: ssDNA protection/transport, not nicking.

Common Pitfalls:
Attributing nicking to virB or virE because they are abundant; abundance does not equal catalytic function—virD performs the cleavage.

Final Answer:
virD