Ti plasmid genetics — Where are the virulence (vir) genes that mediate T-DNA processing and transfer located relative to the T-DNA region?

Introduction / Context:
Agrobacterium tumefaciens uses vir genes to excise, process, and transfer T-DNA into plant cells. These vir genes are essential for infection but are not themselves transferred to the plant; they reside in a distinct region of the Ti plasmid.

Given Data / Assumptions:

• T-DNA borders delimit the DNA that is transferred.
• virA/virG and other vir operons encode proteins for sensing, processing, and transfer.
• Only the T-DNA between border repeats integrates into the plant genome.

Concept / Approach:
Separate the transfer cargo (T-DNA) from the bacterial machinery (vir region). The vir genes operate in trans to mobilize the T-DNA, remaining in the bacterium.

Step-by-Step Solution:

Verification / Alternative check:
Binary vector systems split T-DNA and vir functions onto separate plasmids, proving vir can act in trans.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing border sequences (on T-DNA) with vir genes (outside T-DNA).

Final Answer:
Outside the T-DNA region on the Ti plasmid (separate vir region).