Q.54 For successful transfer of a foreign gene from the engineered Ti-plasmid to the plant genome, few
cis-acting DNA elements and trans-acting protein factors are very much essential. Select the
CORRECT combination from the following
(A) Opine catabolism genes, Left border sequence, Right border sequence
(B) Opine catabolism genes, Left border sequence, Virulence genes
(C) Hormone biosynthetic genes, Right border sequence, Virulence genes
(D) Left border sequence, Right border sequence, Virulence genes
Ti Plasmid Essentials: Successful Foreign Gene Transfer
Agrobacterium tumefaciens uses engineered Ti plasmids to deliver foreign genes into plant genomes via T-DNA. Question 9 identifies the critical cis-acting DNA elements (borders) and trans-acting protein factors (vir products) needed for excision, protection, and integration.
Correct Answer: (D) Left border sequence, Right border sequence, Virulence genes
LB/RB borders flank T-DNA; vir genes process and export it.
25-bp imperfect repeats define T-DNA ends. VirD2 nicks at right border (RB), attaches to 5′ end; virB/virD4 form type IV secretion system; virE2 coats single-stranded T-DNA. In binary vectors, foreign genes replace tumor genes between borders—vir region on separate replicon.
Why Other Options Fail: Precise Breakdown
Wrong choices include irrelevant genes not required for transfer mechanics.
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(A) Opine catabolism genes, Left border, Right border: Opine catabolism (ocs/ocsL) lets bacteria use opines; borders present but lack vir proteins for processing/export.
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(B) Opine catabolism genes, Left border, Virulence genes: Misses RB (nicking starts here); opines unnecessary for disarmed vectors.
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(C) Hormone biosynthetic genes, Right border, Virulence genes: Auxin/cytokinin genes (iaaM, ipt) cause galls in wild Ti but deleted in gene transfer vectors; lacks LB (termination border).
Component Roles Table
| Option | Cis Elements | Trans Factors | Transfer Possible? | Reason |
|---|---|---|---|---|
| (A) | LB, RB | None (opines) | No | No vir export |
| (B) | LB only | Vir genes | No | Missing RB nick site |
| (C) | RB only | Vir genes | No | Tumor genes irrelevant; no LB |
| (D) | LB, RB | Vir genes | Yes | Complete T-DNA machinery |
Mechanism Snapshot
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Plant phenolics → VirA/G activation.
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VirD1/D2 → RB nick, T-strand formation.
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VirE2 coats → VirB/D4 exports to nucleus.
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Integrates randomly via host repair.
Key for exams: Binary vectors split T-DNA (foreign gene) from vir—focus on borders + vir over selectable markers/opines.
