Q.41 Two of the vir operons of Ti plasmid in Agrobacterium tumefaciens are constitutively expressed.

Identify the CORRECT pair.

(A) vir 𝐴 and vir 𝐺

(B) vir 𝐹 and vir 𝐻

(C) vir 𝐶 and vir 𝐷

(D) vir 𝐵 and vir

The correct answer is (A) vir A and vir G. These two vir operons on the Ti plasmid of Agrobacterium tumefaciens are constitutively expressed, meaning they produce proteins at a basal level even without plant signals, enabling initial detection of host cues.​

Option Analysis

• (A) vir A and vir G: VirA acts as a sensor kinase on the membrane, detecting plant phenolics, while VirG serves as a response regulator that activates other vir genes upon phosphorylation. Both operons express constitutively (virA fully, virG basally) to prime the bacterium for infection.​

• (B) vir F and vir H: These are minor operons (present in octopine Ti plasmids); virF encodes an F-box effector for host protein degradation, and virH’s role is less defined. Neither shows constitutive expression; they require induction.​

• (C) vir C and vir D: VirC aids T-strand recruitment, and virD encodes endonucleases for T-DNA processing. Both are inducible only by plant signals via phospho-VirG.​

• (D) vir B and vir ?: VirB forms the type IV secretion system (T4SS) for T-DNA export. It is strictly inducible. The incomplete “vir ?” likely refers to another inducible operon like virE.​

The Ti plasmid in Agrobacterium tumefaciens harbors vir operons essential for plant transformation via T-DNA transfer. Two vir operons of Ti plasmid in Agrobacterium tumefaciens are constitutively expressed—virA and virG—setting the stage for inducible activation of others upon sensing plant wound signals like acetosyringone. This regulation is key for CSIR NET Life Sciences aspirants studying bacterial pathogenesis and genetic engineering.​

Vir Regions Overview

The vir region spans ~35 kb with ~10 operons (virA-G, plus F/H in octopine types). Most (virB, C, D, E) are inducible, activated by phospho-VirG binding vir-box promoters. VirA (sensor kinase) and virG (transcriptional activator) express constitutively: virA fully non-inducible, virG with basal and inducible promoters.​

Functions of Constitutive VirA and VirG

VirA spans the inner membrane, autophosphorylates on histidine upon phenolic binding, then transfers phosphate to VirG’s aspartate. Phospho-VirG induces all other vir operons. Constitutive basal expression ensures readiness for host contact.​

• VirA perceives signals (phenolics, sugars via ChvE, low pH).

• VirG activates ~25 vir genes for T-strand formation, export, and protection.​

Why Other Options Incorrect

VirB (T4SS pilus), virC (T-border binding), virD (nicking), virE (ssDNA coat), virF/H (host modulation) require VirG induction. Mutations confirm: virA/G defects abolish virulence entirely.​

This mechanism enables A. tumefaciens as a biotech tool for stable plant transformation.​