vir gene regulation in Agrobacterium Which vir genes are constitutively expressed at a basal level and together control plant-induced activation of the other vir operons?

Introduction / Context:
Agrobacterium senses wounded plant tissues via phenolic signals and activates a regulatory cascade that turns on DNA transfer functions. The central two-component system virA/virG orchestrates this induction, a concept essential for understanding how transformation efficiency depends on signal perception.

Given Data / Assumptions:

• vir genes reside on the Ti plasmid outside the T-DNA.
• Phenolics (e.g., acetosyringone) from wounded plants trigger the system.
• Two-component regulatory logic is used (sensor kinase + response regulator).

Concept / Approach:
virA encodes a membrane-associated sensor histidine kinase that detects phenolic inducers and phosphorylates virG, the cytoplasmic response regulator. Phosphorylated VirG activates transcription of other vir operons (virB, virC, virD, virE), enabling T-DNA processing and export. virA and virG are expressed constitutively at basal levels so they can respond promptly to plant signals.

Step-by-Step Solution:

Verification / Alternative check:
Genetic studies show that virA or virG mutants fail to induce other vir genes even in the presence of acetosyringone, demonstrating their upstream regulatory role.

Why Other Options Are Wrong:

• virB, virC, virD, virE encode transfer/processing proteins but are downstream and not the primary sensors/regulators.
• virA with virB or virG with virE: mismatched pairs lacking the canonical two-component relationship.

Common Pitfalls:
Assuming all vir genes are induced equally; instead, virA/virG form the master switch that turns on the rest in response to plant cues.

Final Answer:
virA and virG