- A transposon carrying a promoterless ß-galactosidase (lac Z) was used to create insertional mutation in the vir region of Ti-plasmid of Agrobacterium tumefaciens. All the mutants in which lac Z fusion was in frame were divided into the following three groups:
A. The virulence of the bacteria was completely lost and the lac Z was induced by acetosyringone.
B. The virulence of the bacteria was reduced and the lac Z was induced by acetosyringone.
C. The virulence of the bacteria was completely lost and lac Z was not induced by acetosyringone.
Which of the following assumptions are valid about these mutants?
(1) In group A, the insertion could be in vir B, or D; in group B the insertion could be in virC orE; and in group C the insertion could be in virA or G.
(2) In group A, the insertion could be in virA, B, C or D; in group B, the insertion could be in either virC or D; and in group C the insertion could be in virG.
(3) In group A, the insertion could be in virA; in group B, the insertion could be in vir B; and in group C, the insertion could be in vir C.
(4) In group A, the insertion could be in virG; in group B, the insertion could be in virB, D and E; in group C, the insertion could be in virA.The valid assumption is option (1):
-
Group A: insertion in virB or virD
-
Group B: insertion in virC or virE
-
Group C: insertion in virA or virG
Logic behind each mutant group
Group A: virulence completely lost, lacZ induced by acetosyringone
-
Phenotype: no tumor formation but normal induction by acetosyringone (lacZ ON).
-
Interpretation: the regulatory VirA/VirG system is intact, so vir promoters are still induced by acetosyringone.
-
Defect must be in genes downstream of induction, essential for T‑DNA processing/transfer: mainly virB (T4SS channel) or virD (T‑strand processing).
-
Hence in option (1) group A → virB or virD is correct.
Group B: virulence reduced, lacZ induced by acetosyringone
-
Phenotype: partial virulence and normal induction.
-
virC and virE mutants show reduced, not abolished, virulence, and virC/virE are important but not absolutely essential.
-
Their promoters are still induced via VirA/VirG, so lacZ remains acetosyringone‑inducible.
-
Thus group B → virC or virE in option (1) is valid.
Group C: virulence completely lost, lacZ not induced by acetosyringone
-
Phenotype: no tumors and no acetosyringone induction of lacZ.
-
This indicates that the induction system itself is broken: mutations in virA or virG block vir gene induction, so both virulence and acetosyringone‑responsive lacZ expression are lost.
-
Option (1) correctly assigns group C → virA or virG.
Why the other options are wrong
-
Option (2)
-
Puts group A insertions also in virA/C, but virA mutants would not show AS‑induced lacZ; group A clearly has induction, so virA cannot belong there.
-
Assigns group C only to virG, ignoring that virA mutants also lose induction.
-
-
Option (3)
-
Maps group A solely to virA, which contradicts the observed acetosyringone induction (virA mutants abolish induction).
-
Assigns virB to reduced virulence (group B), but virB2–virB11 are absolutely essential, giving complete loss, not just reduction.
-
-
Option (4)
-
Puts group A in virG, again inconsistent with AS‑inducible lacZ.
-
Places virB and virE together in group B, but virB core genes cause complete avirulence, not partial.
-
SEO‑oriented introduction (for article use)
Insertional mutagenesis of the vir region with a promoterless lacZ reporter allows researchers to distinguish mutants that still induce vir genes from those that cannot respond to acetosyringone. When virulence is completely lost but lacZ remains inducible, the lesion lies in structural genes like virB or virD; when virulence is only reduced, genes like virC or virE are typically affected; and when both virulence and acetosyringone induction are absent, the defect is in regulatory genes virA or virG. This reasoning makes option (1) the correct assignment.
-
