Q.85 Which of the following statements are TRUE regarding recA mutants of E. coli?
(P) Exhibit much reduced recombination
(Q) Do not survive UV irradiation
(R) Show no effect on doubling time
(S) Exhibit pleiotropy
(A) P, Q & R only (B) P, Q & S only
(C) P, R & S only (D) Q, R & S only
RecA mutants of E. coli exhibit reduced recombination (P), extreme UV sensitivity (Q), and pleiotropy (S). They have normal doubling time under non-stress conditions. Correct answer: (B) P, Q & S only.
RecA Protein: Master Regulator
RecA is central to homologous recombination, SOS DNA repair, and DNA damage response in E. coli. It forms nucleoprotein filaments on ssDNA, enabling strand invasion and LexA cleavage (SOS induction).
recA null mutants eliminate these functions, causing multiple defects.
Statement Analysis
(P) TRUE: Exhibit much reduced recombination
RecA catalyzes strand exchange during homologous recombination. recA mutants show >1000-fold recombination defect (conjugation, transduction). Classic recA1 mutation = recombination deficient.
(Q) TRUE: Do not survive UV irradiation
UV creates pyrimidine dimers repaired by nucleotide excision repair (uvrABC) + RecA-mediated post-replication repair. recA mutants are extremely UV sensitive (10-100x wild-type sensitivity), cannot induce SOS genes.
(R) FALSE: Show no effect on doubling time
Growth rate normal in rich media without DNA damage. Doubling time unaffected under standard lab conditions (unlike dnaB replication mutants).
(S) TRUE: Exhibit pleiotropy
Multiple phenotypes from single mutation:
-
Recombination deficiency
-
UV sensitivity
-
SOS uninducible
-
Sensitivity to cross-linking agents (mitomycin C)
-
Filamentation (defective cell division via SulA)
Correct Answer: (B) P, Q & S only
| Statement | recA Mutant Effect | TRUE/FALSE |
|---|---|---|
| (P) Reduced recombination | >1000x defect | ✅ TRUE |
| (Q) UV lethal | Cannot repair dimers | ✅ TRUE |
| (R) Doubling time | Normal growth | ❌ FALSE |
| (S) Pleiotropy | Multiple defects | ✅ TRUE |
Why Other Options Wrong?
(A) P, Q & R only ❌
R false—includes normal doubling time myth.
(C) P, R & S only ❌
Q false—excludes hallmark UV sensitivity.
(D) Q, R & S only ❌
P false—excludes primary recombination defect.
RecA Mutant Phenotype Summary
recA⁻ E. coli:
✓ Recombination: DEFECTIVE (P)
✓ UV: LETHAL (Q) ← CSIR favorite!
✗ Growth rate: NORMAL (R)
✓ Pleiotropy: MULTIPLE defects (S)
Exam Mnemonic
“RecA Rules: Recombination, UV, SOS” → P, Q, S true (R = growth = normal)
