Three mutant E. coli strains were tested. Strain 1: lacI- (repressor knockout); Strain 2: lacO- (operator mutation); Strain 3: crp- (CAP knockout). Activity was measured in all four glucose/ lactose combinations.
Which strain shows that CAP activation is required for transcription?
- Acheck_circle
crp-; the operon does not transcribe even when glucose is absent and lactose is unavailable.
- B
All three behave identically.
- C
lacI-; the operon is constitutively on.
- D
lacO-; the operator is missing.
Explanation
Without functional CAP (crp-), even derepressed conditions cannot drive high transcription because CAP-cAMP recruits RNA polymerase to the lac promoter.