AIIMS November 2011 – MCQ 131

CAP in Lac operon is an example of:
A. Positive regulator
B. Negative regulator
C. Constitutive expression
D. Attenuation

(Repeat question – AIPGME 07 MK q40)

Correct answer : Positive regulator

Ref: Harper

Explanation: CAP acts as the positive regulator in LAC operon and is required for gene expression.
An operon is a functioning unit of genomic DNA containing a cluster of genes under the control of a single regulatory signal or promoter. The lac operon is an operon required for the transport and metabolism of lactose in Escherichia coli and some other enteric bacteria. The lac operon is regulated by several factors including the availability of glucose and of lactose.


Parts of LAC operon:

  • Regulator / Inhibitor gene – produces repressor protein when lactose concentration is high
  • Promoter – The RNA polymerase binds here
  • Operator – It lies between the promoter and the structural genes. When repressor protein binds to the operator, the transcription is blocked
  • Structural genes – Z, Y, A
Mechanism of action:
Lac operon produces lactose utilising enzymes. When lactose is absent, the repressor protein binds to the operator and blocks transcription. But when lactose is present, it binds to the repressor protein and inactivates it. Thus the block on transcription is removed.
Image credits : Wikipedia

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