Question

1. In the lac operon, what are the likely effects on operon transcription of the mutations listed below? PLEASE EXPLAIN your answers. I think I answered the first part of the question but need help explaining why.

   1. Mutation of the consensus sequence in the lac promoter.

      i. Transcription is blocked.

2. Mutation of the repressor binding sites in the operator sequences.

   i. Transcription is constitutive.

3. Mutation of the lacI gene affecting the allosteric site of the protein.

   i. Transcription is blocked.

4. Mutation of the lacI gene affecting the DNA-binding site of the protein.

   i. Transcription is constitutive.

Answer 1

First Part: Mutation of the consensus sequence in the lac promoter

Promoter is the binding site of the RNA polymerase. If mutation occur in the promoter region, RNA polymerase will be unable to bind with the operon & thus transcription is blocked.

Second Part: Mutation of the repressor binding sites in the operator sequence

In absence of lactose, repressor is bind with the operator & prevents transcription. Now if mutation occur in the operator, repressor will be unable to bind with the operator in any condition & RNA polymerase will be able to transcribe even in the absence of lactose. Thus, transcription will be constitutive.

Third Part: Mutation of the lacI gene affecting the allosteric site of the protein

When allolactose (an isomer of lactose) is present, it binds with the allosteric site of the repressor (Encoded by lacI gene) & cause repressor to remove from the operator. Thus RNA polymerase is able to start transcription. But if this allosteric site is changed, allolactose will be unable to bind with repressor & thus repressor will bind with operator in all condition. So, RNA polymerase will be unable to transcribe & thus transcription is blocked.

Last Part: Mutation of the lacI gene affecting the DNA-binding site of the protein

Repressor is encoded by lacI gene & bind with the operator in DNA. Now if mutation occur in lacI gene & affect the DNA binding site of the repressor, then repressor will be unable to bind with the operator. In absence of repressor RNA polymerase will be able to transcribe in all condition. Thus, transcription will be constitutive.