Multiple choice

Which of the following statements regarding the lac operon is INCORRECT?

  1. The first control mechanism is the regulatory response to lactose.

  2. Transcription of all genes starts with the binding of the enzyme RNA polymerase.

  3. Specific control of the lac genes depends on the availability of the substrate lactose to the bacterium.

  4. The repressor is an allosteric protein.

  5. The lac repressor is a trimer of identical subunits.
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E Correct answer
Explanation

The lac repressor is a tetramer of identical subunits. Each subunit contains a helix-turn-helix (HTH) motif capable of binding to DNA. The operator site where repressor binds is a DNA sequence with inverted repeat symmetry.

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