Multiple choice

Which of the following acts as the inducer of the lac operon?

  1. Phosphoenolpyruvic acid

  2. Allolactose

  3. ONPG

  4. Phenyl-β-D-galactose (phenyl-Gal)

  5. Isopropyl-β-D-thio-galactoside
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B Correct answer
Explanation

Allolactose is an isomer of lactose and is the inducer of the lac operon. Lactose is galactose-(β1->4)-glucose, whereas allolactose is galactose-(β1->6)-glucose. Lactose is converted to allolactose by β-galactosidase in an alternative reaction to the hydrolytic one.

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