Following are a few statements made regarding the lac operon:

A. The LacZ, LacY, and LacA are encoded by a single transcript.
B. The three proteins are translated as a single precursor and then processed.
C. In the presence of glucose, lactose can upregulate the operon.
D. Isopropyl thio β-D-galactoside (IPTG) is a gratuitous inducer.

Which one of the following options represents the combination of all correct statements?

Correct option is D

Explanation:

1. Statement A: The LacZ, LacY, and LacA are encoded by a single transcript.

   • Correct: The lac operon encodes three proteins (LacZ, LacY, and LacA) from a single polycistronic mRNA transcript.

2. Statement B: The three proteins are translated as a single precursor and then processed.

   • Incorrect: This is not true. While LacZ, LacY, and LacA are encoded by a single transcript, they are not translated as a single precursor. They are translated independently from the polycistronic mRNA as separate proteins in a coordinated manner, not as a single precursor protein that is later processed.

3. Statement C: In the presence of glucose, lactose can upregulate the operon.

   • Incorrect: This is also false. In the presence of glucose, the lac operon is repressed due to catabolite repression, and lactose cannot upregulate the operon. The operon is activated only when glucose is absent and lactose is present.

4. Statement D: Isopropyl thio β-D-galactoside (IPTG) is a gratuitous inducer.

   • Correct: IPTG is a synthetic inducer of the lac operon. It mimics lactose but is not metabolized, which makes it a gratuitous inducer.

The correct answer is Option 4: A and D only, as only A and D are correct.