Which statement best describes the lac operon and its regulation in bacteria?

• A. The lac operon includes lacZ, lacY, lacA genes under a single promoter and operator, regulated by lacI repressor.
• B. The operon uses multiple promoters for each gene and is always expressed.
• C. The lacI gene codes for an activator that binds to enhancers to increase transcription.
• D. The presence of lactose inhibits allolactose formation, decreasing transcription.

Answer: (A)

The lac operon is an inducible, polycistronic unit controlled by a repressor that binds an operator next to a single promoter. When lactose is absent, the repressor (encoded by lacI) sits on the operator and blocks RNA polymerase, keeping transcription off. When lactose is present, some is converted to allolactose, which binds the repressor and inactivates it, allowing RNA polymerase to transcribe lacZ, lacY, and lacA as one mRNA transcript. The lacI gene acts independently to regulate the operon, not as part of its own operon.

This is why the statement describing three genes under a single promoter and operator regulated by lacI repressor is the best fit. The other options don’t match: the operon isn’t driven by multiple promoters for each gene and isn’t always expressed; lacI encodes a repressor, not an activator that binds enhancers; and lactose leads to the production of allolactose, which relieves repression to increase transcription, not inhibit it.