Which bacterial regulatory system controls metabolism of lactose and is a classic model of gene regulation?

• A. The lac promoter
• B. The lac operon (lac repressor, operator, and inducer)
• C. The trp operon
• D. The promoter and terminator regions

Answer: (B)

Lactose metabolism in bacteria is controlled by a coordinated operon that links the genes needed to import and break down lactose under shared regulatory control. The lac operon includes a promoter, an operator, and the structural genes lacZ, lacY, and lacA. A lac repressor binds to the operator to block transcription when lactose is absent; when lactose (converted to allolactose) is present, it binds the repressor, changing its shape and releasing it from the operator, which allows RNA polymerase to transcribe the operon. This setup lets the cell produce the necessary enzymes only when lactose is available. Additional regulation by CAP and cAMP enhances transcription when glucose is scarce, illustrating both negative and positive control in a single system and making this operon a classic model of gene regulation. The lac promoter by itself is only part of the system, not the regulatory mechanism; the trp operon regulates a different pathway (tryptophan synthesis) and operates by a different mode (repression of a biosynthetic pathway), and promoter and terminator regions describe general transcription signals rather than a complete, lactose-specific regulatory system.