Question

How is the functioning of the lac operon different from that of the try operon?

Answer 1

The functioning of the lac operon and trp operon differs primarily in their regulatory logic and biological purpose. While the lac operon is inducible (turned ON by its substrate), the trp operon is repressible (turned OFF by its product).

• Type of Pathway: The lac operon is catabolic, meaning it controls the breakdown of a substance (lactose) for energy. The trp operon is anabolic (biosynthetic), as it controls the synthesis of a necessary molecule (the amino acid tryptophan).
• Default State: The lac operon is normally OFF and is only activated when glucose is scarce and lactose is available. In contrast, the trp operon is normally ON because the cell constantly needs tryptophan for protein synthesis; it only switches off when tryptophan is already abundant.
• Regulatory Molecule: In the lac operon, allolactose (an isomer of lactose) acts as an inducer that inactivates the repressor. In the trp operon, tryptophan acts as a co-repressor that binds to and activates the repressor to block transcription.
• Repressor Behaviour: The lac repressor is active on its own and must be “unlocked” by the inducer to fall off the DNA. The trp repressor is inactive on its own (an apo-repressor) and only binds to the DNA after it is “locked” by tryptophan.
• Complexity of Control: The lac operon uses both negative control (repressor) and positive control (via the CAP-cAMP complex) to sense glucose levels. The trp operon uses negative control and a unique second layer called attenuation, which fine-tunes transcription based on how quickly ribosomes move along the mRNA.