How do we know that the trp operon is a repressible control system, in contrast to the lac operon, which is an inducible control system?

Verified step by step guidance

1. Understand the basic definitions: A repressible operon is typically 'on' and actively transcribing until it is turned 'off' by a specific molecule (corepressor), whereas an inducible operon is usually 'off' and requires an inducer molecule to turn it 'on'.
2. Examine the trp operon: It controls the synthesis of tryptophan. When tryptophan levels are low, the operon is active, allowing the production of enzymes needed to make tryptophan. When tryptophan is abundant, it acts as a corepressor by binding to the repressor protein, enabling the repressor to bind the operator and block transcription.
3. Analyze the lac operon: It controls the breakdown of lactose. In the absence of lactose, the repressor binds the operator and prevents transcription. When lactose is present, it acts as an inducer by binding to the repressor, causing it to release from the operator and allowing transcription to proceed.
4. Compare the mechanisms: The trp operon is repressible because it is normally active and turned off by the presence of a corepressor (tryptophan), while the lac operon is inducible because it is normally inactive and turned on by the presence of an inducer (lactose).
5. Summarize the key difference: The trp operon responds to the end product (tryptophan) to repress gene expression, whereas the lac operon responds to the substrate (lactose) to induce gene expression.

Key Concepts

Repressible Operon

A repressible operon is typically active but can be turned off when a specific molecule, often the end product of a pathway, binds to a repressor protein. This binding enables the repressor to attach to the operator region, blocking transcription. The trp operon is repressible because it is usually on and is turned off when tryptophan is abundant.

Inducible Operon

An inducible operon is usually off and requires the presence of a specific inducer molecule to activate transcription. The inducer inactivates the repressor, allowing gene expression. The lac operon is inducible because it is off until lactose is present, which induces the operon to produce enzymes for lactose metabolism.

Mechanism of Gene Regulation in Operons

Operons regulate gene expression through interactions between repressors, inducers/corepressors, and operator DNA sequences. Understanding how these molecules influence RNA polymerase binding and transcription initiation is key to distinguishing repressible and inducible systems, as seen in the contrasting control of the trp and lac operons.