Contrast the role of the repressor in an inducible system and in a repressible system.

Verified step by step guidance

1. Begin by defining what a repressor protein is: a molecule that can bind to an operator region of DNA to regulate gene expression by preventing RNA polymerase from transcribing the gene.
2. Explain the role of the repressor in an inducible system: the repressor is normally bound to the operator, blocking transcription. When an inducer molecule is present, it binds to the repressor, causing it to change shape and release from the operator, allowing transcription to proceed.
3. Describe the role of the repressor in a repressible system: the repressor is normally inactive and does not bind to the operator, so transcription occurs. When a corepressor molecule is present, it binds to the repressor, activating it. The active repressor then binds to the operator and blocks transcription.
4. Highlight the key difference: in inducible systems, the repressor is active by default and is inactivated by the inducer, whereas in repressible systems, the repressor is inactive by default and is activated by the corepressor.
5. Summarize by noting that inducible systems typically regulate genes involved in catabolism (breaking down molecules), while repressible systems regulate genes involved in anabolism (building molecules), reflecting their different modes of repressor action.

Key Concepts

Inducible Operon System

An inducible operon is typically off and requires an inducer molecule to deactivate the repressor, allowing gene transcription. The repressor binds to the operator to block transcription until the inducer binds, causing the repressor to release and enabling gene expression, as seen in the lac operon.

Repressible Operon System

A repressible operon is usually on and actively transcribed until a corepressor binds to the repressor, activating it. The active repressor then binds to the operator to block transcription, as in the trp operon, which regulates genes involved in amino acid synthesis.

Role of the Repressor Protein

The repressor protein controls gene expression by binding to the operator region of an operon. Its activity is modulated by effectors: in inducible systems, the repressor is inactivated by an inducer, while in repressible systems, it is activated by a corepressor, thus either permitting or blocking transcription.