Regulation of Gene Expression - Induction and Repression

An inducer is a small molecule that activates gene expression by initiating the synthesis of specific enzymes.

Inducible proteins are enzymes that are synthesised only in the presence of a specific inducer or substrate.

A co-repressor is a substance which, when present in excess, binds with a repressor to switch off gene expression.

Constitutive genes are genes that are continuously expressed irrespective of environmental conditions.

Constitutive enzymes are enzymes that are synthesised continuously and are essential for basic cellular metabolism.

• Gene regulation controls when and how much a gene is expressed.
• Cells switch genes ON or OFF to save energy and respond to changing conditions.
• In bacteria, many genes are organised in operons and are regulated together (classic example: lac operon).

• Induction: Turning genes ON in response to a substrate or inducer.
• Inducer: Small molecule that activates gene expression (often the substrate itself or its derivative).
• Inducible enzymes: Enzymes produced only when an inducer is present.
• Inducible systems are usually OFF by default and become ON when a substrate is present.
• Example: In lac operon, lactose acts as an inducer; when lactose is present (and glucose is low), structural genes for lactose metabolism are expressed.
• Induction helps bacteria synthesise enzymes only when the relevant substrate is available.

Induction = substrate present → genes ON → enzyme made.

• Repression: Turning genes OFF to stop unnecessary synthesis of a product.
• Repressible systems: Typically ON by default, switched OFF when the end product is in excess.
• Co-repressor: End product or related molecule that helps the repressor protein block transcription.
• Common in anabolic/biosynthetic pathways.
• Example: In trp operon (tryptophan synthesis), excess tryptophan acts as a co-repressor and turns off genes for its own synthesis.
• Prevents wasteful overproduction of metabolites.

Repression = product excess → genes OFF → enzyme not made.

• Feedback repression: End product of a pathway reduces or stops synthesis of enzymes involved in making that product.
• Acts as a negative feedback loop at the gene level.
• End product accumulation → activates repression → transcription of related genes decreases.
• Maintains metabolic balance and conserves energy.

• Constitutive genes: Genes expressed continuously, irrespective of substrate or product levels.
• Constitutive enzymes: Enzymes produced all the time because they are essential for basic cellular functions.

• Gene regulation turns genes ON or OFF so cells save energy and respond to conditions.
• Induction: genes are normally OFF, but a substrate/inducer makes them switch ON to produce needed enzymes (e.g., lactose in lac operon).
• Repression: genes are normally ON, but excess end product/co-repressor switches them OFF to stop overproduction (e.g., tryptophan in trp operon).
• Inducible operons are linked to catabolic (breakdown) pathways; repressible operons are linked to anabolic (synthesis) pathways.
• Constitutive genes are expressed all the time for basic functions, while regulated genes use induction or repression to control expression only when needed.