Operon Concept

• Operon concept is a transcriptional control mechanism of gene regulation.
• It was explained by François Jacob and Jacques Monod in 1961.
• It explains that metabolic pathways are regulated as a unit.
• The model is based on the lac operon in E. coli.
• An operon is a unit of coordinated control of protein synthesis.
• It consists of an operator gene that controls the activity of structural genes involved in protein synthesis.
• These structural genes synthesise mRNA under the operational control of the operator gene.
• The operator is controlled by a repressor molecule synthesised by a regulator gene.
• The regulator gene is not a part of the operon.

Example: lac operon in E. coli

• When lactose is provided to the culture medium, the cell induces the production of three enzymes necessary for the digestion of lactose.
• These enzymes are β-galactosidase, β-galactoside permease, and transacetylase.

Enzymes produced:

The operon consists of four main components: regulator, promoter, operator, and structural genes.

1. Regulator gene (i)

• The regulator is responsible for synthesising the protein called repressor.
• The active repressor is seen in an inducible system, while the inactive repressor is seen in a repressible system.

2. Promoter (p)

• It is the segment at which RNA polymerase binds.
• It initiates transcription of structural genes and controls the rate of mRNA synthesis.

3. Operator (o)

• This segment of DNA imposes control over transcription.
• This region acts like an ON and OFF switch for protein synthesis.

4. Structural genes (z, y, a)

• This region of DNA codes for the synthesis of proteins.
• These determine the primary structure of the polypeptide chain.
• The three structural genes are z, y and a.

• Gene expression depends on whether the operator is switched ON or switched OFF.
• Switching ON or OFF of the operator is achieved by a protein called repressor.
• When the repressor is attached to the operator and blocks it, the switch is turned OFF and structural genes are not expressed.

When operator is switched ON:

• The three genes z, y and a are transcribed by RNA polymerase into a single mRNA.
• Each structural gene is called a cistron.
• The long mRNA covering various cistrons is called polycistronic mRNA.

On the basis of the activity principle, operons are of two types: inducible systems and repressible systems.

Inducible system:

This operon is inactive in the normal case and can be activated by an inducer.

When the inducer is absent:

• The regulator gene produces an active repressor.
• The active repressor binds to the operator gene.
• It blocks transcription and protein synthesis.

When the inducer is present:

• The regulator gene produces an active repressor.
• This forms an inducer-repressor complex.
• It does not bind to the operator gene.
• Therefore, transcription and translation go on.

Repressible system:

This operon is active in normal cases and can be inactivated by a co-repressor.

When the co-repressor is absent:

• The regulator gene produces apo-repressor.
• It does not have affinity for the operator gene.
• Therefore, it does not bind to the operator to block transcription and translation.

When the co-repressor is present:

• The apo-repressor combines with the co-repressor and becomes active.
• This active repressor binds to the operator gene.
• It blocks both transcription and translation.

• The operon concept (Jacob & Monod, 1961) explains how related genes are regulated together as one unit, based on the lac operon in E. coli.
• An operon has four parts: regulator (i), promoter (p), operator (o), and structural genes (z, y, a) - coding for β-galactosidase, permease, and transacetylase.
• The operator acts as an ON/OFF switch; when active, all three genes are transcribed into a single polycistronic mRNA (each gene = one cistron).
• The repressor (from the regulator gene) controls the operator - bound = OFF, unbound = ON.
• Inducible system (lac operon): the inducer (lactose) inactivates the repressor → transcription ON.
• Repressible system: a co-repressor activates the repressor → transcription OFF.