Regulation of Gene Expression in Prokaryotes

Overview of Gene Regulation

Gene expression in prokaryotes, such as E. coli, is tightly regulated to ensure that cellular resources are used efficiently. Regulation occurs in response to environmental conditions and internal cellular needs, allowing bacteria to adapt rapidly to changes.

• Inducible enzymes: Produced only when specific substrates are present.

• Constitutive enzymes: Produced continuously, regardless of environmental conditions.

• Repressible enzymes: Not produced when a specific molecule is present.

• Negative control: Gene expression occurs unless shut off by a regulator molecule.

• Positive control: Transcription occurs only if a regulator molecule stimulates RNA production.

Example: Feedback inhibition and gene regulation are two ways cells control enzyme production.

The Lac Operon: An Inducible System

Structure and Function of the Lac Operon

The lac operon in E. coli is a classic example of an inducible gene system. It consists of three structural genes (lacZ, lacY, lacA) and an upstream regulatory region (operator and promoter). These genes are transcribed as a single polycistronic mRNA.

• lacZ: Encodes β-galactosidase, which cleaves lactose into glucose and galactose.

• lacY: Encodes permease, which facilitates lactose entry into the cell.

• lacA: Encodes transacetylase, which may remove toxic by-products of lactose digestion.

• Operator: DNA segment where the repressor binds to regulate transcription.

• Promoter: DNA segment where RNA polymerase binds to initiate transcription.

Example: In the presence of lactose, the concentration of enzymes responsible for lactose metabolism increases.

Polycistronic mRNA and Translation

All three structural genes are transcribed as a single unit, resulting in polycistronic mRNA. This mRNA is translated to produce the three enzymes required for lactose metabolism.

Regulatory Components of the Lac Operon

The lac operon is regulated by the lacI gene, which produces a repressor protein. This repressor is allosteric, meaning it can change shape and activity upon binding to lactose.

Negative Control of the Lac Operon

Transcription of the lac operon occurs only when the repressor fails to bind to the operator region. In the absence of lactose, the repressor binds to the operator, blocking transcription.

When lactose is present, it binds to the repressor, causing a conformational change that prevents the repressor from binding to the operator. Transcription proceeds, and the enzymes are produced.

Genetic Proof of the Operon Model

Mutations in the lac operon provide evidence for the operon model. For example, mutations in the repressor gene or operator gene can lead to constitutive expression, where transcription occurs regardless of lactose presence.

• Mutant repressor gene: No binding occurs; transcription proceeds (constitutive).

• Mutant operator gene: Nucleotide sequence altered; no binding occurs; transcription proceeds (constitutive).

• Mutant repressor unable to bind lactose: Repressor always bound; transcription blocked (repressed).

Positive Control: Catabolite Repression and CAP

The catabolite-activating protein (CAP) exerts positive control over the lac operon. In the presence of glucose, CAP does not bind, and transcription is repressed. In the absence of glucose and presence of lactose, CAP binds to the CAP-binding site, facilitating RNA polymerase binding and maximal expression.

• cAMP: Required for CAP binding. Glucose inhibits adenylyl cyclase, reducing cAMP levels.

The trp Operon: A Repressible System

Structure and Function of the trp Operon

The trp operon in E. coli is a repressible gene system responsible for the biosynthesis of tryptophan. It consists of five structural genes (trpE, trpD, trpC, trpB, trpA) and regulatory regions (promoter, operator, leader, attenuator).

• trpR: Encodes the repressor protein.

• Operator: DNA segment where the repressor binds.

• Promoter: DNA segment where RNA polymerase binds.

• Leader and attenuator: Regions involved in attenuation, a secondary regulatory mechanism.

Regulation of the trp Operon

When tryptophan is absent, the repressor cannot bind to the operator, and transcription proceeds. When tryptophan is present, it acts as a corepressor, enabling the repressor to bind to the operator and block transcription.

Attenuation: Secondary Regulation of the trp Operon

Attenuation is a form of regulation where transcription is greatly reduced but not completely prevented. The leader region of the trp operon can form different stem-loop structures depending on tryptophan availability.

• Presence of tryptophan: Terminator hairpin forms, stopping transcription.

• Absence of tryptophan: Antiterminator hairpin forms, allowing transcription to proceed.

Key Terms and Concepts

• Operon: A cluster of genes under coordinated control by a single regulatory region.

• Operator: Regulatory region where repressor proteins bind to control transcription.

• Inducer: Molecule that inactivates the repressor, allowing transcription.

• Corepressor: Molecule that activates the repressor, blocking transcription.

• Polycistronic mRNA: Single mRNA molecule encoding multiple proteins.

• Attenuation: Regulatory mechanism that reduces transcription via formation of specific mRNA structures.

Summary Table: Comparison of Lac and Trp Operons

Additional info: The lac operon is a model for understanding negative and positive control of gene expression, while the trp operon illustrates repressible systems and attenuation. Both operons are transcribed as polycistronic mRNA, allowing coordinated regulation of multiple genes.