Regulation of Gene Expression (Chapter 15) – Campbell Biology in Focus

Regulation of Gene Expression

Introduction

Gene expression is the process by which information from a gene is used to synthesize functional gene products, such as proteins. Regulation of gene expression allows cells to respond to environmental changes and differentiate into specialized cell types, despite having identical genetic material.

Regulation of Gene Expression in Prokaryotes

Bacteria Respond to Environmental Change by Regulating Transcription

• Natural selection favors bacteria that produce only the gene products needed by the cell.

• Cells regulate enzyme production by feedback inhibition or gene regulation.

• Gene expression in bacteria is often controlled by the operon model.

Operons: The Basic Concept

• An operon is a stretch of DNA including a promoter, operator, and genes they control.

• A single promoter serves a set of functionally related genes, coordinately controlled by an "on-off switch" called the operator.

• The repressor protein can switch the operon off by binding to the operator, blocking RNA polymerase attachment.

• The repressor is produced by a separate regulatory gene.

• The operator alternates between two states: repressor bound and unbound.

Repressible and Inducible Operons: Negative Gene Regulation

• Repressible operon: Usually on; binding of a repressor to the operator shuts off transcription (e.g., trp operon).

• Inducible operon: Usually off; an inducer inactivates the repressor and turns on transcription (e.g., lac operon).

• For the lac operon, the inducer is allolactose, formed from lactose.

• Inducible enzymes function in catabolic pathways; repressible enzymes function in anabolic pathways.

Positive Gene Regulation

• E. coli prefers glucose; when glucose is scarce, CRP (cAMP receptor protein) acts as a transcription activator.

• CRP is activated by binding with cyclic AMP (cAMP).

• Activated CRP increases RNA polymerase affinity for the lac operon promoter, accelerating transcription.

• When glucose levels rise, cAMP falls, CRP detaches, and transcription slows.

Regulation of Gene Expression in Eukaryotes

Gene Expression Is Regulated at Many Stages

• All organisms must regulate which genes are expressed at any time.

• In multicellular organisms, regulation is essential for cell specialization.

• Differential gene expression leads to different cell types from the same genome.

• Gene expression is regulated at multiple stages: transcription, RNA processing, translation, and post-translation.

Regulation of Chromatin Structure

• Chromatin organization packs DNA and helps regulate gene expression.

• Gene promoter location relative to nucleosomes and scaffold attachment sites influences transcription.

• Genes in condensed heterochromatin are usually not expressed.

• Chemical modifications to histone proteins and DNA affect chromatin structure and gene expression.

Histone Modifications and DNA Methylation

• Histone acetylation: Acetyl groups attach to histone tails, loosening chromatin and promoting transcription.

• Methylation: Addition of methyl groups condenses chromatin, reducing transcription.

• DNA methylation: Methyl groups added to cytosine bases; heavily methylated genes are not expressed.

• Methylation patterns are inherited after cell division.

Epigenetic Inheritance

• Chromatin modifications can be inherited without changing DNA sequence (epigenetic inheritance).

• Epigenetic modifications are reversible, unlike DNA mutations.

• Example: Maternal diet can affect gene expression in offspring (see Figure 15.8).

Regulation of Transcription Initiation

• Chromatin-modifying enzymes control gene expression by altering DNA accessibility for transcription machinery.

Organization of a Typical Eukaryotic Gene and Its Transcript

• Most eukaryotic genes have multiple control elements (noncoding DNA segments) that bind transcription factors.

• Control elements and transcription factors are critical for precise gene regulation in different cell types.

The Roles of General and Specific Transcription Factors

• General transcription factors act at all gene promoters.

• Specific transcription factors bind to control elements near or far from the promoter.

• Initiation of transcription requires RNA polymerase II and transcription factors.

Enhancers and Specific Transcription Factors

• Proximal control elements are close to the promoter; distal control elements (enhancers) may be far away or in introns.

• An activator protein binds to an enhancer and stimulates transcription.

• Activators have DNA-binding and activation domains; they facilitate protein-protein interactions for transcription initiation.

• Some transcription factors act as repressors, inhibiting gene expression by blocking activator binding or altering chromatin structure.

Combinatorial Control of Gene Activation

• Precise transcription control depends on the combination of activators binding to control elements.

• With a limited number of control element sequences, many combinations are possible, allowing complex regulation.

Coordinately Controlled Genes in Eukaryotes

• Co-expressed eukaryotic genes are usually not organized into operons.

• Genes with the same control elements can be scattered across chromosomes but are activated by the same signals.

Summary Table: Operon Types and Regulation

Operon Type	Default State	Regulation Mechanism	Example
Repressible	On	Repressor + Corepressor turns off	trp operon
Inducible	Off	Inducer inactivates repressor	lac operon

Key Terms and Definitions

• Operon: A unit of DNA containing a cluster of genes under control of a single promoter and operator.

• Repressor: Protein that binds to the operator to block transcription.

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

• Corepressor: Molecule that activates a repressor to shut off transcription.

• Enhancer: DNA sequence that increases transcription when bound by activators.

• Epigenetics: Study of heritable changes in gene function not involving changes in DNA sequence.

Equations and Models

• Feedback inhibition (metabolic pathway):

$\text{End product inhibits enzyme activity:} \\ E_1 \xrightarrow{\text{Enzyme 1}} E_2 \xrightarrow{\text{Enzyme 2}} E_3 \xrightarrow{\text{Enzyme 3}} \text{End Product}$

• Gene regulation by operon:

$\text{Transcription} \rightarrow \text{Translation} \rightarrow \text{Protein}$

Example

• trp operon: When tryptophan is absent, the operon is on and enzymes for tryptophan synthesis are produced. When tryptophan is present, it acts as a corepressor, activating the repressor and turning the operon off.

• lac operon: In the absence of lactose, the repressor is active and the operon is off. When lactose is present, allolactose (inducer) inactivates the repressor, turning the operon on and allowing enzymes for lactose metabolism to be produced.

Additional info:

• Regulation of gene expression is fundamental for development, adaptation, and disease prevention in all organisms.

• Epigenetic changes, such as DNA methylation and histone modification, can be influenced by environmental factors and are reversible.