Regulation of Gene Expression (Ch. 18): Prokaryotic and Eukaryotic Mechanisms

Regulation of Gene Expression

Introduction to Regulation of Gene Expression

Both prokaryotic and eukaryotic cells can regulate or control their gene expression, allowing them to respond to environmental changes and cellular needs. Gene expression can be controlled at multiple stages, from DNA to functional protein.

• Chromatin Remodeling: Modifies DNA accessibility for transcription.

• Transcriptional Control: Regulates the initiation of transcription.

• Post-Transcriptional Control: Regulates mRNA after transcription.

• Translational Control: Regulates mRNA translation into protein.

• Post-Translational Control: Modifies proteins after translation.

Example: Regulation can occur at any of these five stages, as illustrated in the central dogma of molecular biology.

Positive vs. Negative Gene Regulation

• Negative Regulation: Inhibits gene expression by turning "off" the gene.

• Positive Regulation: Promotes gene expression by turning "on" the gene.

Example: A gene regulatory system can act as a "light switch" to turn gene expression on or off.

Prokaryotic Gene Regulation via Operons

Structure and Function of Operons

Prokaryotes often regulate groups of related genes using operons, which are clusters of genes transcribed as a single mRNA under the control of a single promoter.

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

• Operator: DNA segment within the promoter that acts as an on/off switch.

• Regulatory Gene: Encodes a repressor protein that can bind to the operator to block transcription.

Example: The lac operon and trp operon are classic examples of prokaryotic gene regulation.

Inducible vs. Repressible Operons

• Inducible Operon: Normally off but can be turned on (induced) by an inducer molecule (e.g., lac operon).

• Repressible Operon: Normally on but can be turned off (repressed) by a corepressor molecule (e.g., trp operon).

Type	Normally	Can be turned	Regulatory Molecule	Effect of Molecule
Inducible	OFF	ON	Inducer	Inactivates repressor
Repressible	ON	OFF	Corepressor	Activates repressor

The Lac Operon

The lac operon in E. coli encodes enzymes for lactose metabolism. It is an inducible operon regulated by the presence or absence of lactose and glucose.

• In the absence of lactose, the repressor binds to the operator, blocking transcription.

• In the presence of lactose, an inducer molecule (allolactose) inactivates the repressor, allowing transcription.

• Glucose levels also affect the operon via cAMP and CRP (catabolite activator protein).

Environmental Levels	cAMP Level	Lac Operon Expression
High Glucose, Low Lactose	Low	OFF
Low Glucose, High Lactose	High	ON

The trp Operon

The trp operon encodes enzymes for tryptophan synthesis. It is a repressible operon regulated by tryptophan levels.

• In the absence of tryptophan, the repressor is inactive, and the operon is transcribed.

• In the presence of tryptophan, tryptophan acts as a corepressor, activating the repressor to block transcription.

Review: Lac vs. trp Operons

Operon Type	Function	Regulatory Molecule	Effect
lac	Lactose metabolism	Inducer (allolactose)	Turns ON operon
trp	Tryptophan synthesis	Corepressor (tryptophan)	Turns OFF operon

Introduction to Eukaryotic Gene Regulation

Gene regulation in eukaryotes is essential for cellular differentiation and development. All cells in a multicellular organism have the same DNA, but different genes are expressed in different cell types.

Eukaryotic Chromatin Modifications

• Heterochromatin: Densely packed, transcriptionally inactive chromatin.

• Euchromatin: Loosely packed, transcriptionally active chromatin.

• Histone Acetylation: Addition of acetyl groups to histones, loosening chromatin and promoting transcription.

• DNA Methylation: Addition of methyl groups to DNA (usually cytosine), repressing transcription.

Eukaryotic Transcriptional Control

• Transcription Factors: Proteins that bind to specific DNA sequences to regulate transcription.

• General Transcription Factors: Required for transcription of all protein-coding genes.

• Specific Transcription Factors: Bind to enhancers or proximal control elements to regulate specific genes.

Type	Binding Site	Function
General	Promoter (TATA box)	Initiate transcription
Specific	Enhancer/Proximal Elements	Increase/decrease transcription rate

Eukaryotic Post-Transcriptional Regulation

• Alternative RNA Splicing: Different mRNAs produced from the same transcript.

• mRNA Protection: 5' cap and poly-A tail protect mRNA from degradation.

• RNA Interference (RNAi): Small RNAs (siRNA, miRNA) block translation or degrade mRNA.

Type	Function
miRNA	Degrades mRNA or blocks translation
siRNA	Degrades mRNA

Eukaryotic Post-Translational Regulation

• Protein Modification: Proteins can be activated, inactivated, or degraded after translation.

• Ubiquitination: Proteins tagged with ubiquitin are targeted for degradation by proteasomes.

Example: Ubiquitin ligase adds a ubiquitin peptide to a misfolded or non-functioning protein, marking it for destruction.

Summary Table: Key Mechanisms of Gene Regulation

Stage	Mechanism	Effect
Chromatin	Acetylation, Methylation	Open/close chromatin, regulate access
Transcription	Transcription factors, enhancers	Initiate or repress transcription
Post-Transcription	RNA splicing, RNAi	Modify mRNA, regulate stability
Translation	Regulatory proteins, miRNA	Control translation rate
Post-Translation	Ubiquitination, phosphorylation	Modify or degrade protein

Additional info: These notes include both foundational concepts and specific examples (lac and trp operons, chromatin modifications, transcription factors, RNA interference, and protein degradation) relevant to the regulation of gene expression in prokaryotes and eukaryotes, as covered in a General Biology course.