Regulation of Gene Expression in Eukaryotes (Chapter 16) — Genetics Study Notes

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Regulation of Gene Expression in Eukaryotes

Introduction

Gene expression regulation in eukaryotes is a complex, multi-level process that ensures genes are expressed in the right cell type, at the right time, and in the correct amount. This regulation is essential for normal development, cellular function, and adaptation to environmental changes.

Cell-type specific gene expression: Different cell types express unique sets of genes (e.g., keratin in skin cells, myosin in muscle cells).
Conditional gene expression: Genes may be upregulated or downregulated in response to physiological conditions (e.g., low oxygen increases erythropoietin production).
Misregulation: Abnormal gene expression can cause developmental defects, disease, or cancer.
Cis-acting and trans-acting factors: These elements regulate transcription initiation and overall gene expression.

16.1 Organization of the Eukaryotic Cell Facilitates Gene Regulation at Several Levels

Comparison of Bacterial and Eukaryotic Gene Regulation

Eukaryotic gene regulation is more complex than bacterial regulation due to additional steps and compartmentalization.

Bacterial regulation: Transcription and translation occur simultaneously in the cytoplasm; regulation is closely tied to metabolic needs.
Eukaryotic regulation: DNA is packaged with histones into chromatin, affecting accessibility. Transcription occurs in the nucleus, translation in the cytoplasm, and mRNA undergoes modifications (splicing, capping, poly-A tail addition).

16.2 Eukaryotic Gene Expression Is Influenced by Chromatin Modifications

Chromatin Structure and Territories

Chromatin is composed of DNA, histones, and non-histone proteins, forming nucleosomes. Chromatin compaction regulates access to DNA for transcription, replication, and repair.

Chromosome territories: Each chromosome occupies a distinct domain in the nucleus, with active genes located at the edges for efficient transcription and RNA processing.
Interchromatin compartments: Channels between chromosome territories facilitate mRNA modification and export.

Chromatin Conformation and Remodeling

Closed chromatin (heterochromatin): Inhibits transcription due to tight DNA-histone association.
Open chromatin (euchromatin): Permits transcription; can be promoted by histone variant H2A.Z.
Chromatin remodeling: Repositioning or removal of nucleosomes by remodeling complexes increases DNA accessibility.

Histone Modifications

Acetylation: Addition of acetyl groups by Histone Acetyltransferases (HATs) neutralizes positive charges on histone tails, loosening DNA-histone interaction and promoting transcription.
Deacetylation: Histone Deacetylases (HDACs) remove acetyl groups, tightening DNA-histone interaction and repressing transcription.
Methylation and phosphorylation: Other covalent modifications that influence chromatin state and gene expression.

DNA Methylation

CpG islands: Regions with high frequency of CG dinucleotides; methylation of cytosine in these regions typically represses gene expression.

16.3 Eukaryotic Transcription Initiation Requires Specific Cis-Acting Sites

Core Promoters and Elements

Transcription initiation in eukaryotes requires specific DNA sequences called cis-acting elements.

Core promoters: Minimal DNA region required for accurate transcription initiation, including the transcription start site.
Focused core promoters: Direct transcription from a single start site; common in lower eukaryotes.
Dispersed core promoters: Initiate transcription from multiple weak start sites; prevalent in housekeeping genes.
Core-promoter elements: DNA motifs such as the TATA box and Initiator (Inr) element bind regulatory proteins.

Proximal-Promoter Elements

Located upstream of the TATA box and TFIIB Recognition Element (BRE).
Enhance basal transcription levels and provide binding sites for sequence-specific DNA-binding proteins.
Examples: CAAT box and GC boxes; mutations can significantly affect transcription.

16.4 Eukaryotic Transcription Initiation Is Regulated by Transcription Factors That Bind to Cis-Acting Sites

Transcription Factors and Regulation

Transcription factors are proteins that bind to cis-acting elements to regulate gene expression.

General Transcription Factors (GTFs): Required for the assembly of the pre-initiation complex (PIC) at the promoter.
Specific transcription factors: Bind to enhancers or silencers to modulate transcription rates.

16.5 Activators and Repressors Interact with General Transcription Factors and Affect Chromatin Structure

Mechanisms of Transcription Activation and Repression

Activators and repressors: Modulate transcription by interacting with GTFs and chromatin.
DNA loops: Bring activators and GTFs to the promoter region, facilitating PIC assembly.
Enhanceosomes: Large complexes of activators and coactivators that direct transcription activation.
Repressor proteins: Bind to silencer elements, decreasing PIC assembly and RNA polymerase II release.

16.6 Regulation of Alternative Splicing Determines Which RNA Spliceforms of a Gene Are Translated

Alternative Splicing

Alternative splicing allows a single gene to produce multiple protein isoforms with different functions.

Isoforms: Different protein variants produced from the same gene.
Spliceforms: mRNA variants generated by including or omitting different exons during splicing.
Defects in splicing: Associated with diseases such as myotonic dystrophy.

16.7 Gene Expression Is Regulated by mRNA Stability and Degradation

mRNA Stability and Decay

Steady-state level: Amount of mRNA available for translation depends on rates of transcription and degradation.
Exoribonucleases: Enzymes that degrade mRNA by removing terminal nucleotides; 5' cap and 3' poly-A tail protect mRNA.
Deadenylation-dependent decay: Initiated by deadenylase shortening the poly-A tail, followed by decapping and degradation.
Nonsense-mediated decay (NMD): Surveillance mechanism that degrades mRNAs with premature stop codons.

16.8 Noncoding RNAs Play Diverse Roles in Posttranscriptional Regulation

Types and Functions of Noncoding RNAs (ncRNAs)

Noncoding RNAs (ncRNAs): RNAs that do not encode proteins but regulate gene expression post-transcriptionally (e.g., rRNA, tRNA, miRNA, siRNA, lncRNA).
RNA interference (RNAi): ncRNAs guide sequence-specific silencing of mRNAs.
Small noncoding RNAs (sncRNAs): siRNAs and miRNAs (20-31 nt, double-stranded with 2-nt 3' overhangs).
siRNAs: Derived from long double-stranded RNAs (e.g., viruses, transposons); protect against foreign genetic elements.
miRNAs: Produced in the nucleus, processed by Drosha and Dicer; bind to mRNA response elements (MREs) to block translation or promote degradation.
RNA-Induced Silencing Complex (RISC): Contains Argonaute (AGO); uses one strand of siRNA/miRNA as a guide to target complementary mRNA.
Long noncoding RNAs (lncRNAs): Regulate gene expression and chromatin modifications; can act as competing endogenous RNAs (ceRNAs) to sequester miRNAs.

16.10 Posttranslational Modifications Regulate Protein Activity

Types of Posttranslational Modifications

Phosphorylation: Addition of phosphate groups to serine, threonine, or tyrosine residues by kinases; removed by phosphatases. Regulates protein activity, localization, and interactions.
Ubiquitination: Covalent attachment of ubiquitin molecules to proteins, targeting them for degradation by the proteasome.
Proteasome: Degrades poly-ubiquitinated proteins, recycling amino acids and removing damaged or misfolded proteins.

Protein Folding and Disease

Chaperones: Proteins that assist in proper folding of other proteins.
Misfolded proteins: Can aggregate and cause diseases such as Huntington's, Alzheimer's, and Parkinson's.
Prions: Infectious protein aggregates causing neurodegenerative diseases (e.g., mad cow disease).

Summary Table: Key Regulatory Mechanisms in Eukaryotic Gene Expression

Regulatory Level	Mechanism	Key Features
Chromatin Structure	Remodeling, Histone Modification, DNA Methylation	Controls DNA accessibility
Transcription Initiation	Core Promoters, Proximal Elements, Transcription Factors	Determines where and how much transcription occurs
Post-transcriptional	Alternative Splicing, mRNA Stability, ncRNAs	Generates protein diversity, regulates mRNA lifespan
Post-translational	Phosphorylation, Ubiquitination	Modifies protein activity and degradation

Example: Myotonic Dystrophy as a Spliceopathy

Myotonic dystrophy: A disease caused by defects in alternative splicing, leading to muscle weakness, cardiac abnormalities, and other symptoms.

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