Gene Regulation in Eukaryotes

Introduction to Gene Regulation

Gene regulation refers to the control of the level and timing of gene expression in response to internal and external cues. In eukaryotes, gene regulation is essential for cellular responses to environmental changes, the development of different cell types, and the progression through developmental stages.

• Environmental Response: Cells adjust gene expression in response to nutrients, stress, and other environmental factors.

• Cell Differentiation: Multicellular organisms use gene regulation to produce specialized cell types.

• Developmental Changes: Certain genes are expressed only during specific developmental stages (e.g., embryonic vs. adult).

Levels of Gene Expression Regulation

Gene expression in eukaryotes can be regulated at multiple levels:

• Transcriptional Regulation: Involves regulatory transcription factors, nucleosome arrangement, and DNA methylation.

• RNA Modification: Includes alternative splicing and RNA editing.

• Translational Regulation: Involves control of translation initiation, mRNA stability, and RNA interference.

• Post-Translational Regulation: Includes feedback inhibition and covalent modifications of proteins.

Combinatorial Control of Gene Expression

Definition and Mechanisms

Most eukaryotic genes are regulated by the combined action of multiple factors, a phenomenon known as combinatorial control.

• Multiple activator proteins can stimulate transcription.

• Multiple repressor proteins can inhibit transcription.

• Regulatory proteins may be modulated by small effector molecules, protein-protein interactions, or covalent modifications.

• Regulatory proteins can alter nucleosome positioning near promoters.

• DNA methylation and heterochromatin formation can inhibit transcription.

Regulatory Transcription Factors and Enhancers

Types and Structure of Transcription Factors

Transcription factors are proteins that influence the ability of RNA polymerase to transcribe genes. They are classified as:

• General Transcription Factors (GTFs): Required for the transcription of all protein-coding genes.

• Regulatory Transcription Factors: Modulate the expression of specific genes.

Transcription factors contain functional regions called domains (e.g., DNA-binding domain, dimerization domain, effector molecule binding site). A motif is a structural element within a domain that is conserved across different proteins.

Common Structural Motifs in Transcription Factors

• Helix-Turn-Helix: Two α-helices connected by a short turn; recognition helix fits into the major groove of DNA.

• Helix-Loop-Helix: Two α-helices connected by a longer, flexible loop; often mediates dimerization and DNA binding.

• Zinc Finger: Contains zinc ions stabilizing finger-like loops; multiple zinc fingers allow recognition of longer DNA sequences.

• Leucine Zipper: Leucine residues at every seventh position form a dimerization interface; DNA-binding domains extend into the major groove.

Regulatory Elements and Enhancers

Regulatory transcription factors recognize specific DNA sequences called cis-regulatory elements within enhancers (50–1000 bp regions). Binding of activators increases transcription (up-regulation), while repressors decrease transcription (down-regulation).

Mechanisms of Modulating Transcription Factor Activity

• Small Effector Molecule Binding: e.g., hormones can activate transcription factors.

• Protein-Protein Interactions: Dimerization may be required for DNA binding.

• Covalent Modification: e.g., phosphorylation can activate or inactivate transcription factors.

Chromatin Structure and Gene Regulation

Chromatin Conformations

• Closed Conformation: Chromatin is tightly packed; transcription is difficult or impossible.

• Open Conformation: Chromatin is accessible to transcription factors; transcription can occur.

ATP-Dependent Chromatin Remodeling

Chromatin remodeling complexes use ATP hydrolysis to reposition, evict, or replace nucleosomes, altering DNA accessibility. Major families include SWI/SNF, ISWI, INO80, and Mi-2.

• Change in Nucleosome Position: Nucleosomes can be slid along DNA locally or over long distances.

• Eviction of Histone Octamers: Removal of nucleosomes creates nucleosome-free regions (NFRs).

• Change in Nucleosome Composition: Replacement of standard histones with histone variants can affect gene expression.

Histone Variants and the Histone Code

• Histone Variants: Modified forms of core histones (H1, H2A, H2B, H3, H4) with specialized roles in chromatin structure and function.

• Histone Code Hypothesis: Specific patterns of histone modifications (acetylation, methylation, phosphorylation) serve as binding sites for regulatory proteins, influencing chromatin structure and gene expression.

Histone Acetylation and Deacetylation

• Acetylation: Addition of acetyl groups (COCH3) to lysine residues by histone acetyltransferases (HATs) reduces positive charge, loosening DNA-histone interaction and favoring open chromatin.

• Deacetylation: Removal of acetyl groups by histone deacetylases (HDACs) restores positive charge, tightening DNA-histone interaction and promoting closed chromatin.

DNA Methylation

Mechanism and Effects

DNA methylation is the covalent addition of methyl groups (CH3) to cytosine bases, typically at CpG dinucleotides, by DNA methyltransferases. Methylation generally inhibits gene transcription in eukaryotes.

• Unmethylated DNA: Both DNA strands lack methyl groups.

• Hemimethylated DNA: Only one strand (parental) is methylated after DNA replication.

• Fully Methylated DNA: Both strands are methylated after maintenance methylation.

CpG Islands and Gene Expression

• CpG Islands: Regions rich in CpG dinucleotides near gene promoters.

• Housekeeping Genes: CpG islands are typically unmethylated, allowing broad expression.

• Tissue-Specific Genes: CpG islands may be methylated, silencing gene expression in certain tissues.

Methylation can inhibit transcription by blocking activator binding or by recruiting methyl-CpG-binding proteins that promote chromatin compaction.

Inheritance of DNA Methylation

• De Novo Methylation: Addition of methyl groups to previously unmethylated DNA (rare, highly regulated).

• Maintenance Methylation: After DNA replication, hemimethylated DNA is recognized and methylated on the new strand to restore full methylation.

Insulators

Functions of Insulators

• Barrier Insulators: Prevent the spread of chromatin modifications (e.g., acetylation) into neighboring regions.

• Enhancer-Blocking Insulators: Prevent enhancers from activating unintended genes by blocking enhancer-promoter communication across the insulator.

Gene Activation and Repression

Steps in Gene Activation

1. Regulatory transcription factors (activators) bind to enhancers.

2. Activators recruit coactivators, including chromatin remodeling complexes and histone-modifying enzymes.

3. RNA polymerase II and general transcription factors assemble at the core promoter to form the preinitiation complex.

4. RNA polymerase II initiates and elongates the RNA transcript.

Nucleosome Organization

• Nucleosome-Free Regions (NFRs): Found at the transcriptional start site (core promoter) and termination site; required but not sufficient for transcription.

• Well-Positioned Nucleosomes: Flank the NFRs at gene boundaries; less regular spacing within gene bodies.

Preinitiation Complex Formation

1. Activator binds to enhancer (often in an NFR).

2. Activator recruits coactivators (chromatin remodelers, histone-modifying enzymes).

3. Chromatin remodeling and histone modification occur (nucleosome repositioning, eviction, or variant incorporation).

4. General transcription factors and RNA polymerase II assemble at the core promoter.

Transcription Elongation

Key Events During Elongation

• Open Complex Formation: TFIIH unwinds DNA at the promoter, allowing template strand access.

• Promoter Escape: Phosphorylation of the C-terminal domain (CTD) of RNA polymerase II enables it to leave the promoter and begin elongation.

• Proximal Promoter Pausing: RNA polymerase II pauses near the start site, regulated by DSIF and NELF; release requires phosphorylation by P-TEFb.

• Histone Modifications: Histones ahead of the polymerase are acetylated and evicted; behind, they are deacetylated and reassembled.

Additional Roles of Promoter Pausing

• Regulates transcription levels.

• Maintains nucleosome-free regions.

• Allows time for recruitment of RNA modification factors and elongation factors.

Hormone-Responsive Gene Regulation

Steroid Hormones and Steroid Receptors

• Steroid Receptors: Regulatory transcription factors activated by steroid hormones (e.g., glucocorticoids).

• Glucocorticoid Response Elements (GREs): DNA sequences in enhancers that bind glucocorticoid receptor dimers, activating multiple genes simultaneously.

• Mechanism: Hormone enters cell, binds receptor (releasing HSP 90), receptor dimerizes, enters nucleus, binds GRE, and activates transcription.

CREB Protein and cAMP Response

• CREB (cAMP Response Element-Binding Protein): Activated by phosphorylation in response to increased cAMP.

• CRE (cAMP Response Element): DNA sequence (5'-TGACGTCA-3') bound by CREB.

• Activation Pathway: Extracellular signal → G protein-coupled receptor → adenylyl cyclase → cAMP → protein kinase A → CREB phosphorylation → CBP recruitment → transcription activation.

• Unphosphorylated CREB binds DNA but cannot activate transcription.

Gene Repression Mechanisms

• Repressors: Proteins that bind regulatory elements and inhibit transcription, often by recruiting corepressors.

• Short-Term Repression: Direct inhibition of gene activation steps.

• Long-Term Repression: Gene silencing via heterochromatin formation (see Chapter 16).

Chromatin Immunoprecipitation Sequencing (ChIP-Seq)

Purpose and Procedure

ChIP-Seq is a technique for mapping the locations of specific nucleosomes, histone variants, or histone modifications across the genome.

1. Crosslink proteins to DNA with formaldehyde; lyse cells and fragment DNA.

2. Add antibodies specific to the protein of interest; immunoprecipitate complexes.

3. Reverse crosslinks and purify DNA.

4. Add sequencing linkers and perform DNA sequencing.

5. Map sequences to the genome to determine protein binding sites.

Comparison of Transcriptional Regulation Across Domains of Life

Example: Glucocorticoid Hormone Action

• Glucocorticoid hormone enters the cell, binds to its receptor, causing dimerization and nuclear import.

• The receptor dimer binds to GREs in enhancers, activating transcription of target genes involved in metabolism.

Example: CREB Protein Activation

• Extracellular signal (e.g., epinephrine) increases cAMP, activating protein kinase A.

• Protein kinase A phosphorylates CREB, which then recruits CBP and activates transcription at CRE sites.

Additional info: The above notes synthesize and expand upon the accessible reading version of the Chapter 15 slide deck, providing definitions, mechanisms, and examples for key concepts in eukaryotic gene regulation. For further details on heterochromatin and epigenetic regulation, see Chapter 16.