# Lecture 13: Regulation of Gene Expression: Eukaryotes
## Genetics
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Describe the multiple levels at which eukaryotic gene expression can be regulated
2. Explain the roles of enhancers, silencers, and transcription factors in transcriptional regulation
3. Describe chromatin remodeling and its impact on gene accessibility
4. Explain post-transcriptional regulation including mRNA stability, miRNA-mediated silencing, and translational control
5. Discuss the role of signal transduction pathways in regulating gene expression
6. Compare prokaryotic and eukaryotic gene regulation strategies
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## Lecture Content
### I. Overview: Multiple Levels of Regulation
Eukaryotic gene regulation is far more complex than prokaryotic regulation and can occur at every step from chromatin to functional protein. At the **chromatin level**, regulation involves chromatin remodeling, histone modifications, and DNA methylation. At the **transcriptional level**, transcription factors, enhancers, silencers, and the Mediator complex determine whether and how efficiently a gene is transcribed. **Post-transcriptional regulation** encompasses RNA processing events such as alternative splicing and polyadenylation, as well as mRNA export from the nucleus. **mRNA stability** is controlled through degradation rates, AU-rich elements, and miRNA targeting. **Translational regulation** operates through initiation factor modification, upstream ORFs, and internal ribosome entry sites (IRES). Finally, **post-translational regulation** governs protein folding, modification, localization, and degradation via the ubiquitin-proteasome system. Although most regulation occurs at the transcriptional level, all of these layers are functionally important.
### II. Transcriptional Regulation: Cis-Regulatory Elements
The **core promoter** contains the TATA box, Inr, and DPE, which together provide the minimal elements for basal transcription. **Proximal promoter elements** (approximately 100-200 base pairs upstream), including the CAAT box and GC box, are bound by ubiquitous transcription factors such as Sp1, NF-Y, and CTF.
**Enhancers** are remarkable regulatory elements that can be located thousands or even millions of base pairs upstream, downstream, or within introns of their target gene. They function in either orientation and contain binding sites for multiple transcription factor activators. Enhancers contact their target promoters through **DNA looping**, a process mediated by the **Mediator complex** and facilitated by **cohesin/CTCF**. Enhancer activity is often tissue-specific and developmental stage-specific, and clusters of enhancers called **super-enhancers** drive high expression of cell identity genes. **Silencers** are analogous to enhancers but repress transcription through the binding of repressor proteins. **Insulators/boundary elements** block enhancer-promoter communication, with **CTCF** (CCCTC-binding factor) serving as the key insulator-binding protein that prevents enhancers from activating inappropriate promoters and helps organize topologically associating domains (TADs).
### III. Transcription Factors
**General transcription factors** (GTFs) including TFIIA, TFIIB, TFIID, TFIIE, TFIIF, and TFIIH are required for all Pol II transcription and assemble the pre-initiation complex (PIC) at the promoter. **Specific transcription factors** bind enhancers or silencers and can function as activators or repressors. They have a modular structure with distinct functional domains: a **DNA-binding domain** (common motifs include helix-turn-helix, zinc finger, leucine zipper, helix-loop-helix, and homeodomain), an **activation domain** (acidic, glutamine-rich, or proline-rich types that interact with coactivators, Mediator, or the general transcription machinery), and often a **dimerization domain** (many transcription factors function as homo- or heterodimers, particularly those with leucine zippers and bHLH domains).
**Activators** enhance transcription by recruiting coactivators, HATs, chromatin remodelers, or the Mediator complex. **Repressors** silence genes by recruiting HDACs, corepressors, or by directly blocking activator binding. The **Mediator complex** is a large multi-subunit assembly of approximately 26-30 subunits that bridges specific transcription factors bound at enhancers to the general transcription machinery at the promoter and is essential for regulated transcription.
Panel A: Diagram showing enhancer-promoter communication through DNA looping, with multiple transcription factors bound to an enhancer element, the Mediator complex bridging the loop to the promoter-bound general transcription factors and RNA Pol II, with cohesin/CTCF organizing the loop domain. Panel B: Structural motifs of DNA-binding domains illustrated: zinc finger (Cys2His2 coordinating a zinc ion), leucine zipper (two alpha-helices with leucine residues interdigitating), helix-turn-helix, and basic helix-loop-helix — each shown binding to DNA. Panel C: Overview diagram showing all levels of eukaryotic gene regulation from chromatin to protein, with arrows indicating where regulation occurs: chromatin remodeling, transcription, RNA processing, mRNA export, mRNA stability, translation, and post-translational modification/degradation.
### IV. Chromatin Remodeling and Histone Modifications
Chromatin structure controls DNA accessibility: tightly packed heterochromatin is transcriptionally silent, while open euchromatin is accessible to the transcription machinery. **Chromatin remodeling complexes** use ATP hydrolysis to alter nucleosome positioning. The **SWI/SNF family** slides, ejects, or restructures nucleosomes to expose DNA. The **ISWI family** spaces nucleosomes regularly and can either activate or repress transcription. The **CHD family** contains chromodomains that recognize methylated histones. The **INO80 family** incorporates histone variants and participates in DNA repair.
**Histone modifications** constitute the "histone code." **Acetylation** by HATs such as p300/CBP and GCN5 neutralizes positive charges on lysines, loosening chromatin and promoting gene activation. **Deacetylation** by HDACs restores positive charge, tightening chromatin and promoting gene repression. **Methylation** is context-dependent: H3K4me3 and H3K36me3 mark active promoters and gene bodies respectively, while H3K9me3 marks constitutive heterochromatin and H3K27me3 marks Polycomb-mediated repression. **Reader proteins** recognize these modifications through specific domains: bromodomains read acetylated lysines, chromodomains read methylated lysines, and PHD fingers and Tudor domains read various other modifications. **Histone variants** such as H2A.Z (transcription regulation), H3.3 (active chromatin), CENP-A (centromeres), and macroH2A (X-inactivation) add yet another layer of regulation.
### V. DNA Methylation
DNA methylation involves the addition of a methyl group to the 5' position of cytosine, producing 5-methylcytosine (5mC). In mammals, this occurs predominantly at **CpG dinucleotides**. **CpG islands**, regions of approximately 300-3,000 base pairs with high CpG density, are frequently found at gene promoters. Unmethylated CpG islands are associated with active genes, while methylated CpG islands are associated with silenced genes.
Three DNA methyltransferases carry out this modification: **DNMT1** (the maintenance methyltransferase) copies methylation patterns to the newly synthesized strand during replication, ensuring epigenetic inheritance through cell division; **DNMT3A and DNMT3B** (de novo methyltransferases) establish new methylation patterns. Methylated DNA is silenced through two mechanisms: methylated CpG sites recruit **methyl-CpG-binding proteins** (such as MeCP2 and MBD proteins) that in turn recruit HDACs and corepressors to compact chromatin, and 5mC can directly interfere with transcription factor binding. DNA methylation plays essential roles in X-inactivation, genomic imprinting, transposon silencing, and tissue-specific gene expression. In cancer, aberrant methylation manifests as global hypomethylation (promoting genomic instability) coupled with local hypermethylation of tumor suppressor gene promoters.
### VI. Post-Transcriptional Regulation
**Alternative splicing**, covered in detail in Lecture 10, is a major mechanism for generating protein diversity from a single gene. **mRNA stability and degradation** are regulated by the intrinsic half-life of each transcript, which can vary from minutes to days. **AU-rich elements (AREs)** in the 3' UTR serve as destabilizing signals that recruit proteins promoting deadenylation and decay. The primary degradation pathway, **deadenylation-dependent decay**, involves poly(A) tail shortening followed by decapping and then either 5' to 3' degradation by XRN1 or 3' to 5' degradation by the exosome. **Nonsense-mediated decay (NMD)** is a quality control mechanism that degrades mRNAs containing premature stop codons.
**MicroRNAs (miRNAs)** are small (approximately 22 nucleotide) non-coding RNAs that regulate gene expression post-transcriptionally. Their biogenesis proceeds from pri-miRNA through Drosha processing to pre-miRNA, which is exported to the cytoplasm and processed by Dicer into a mature miRNA duplex. One strand is loaded into **RISC** (RNA-induced silencing complex, containing Argonaute protein), which guides RISC to complementary sequences in the 3' UTR of target mRNAs. Imperfect complementarity leads to translational repression and/or mRNA destabilization, while perfect complementarity (more common in plants) leads to mRNA cleavage. Each miRNA can target hundreds of mRNAs, and each mRNA can be targeted by multiple miRNAs. Over 2,500 human miRNAs have been identified, and they are estimated to regulate more than 60% of all protein-coding genes.
### VII. Translational and Post-Translational Regulation
**Translational control** provides rapid, reversible regulation of protein production. Phosphorylation of eIF2-alpha inhibits global translation initiation as part of the stress response. **4E-BPs** (eIF4E-binding proteins) block the eIF4E-eIF4G interaction to repress cap-dependent translation and are regulated by the mTOR signaling pathway. The **iron response element (IRE)** system elegantly coordinates iron metabolism: when iron is low, IRP binds an IRE stem-loop in the 5' UTR of ferritin mRNA, blocking its translation, while simultaneously binding IREs in the 3' UTR of transferrin receptor mRNA to stabilize it. **Upstream open reading frames (uORFs)** in the 5' UTR can reduce translation of the main downstream ORF, and **IRES (Internal Ribosome Entry Sites)** enable cap-independent translation, used by some cellular mRNAs under stress and by viral mRNAs.
**Post-translational regulation** operates through protein phosphorylation and dephosphorylation (providing rapid, reversible modification), the **ubiquitin-proteasome pathway** (which tags proteins with polyubiquitin chains for degradation by the 26S proteasome, controlling the levels of cyclins during the cell cycle, p53 via MDM2, and IkB-alpha to activate NF-kB), and additional modifications including sumoylation, acetylation, and glycosylation.
Panel A: miRNA biogenesis and function pathway showing: transcription of pri-miRNA, Drosha processing in the nucleus to pre-miRNA (hairpin), nuclear export by Exportin-5, Dicer processing in the cytoplasm to miRNA duplex, loading of guide strand into RISC/Argonaute, and targeting of mRNA 3' UTR leading to translational repression or mRNA degradation. Panel B: Iron regulatory system diagram showing dual regulation: ferritin mRNA with IRE in the 5' UTR (IRP blocks translation when iron is low, translation proceeds when iron is high) and transferrin receptor mRNA with IREs in the 3' UTR (IRP stabilizes mRNA when iron is low, mRNA degraded when iron is high). Panel C: The ubiquitin-proteasome pathway showing E1 (ubiquitin-activating), E2 (ubiquitin-conjugating), and E3 (ubiquitin ligase) enzymes attaching polyubiquitin chains to a target protein, which is then recognized and degraded by the 26S proteasome.
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