# Lecture 24: Regulation of Gene Expression in Eukaryotes ## General Biology I — Molecular & Cellular --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Explain why eukaryotic gene regulation is more complex than prokaryotic regulation 2. Describe the role of chromatin remodeling, histone modifications, and DNA methylation in regulating transcription 3. Explain transcriptional regulation by transcription factors, enhancers, silencers, and the mediator complex 4. Describe post-transcriptional mechanisms of gene regulation, including alternative splicing, mRNA stability, and RNA interference 5. Explain the principles of epigenetics and how gene expression patterns can be inherited without changes to DNA sequence --- ## Lecture Content ### I. Overview -- Why Is Eukaryotic Regulation More Complex? Eukaryotic gene regulation is far more elaborate than its prokaryotic counterpart, and several features of eukaryotic biology explain why. Most eukaryotes are **multicellular**, meaning that every cell in the organism contains the same genome, yet different cell types--a liver cell and a neuron, for instance--express vastly different subsets of genes. This **differential gene expression** is the basis of cellular specialization. Eukaryotic genomes are also substantially **larger** and contain far more genes than prokaryotic genomes, requiring more sophisticated control systems. Furthermore, eukaryotic DNA is packaged into **chromatin**, adding an entire layer of regulation--accessibility of DNA to the transcription machinery--that simply does not exist in prokaryotes. Eukaryotic gene expression is regulated at **multiple levels**: chromatin structure and accessibility, transcriptional initiation, post-transcriptional processing (including RNA splicing, transport, and stability), translational control, and post-translational modifications and protein degradation. Unlike prokaryotes, eukaryotes **lack operons**; each gene typically has its own promoter and is transcribed as a **monocistronic mRNA**. Coordinated expression of functionally related genes is achieved not through physical clustering but through shared regulatory sequences recognized by common transcription factors. ### II. Chromatin Structure and Regulation #### Chromatin Packing -- Review DNA in eukaryotic cells is wrapped around **histone octamers** (composed of two copies each of H2A, H2B, H3, and H4) to form **nucleosomes**, the fundamental repeating unit of chromatin. **Linker histone H1** helps compact nucleosomes into higher-order structures such as the 30-nm fiber. Chromatin exists in two general states: **euchromatin**, which is loosely packed and transcriptionally active or potentially active, and **heterochromatin**, which is tightly compacted and transcriptionally silent. **Constitutive heterochromatin** remains permanently condensed (as at centromeres and telomeres), while **facultative heterochromatin** is condensed in some cells or at some developmental stages but not others--the inactive X chromosome forming the Barr body is a classic example. #### Histone Modifications The N-terminal **tails** of histones protrude from the nucleosome and are subject to a variety of covalent modifications that profoundly influence chromatin structure and gene accessibility. **Acetylation**, catalyzed by **histone acetyltransferases (HATs)**, adds acetyl groups to lysine residues on histone tails. This neutralizes the positive charges on the histones, weakening their electrostatic interaction with the negatively charged DNA and opening the chromatin--acetylation is therefore associated with **gene activation**. The reverse reaction, carried out by **histone deacetylases (HDACs)**, removes acetyl groups, restores positive charges, compacts chromatin, and is associated with **gene repression**. **Methylation** of lysine or arginine residues by histone methyltransferases has context-dependent effects. Trimethylation of lysine 4 on histone H3 (H3K4me3) is an activating mark, while trimethylation of H3K9 or H3K27 recruits heterochromatin proteins and is associated with repression. **Phosphorylation** of serine and threonine residues participates in chromosome condensation during mitosis and in the DNA damage response. **Ubiquitination** of lysine residues has effects that depend on which histone and which residue is modified. The **histone code hypothesis** proposes that the specific combination of modifications on a nucleosome constitutes a "code" that is read by effector proteins to determine the transcriptional state of the underlying DNA. A multi-panel figure of histone modifications and their effects on chromatin. Panel A: A nucleosome with histone tails protruding, showing specific residues labeled with their modifications (acetylation on K9, K14 of H3; methylation on K4, K9, K27 of H3). Panel B (Active chromatin): Histones are heavily acetylated by HATs, DNA is loosely wrapped, nucleosomes are spaced apart, and RNA polymerase with transcription factors access the promoter. Panel C (Silent chromatin): Histones are deacetylated by HDACs and methylated at H3K9, chromatin is tightly compacted into heterochromatin, and HP1 (heterochromatin protein 1) is recruited, blocking transcription factor access. #### Chromatin Remodeling Complexes **ATP-dependent chromatin remodeling complexes** such as **SWI/SNF**, **ISWI**, and **Mi-2/NuRD** use the energy of ATP hydrolysis to physically alter the relationship between DNA and histones. They can **slide** nucleosomes along the DNA to expose or conceal regulatory sequences, **eject** nucleosomes entirely, or **replace** standard histones with specialized histone variants (such as H2A.Z or H3.3) that have different biophysical properties. By controlling which stretches of DNA are accessible to transcription factors and RNA polymerase, these complexes play an essential role in regulating gene expression. #### DNA Methylation The addition of a methyl group to the **5-carbon of cytosine**, forming **5-methylcytosine (5mC)**, represents a major epigenetic regulatory mechanism. Methylation occurs primarily at **CpG dinucleotides** (a cytosine immediately followed by a guanine). Many gene promoters, particularly in mammals, contain **CpG islands**--regions enriched in CpG dinucleotides, found near approximately 60-70% of human gene promoters. Methylation of CpG islands is generally associated with **gene silencing**: methyl-CpG-binding proteins such as **MeCP2** recruit HDACs and chromatin remodeling complexes that compact the chromatin, and methylation can also directly block transcription factor binding. DNA methylation patterns are faithfully maintained through cell division by **DNMT1 (maintenance methyltransferase)**, which recognizes hemimethylated DNA at replication forks and methylates the newly synthesized strand. New methylation patterns are established during development by the **de novo methyltransferases DNMT3a and DNMT3b**. DNA methylation plays critical roles in **X-chromosome inactivation** (silencing one X in female mammals), **genomic imprinting** (parent-of-origin-specific gene expression), **silencing of transposable elements** (protecting genome stability), and **cancer** (where aberrant methylation patterns--hypermethylation of tumor suppressor promoters and global hypomethylation--are hallmarks of many malignancies). ### III. Transcriptional Regulation #### The Eukaryotic Promoter and Basal Transcription RNA Polymerase II transcribes protein-coding genes, and its **core promoter** includes elements such as the **TATA box** (located approximately 25-30 bp upstream of the transcription start site and recognized by **TBP**, the TATA-binding protein subunit of **TFIID**), the **Inr (initiator) element** surrounding the start site, and the **DPE (downstream promoter element)** found in TATA-less promoters. The **general (basal) transcription factors** (TFIIA, TFIIB, TFIID, TFIIE, TFIIF, and TFIIH) assemble at the core promoter in an ordered sequence with RNA Polymerase II to form the **pre-initiation complex (PIC)**. On its own, however, this complex supports only a **low, basal level** of transcription--the regulatory transcription factors are what determine whether a gene is transcribed at high levels or silenced. #### Regulatory Transcription Factors Gene-specific **activators** and **repressors** modulate transcription dramatically above or below basal levels. Activators bind to **enhancer** sequences and stimulate transcription. They typically have a modular architecture comprising a **DNA-binding domain** (which recognizes a specific DNA sequence) and an **activation domain** (which interacts with coactivators or the basal transcription machinery). Common DNA-binding motifs include the **zinc finger** (stabilized by zinc ions), the **helix-turn-helix** (including the homeodomain found in developmental regulators), the **leucine zipper** (which mediates dimerization, with the adjacent basic region contacting DNA), and the **helix-loop-helix (bHLH)** motif. Repressors bind to **silencer** sequences and inhibit transcription, often by recruiting HDACs or by competing with activators for overlapping binding sites. #### Enhancers and Silencers **Enhancers** are regulatory DNA sequences that can be located **thousands of base pairs** from the gene they regulate--upstream, downstream, or even within introns--and function in an orientation-independent manner. They contain clusters of binding sites for multiple transcription factors. **Silencers** are analogous elements that mediate repression. The key question is how a distant enhancer communicates with the promoter. The answer involves **DNA looping**: the intervening DNA loops out, bringing the enhancer-bound activators into direct physical contact with the transcription machinery at the promoter. The **Mediator complex**, a large multi-subunit assembly, serves as a molecular bridge between the activators and RNA Polymerase II. **Insulators (boundary elements)** are DNA sequences that, when positioned between an enhancer and a promoter, block their interaction, ensuring that enhancers activate only their intended target genes. A diagram showing the mechanism of enhancer-mediated transcriptional activation through DNA looping. At the top, a linear DNA molecule is shown with an enhancer element located 10 kb upstream of the gene promoter. Activator proteins are bound to the enhancer. Below, the DNA loops so that the enhancer-bound activators physically contact the Mediator complex, which in turn interacts with RNA polymerase II and the general transcription factors (TFIID/TBP on the TATA box, TFIIB, TFIIF, TFIIE, TFIIH) at the core promoter. Coactivators (including HATs) are also shown modifying nearby histones to open the chromatin. The looped-out DNA is stabilized by cohesin ring complexes. #### Combinatorial Control Eukaryotic gene expression is governed by the **combinatorial action** of multiple transcription factors. The specific combination of activators and repressors present in a given cell determines which genes are expressed--different cell types express different combinations of transcription factors, and this is the molecular basis of **cell-type-specific gene expression**. A single transcription factor may regulate many genes, and a single gene may be regulated by many transcription factors. This combinatorial logic allows a relatively limited number of transcription factors to generate the enormous diversity of gene expression patterns required to build and maintain a complex organism. ### IV. Post-Transcriptional Regulation #### Alternative Splicing A single pre-mRNA can be spliced in **multiple ways** to produce different mature mRNAs and therefore different proteins from the same gene. Which exons are included or excluded depends on the cell type, developmental stage, or environmental signals, and is controlled by **splicing factors** (SR proteins and hnRNPs) that bind to **exonic and intronic splicing enhancers (ESE, ISE) and silencers (ESS, ISS)**. The *Drosophila* **Dscam gene** can generate over **38,000 different mRNA variants** through alternative splicing, an extreme example of the principle. The **calcitonin/CGRP gene** illustrates tissue-specific splicing: in the thyroid it is spliced to produce calcitonin, while in neurons it produces CGRP (calcitonin gene-related peptide). Alternative splicing is extraordinarily prevalent in humans--an estimated **95% of multi-exon genes** undergo it--and is a major mechanism for expanding **proteome diversity** far beyond what the gene count alone would suggest. #### mRNA Stability and Degradation The **half-life** of an mRNA--ranging from minutes for rapidly regulated transcripts to hours or days for stable ones--determines how long it is available for translation and thus how much protein it produces. The **5' cap and 3' poly-A tail** protect mRNA from exonuclease degradation; shortening of the poly-A tail (**deadenylation**) is typically the first step in mRNA decay. **AU-rich elements (AREs)** in the 3' UTR target mRNAs for rapid degradation and are found in many cytokine and proto-oncogene transcripts. **RNA-binding proteins** can either stabilize or destabilize specific mRNAs. Additionally, **nonsense-mediated mRNA decay (NMD)** serves as a quality control pathway that identifies and degrades mRNAs containing premature stop codons, preventing translation of truncated, potentially harmful proteins. #### RNA Interference (RNAi) **RNA interference** is a powerful mechanism of **post-transcriptional gene silencing** mediated by small RNA molecules. Discovered by **Andrew Fire and Craig Mello (1998)** in *C. elegans* (Nobel Prize in 2006), RNAi operates through two major classes of small regulatory RNAs. **MicroRNAs (miRNAs)** are endogenous, approximately 22-nucleotide single-stranded RNAs encoded in the genome. They are transcribed as longer precursors (pri-miRNA), processed by **Drosha** in the nucleus to form pre-miRNA, exported to the cytoplasm by Exportin-5, and further processed by **Dicer** into a mature miRNA duplex. One strand (the guide strand) is loaded into the **RISC (RNA-Induced Silencing Complex)**, which contains the **Argonaute** protein. The miRNA guides RISC to complementary sequences in the **3' UTR** of target mRNAs. In animals, where complementarity is typically imperfect, the result is **translational repression** and/or **mRNA destabilization** rather than outright cleavage. It is estimated that miRNAs regulate more than **60% of human protein-coding genes**. **Small interfering RNAs (siRNAs)** are derived from double-stranded RNA of exogenous origin (such as viral RNA) or from transposons. They are processed by **Dicer** into approximately 21-nucleotide duplexes and loaded into RISC. Because siRNAs typically show **perfect complementarity** to their targets, RISC cleaves and degrades the target mRNA. The siRNA pathway is an important defense against viruses and transposable elements and is widely used as a **research tool** for experimental gene knockdown. A diagram of the RNA interference (RNAi) pathway. On the left, the miRNA pathway: a pri-miRNA transcript forms a hairpin structure, which is cleaved by Drosha in the nucleus to form pre-miRNA. The pre-miRNA is exported to the cytoplasm by Exportin-5 and cleaved by Dicer into a miRNA duplex. One strand (guide strand) is loaded into RISC/Argonaute. RISC binds to a partially complementary site in the 3' UTR of a target mRNA, leading to translational repression or deadenylation and mRNA decay. On the right, the siRNA pathway: long double-stranded RNA (from a virus or transgene) is cleaved by Dicer into siRNA duplexes. The guide strand is loaded into RISC, which binds to a perfectly complementary target mRNA and cleaves it, leading to mRNA degradation. ### V. Translational and Post-Translational Regulation #### Translational Control Cells also regulate how efficiently existing mRNAs are translated. **Phosphorylation of eIF2**, a key translation initiation factor, reduces global translation during stress, viral infection, or amino acid starvation. The **mTOR pathway** integrates signals from growth factors, nutrients, and cellular energy status to regulate translation initiation through phosphorylation of downstream targets including 4E-BP and S6 kinase. A beautiful example of mRNA-specific translational control involves **iron regulation**. **Ferritin mRNA** contains an **iron response element (IRE)** in its 5' UTR; when iron is low, **IRP (iron regulatory protein)** binds the IRE and blocks translation, but when iron is abundant, IRP releases and ferritin is translated to store the excess. Conversely, **transferrin receptor mRNA** has IREs in its 3' UTR, where IRP binding stabilizes the transcript when iron is low, increasing iron uptake--two opposite outcomes from the same regulatory protein, depending on the position of its binding site. Some mRNAs are stored with short poly-A tails in a translationally silent state and are activated by **cytoplasmic polyadenylation** at specific developmental stages, as occurs during oocyte maturation. #### Post-Translational Regulation After translation, protein levels and activity are controlled through several mechanisms. **Chaperones** (including Hsp70, Hsp90, and the chaperonins) assist proteins in folding correctly. The **ubiquitin-proteasome pathway** is the cell's primary system for targeted protein degradation: proteins are tagged with chains of **ubiquitin** (a small, 76-amino-acid protein) by a cascade of E1 (activating), E2 (conjugating), and E3 (ligating) enzymes, and polyubiquitinated proteins are then recognized and degraded by the **26S proteasome**, a barrel-shaped protease complex. This system controls the levels of key regulatory proteins including cyclins, p53, and various transcription factors. **Regulated proteolysis** also activates certain transcription factors--the **Notch** signaling pathway and the release of **NF-kB** from its inhibitor IkB both depend on controlled protein cleavage. ### VI. Epigenetics **Epigenetics** refers to heritable changes in gene expression that occur **without changes to the DNA nucleotide sequence**. The principal epigenetic mechanisms include **DNA methylation** (whose patterns are maintained through cell division by DNMT1), **histone modifications** (which can be propagated by recruitment of modifying enzymes to pre-existing marks on neighboring nucleosomes), and **non-coding RNAs** (some of which participate in establishing and maintaining epigenetic states, as exemplified by **Xist RNA** in X-inactivation). **Genomic imprinting** is a striking epigenetic phenomenon in which certain genes are expressed only from the maternal or paternal allele. The silenced allele is marked by DNA methylation established during gametogenesis. The gene **IGF2** is expressed only from the paternal chromosome, while **H19** is expressed only from the maternal chromosome. When deletions in the chromosome 15q region affect the paternal copy, the result is **Prader-Willi syndrome**; when the same region is deleted on the maternal copy, the result is **Angelman syndrome**--two entirely different disorders from the same chromosomal deletion, depending on parent of origin. **X-chromosome inactivation** in female mammals is mediated by **Xist (X-inactive specific transcript)**, a long non-coding RNA expressed from the X-inactivation center. Xist coats the chromosome from which it is transcribed, recruiting repressive histone modifications (H3K27me3) and DNA methylation to form the transcriptionally silent **Barr body**. Inactivation is random with respect to which X is silenced in any given cell but is **clonally inherited**--all daughter cells maintain the same inactive X. Environmental factors including diet, toxins, and stress can alter epigenetic marks, potentially affecting gene expression patterns not only within an individual but across generations. The **Dutch Hunger Winter** studies provided compelling evidence for this: prenatal famine exposure during World War II was associated with altered DNA methylation patterns and increased disease risk decades later in the affected individuals.