Difficulty: Intermediate | Prerequisites: Part 1 notes (chromatin structure, regulatory sequences, transcriptional machinery). Familiarity with basic signal transduction concepts is helpful.
Part 1 covered how eukaryotes open chromatin and assemble the transcription machinery. This second set of notes covers the other side: how genes are actively silenced, how external signals reach the nucleus to switch genes on or off, and a detailed case study of the yeast GAL gene system. The GAL system is a favourite exam topic because it ties together activators, repressors, co-repressors, and signal-dependent regulation in one clean, well-characterised pathway. If you are comfortable with both parts, you can reason through almost any eukaryotic gene regulation question.
Genes are silenced through repressor binding, HDAC recruitment, DNA methylation, and imprinting. External signals (steroid and non-steroid hormones) reach the transcription machinery via distinct pathways. The yeast GAL gene system is a textbook example of how an activator (Gal4p), a repressor (Gal80p), and a signal-responsive inducer (Gal3p) work together to sense galactose and glucose and toggle gene expression accordingly.
Repressor (transcriptional)
A protein that reduces or prevents transcription of a target gene. Repressors can work by blocking activator binding, interacting directly with activators to neutralise them, or recruiting chromatin-modifying complexes. Think of a repressor as a lock that keeps the transcription machinery from engaging.
Histone deacetylase (HDAC)
An enzyme that removes acetyl groups from histone tails, promoting chromatin condensation and transcriptional silencing. Covered in Part 1, but critical here because many repressors function by recruiting HDACs to target genes.
Gene imprinting
The epigenetic silencing of one parental allele so that only the allele from the other parent is expressed. In simple terms, the cell "remembers" which copy of a gene came from mum and which from dad, and deliberately shuts one of them off.
DNA methylation
The addition of a methyl group to cytosine bases in DNA (typically at CpG dinucleotides), carried out by DNA methyltransferases. Methylation inhibits transcription factor binding and can recruit proteins that promote heterochromatin formation. Think of it as a chemical "do not read" stamp on the DNA.
Steroid hormone
A lipophilic signalling molecule (e.g. oestrogen, testosterone, cortisol) that crosses the cell membrane, binds intracellular nuclear receptors, and directly modulates transcription by binding to hormone response elements (HREs) on DNA.
Hormone response element (HRE)
A specific DNA sequence recognised by a hormone-receptor complex. When the receptor binds its hormone and then binds the HRE, it activates (or represses) transcription of nearby genes.
Non-steroid hormone
A signalling molecule (e.g. peptide hormones, epinephrine) that cannot cross the membrane and instead binds to cell-surface receptors, triggering intracellular signalling cascades that ultimately alter transcription factor activity.
G protein-coupled receptor (GPCR)
A membrane receptor that, upon ligand binding, activates intracellular G proteins, which in turn initiate signalling cascades involving second messengers and protein kinases, eventually influencing gene expression.
Gal4p
The transcriptional activator in yeast that binds UAS_GAL_ sequences to drive expression of galactose metabolism genes. It contains a DNA-binding domain and a separate activation domain.
Gal80p
The repressor protein that binds directly to the activation domain of Gal4p in the absence of galactose, preventing Gal4p from activating transcription. Think of it as a cap that sits on Gal4p and blocks its "on" switch.
Gal3p
A galactose-sensing protein. When galactose is present, Gal3p binds to Gal80p, pulling it away from Gal4p and freeing the activation domain.
Mig1
A transcriptional repressor in yeast that mediates glucose repression. When glucose is present, Mig1 is dephosphorylated, enters the nucleus, and represses GAL gene transcription, ensuring the cell preferentially uses glucose over galactose.
Blocking activator binding: some repressors bind to DNA sites that overlap with or sit adjacent to activator binding sites, physically preventing the activator from attaching.
Direct interaction with activators: certain repressors bind directly to an activator protein, masking its activation domain or altering its conformation so it can no longer recruit the transcription machinery. Gal80p's inhibition of Gal4p is a classic example of this.
Recruitment of HDACs: repressors can recruit histone deacetylases to the promoter region. HDACs remove acetyl groups from nearby histones, causing chromatin to condense and shutting down transcription. This is a common and potent silencing mechanism.
Imprinting is essential for normal mammalian development. Certain genes are expressed from only the maternal or the paternal allele; the other is epigenetically silenced.
DNA methylation is a primary mechanism of imprinting. Methylation patterns are established during gametogenesis and maintained through cell division by maintenance methyltransferases (e.g. DNMT1).
Methylated DNA can recruit methyl-CpG-binding proteins (e.g. MeCP2), which in turn recruit HDACs and other repressive complexes, reinforcing the silent state.
Aberrant DNA methylation is a hallmark of many cancers: hypermethylation of tumour suppressor promoters silences them, while global hypomethylation can lead to genomic instability.
Steroid hormones are lipophilic, so they pass directly through the plasma membrane without needing a surface receptor.
Inside the cell, they bind to nuclear receptors (a family of ligand-activated transcription factors). The hormone-receptor complex then binds to HREs in the promoter or enhancer regions of target genes.
This binding can recruit coactivators (and their associated HAT activity) to activate transcription, or corepressors to silence it, depending on the receptor and the context.
Non-steroid hormones (peptides, amines) are hydrophilic and cannot cross the membrane. They bind to cell-surface receptors, commonly GPCRs.
GPCR activation triggers a cascade: the G protein activates effector enzymes (e.g. adenylyl cyclase), producing second messengers (e.g. cAMP), which activate protein kinases (e.g. PKA).
These kinases phosphorylate transcription factors (e.g. CREB), enabling them to enter the nucleus and activate or repress target genes.
The key difference from steroid signalling: the hormone never enters the cell. The signal is relayed entirely through intracellular intermediaries.
This system is a model for understanding how eukaryotic cells integrate multiple environmental signals into a single transcriptional decision.
Gal4p: constitutively bound to UAS_GAL_ sequences upstream of galactose metabolism genes (GAL1, GAL7, GAL10). It has a DNA-binding domain (Zn₂Cys₆ zinc cluster) and a potent activation domain.
Gal80p: binds directly to Gal4p's activation domain, masking it. When Gal80p is bound, Gal4p remains on the DNA but cannot activate transcription.
Gal3p: a cytoplasmic sensor. When galactose is present, Gal3p binds galactose and ATP, undergoes a conformational change, and interacts with Gal80p.
Gal4p sits on UAS_GAL_, but Gal80p covers its activation domain.
GAL genes are off (or at very low basal levels).
Galactose-bound Gal3p sequesters Gal80p away from Gal4p (in some models Gal3p pulls Gal80p into the cytoplasm; in others it simply disrupts the Gal80p-Gal4p interaction on the DNA).
Gal4p's activation domain is exposed, recruiting coactivators and the transcription machinery.
GAL genes are fully induced.
Glucose is the preferred carbon source. Even if galactose is available, the cell represses GAL genes when glucose is abundant.
Mig1-mediated repression: in the presence of glucose, the Snf1 kinase is inactive, so Mig1 remains dephosphorylated. Dephosphorylated Mig1 enters the nucleus and binds to the GAL1 promoter region, recruiting the Tup1-Ssn6 corepressor complex to repress transcription.
When glucose is depleted, Snf1 kinase is activated, phosphorylates Mig1, and Mig1 is exported from the nucleus, lifting repression.
This dual-input system ensures GAL genes are expressed only when galactose is available and glucose is not.
DNA methylation patterns are used clinically as cancer biomarkers. Hypermethylation of specific gene promoters (e.g. MGMT in glioblastoma) guides treatment decisions, since MGMT methylation status predicts response to the chemotherapy agent temozolomide. The steroid hormone signalling pathway is the basis for widely used drugs: glucocorticoids (e.g. dexamethasone) are synthetic steroid hormones that modulate gene expression to suppress inflammation. The GAL system, while a yeast model, provided foundational insights into the Gal4-UAS system now used globally as a genetic tool in Drosophila research to drive expression of any gene in specific tissues.
Students sometimes think DNA methylation is always permanent. It is not. Methylation can be reversed by demethylase enzymes, and patterns are dynamically regulated during development and in response to environmental cues.
A common error is confusing imprinting with X-inactivation. Imprinting silences one allele based on parental origin (which parent it came from); X-inactivation silences one entire X chromosome at random in each cell. Both are epigenetic, but the logic is different.
Students often assume steroid and non-steroid hormones use the same signalling strategy. They do not. Steroids enter the cell and bind intracellular receptors; non-steroids signal from outside through surface receptors and second-messenger cascades.
In the GAL system, students sometimes think Gal4p leaves the DNA when the gene is repressed. It does not. Gal4p stays bound to the UAS; Gal80p simply masks its activation domain.
⚠️ Be able to explain all three mechanisms of transcriptional repression (blocking activator binding, direct activator interaction, HDAC recruitment).
⚠️ The GAL gene system is a very likely exam topic. Know the roles of Gal4p, Gal80p, and Gal3p, and what happens under each sugar condition.
⚠️ Understand the difference between steroid and non-steroid hormone signalling in terms of where the receptor is and how the signal reaches the nucleus.
⚠️ DNA methylation: know how it silences genes (blocks TF binding, recruits repressive complexes) and its connection to imprinting and cancer.
⚠️ Mig1 and glucose repression: know that glucose repression overrides galactose induction, and understand the Snf1/Mig1 phosphorylation switch.
True or False: Gal4p dissociates from DNA when GAL genes are repressed.
Fill in the blank: In the presence of galactose, ______ interacts with Gal80p to free the activation domain of Gal4p.
True or False: Steroid hormones require cell-surface receptors to initiate their signalling cascade.
Fill in the blank: DNA methylation typically occurs at ______ dinucleotides.
True or False: When glucose is present, Mig1 is phosphorylated and enters the nucleus to repress GAL genes.
Answers: 1. False (Gal4p stays bound; Gal80p masks its activation domain). 2. Gal3p. 3. False (steroid hormones cross the membrane and bind intracellular/nuclear receptors). 4. CpG. 5. False (Mig1 is dephosphorylated in the presence of glucose; phosphorylation by Snf1 causes it to leave the nucleus).
Q: Describe the three main mechanisms by which repressors silence transcription in eukaryotes.
A: (1) Repressors can bind to DNA at sites overlapping or adjacent to activator binding sites, physically blocking activator access. (2) Repressors can interact directly with an activator protein, masking its activation domain or otherwise neutralising it (as Gal80p does to Gal4p). (3) Repressors can recruit HDACs to the promoter, removing acetyl groups from histones and driving chromatin condensation, which makes the DNA inaccessible to the transcription machinery.
Q: Walk through the GAL gene regulation system under three conditions: (a) no galactose, (b) galactose only, (c) galactose plus glucose.
A: (a) Without galactose, Gal4p remains bound to UAS_GAL_ but Gal80p covers its activation domain, so transcription is off. (b) With galactose (and no glucose), Gal3p binds galactose, interacts with Gal80p, and relieves its inhibition of Gal4p. The activation domain is now exposed, and GAL genes are transcribed at high levels. (c) With both sugars present, glucose repression dominates. Mig1 (dephosphorylated because Snf1 is inactive when glucose is abundant) translocates to the nucleus and recruits the Tup1-Ssn6 corepressor to the GAL1 promoter, repressing transcription even if Gal4p is active.
Q: How does steroid hormone signalling differ from non-steroid hormone signalling at the mechanistic level?
A: Steroid hormones are lipophilic and pass through the cell membrane to bind intracellular nuclear receptors. The hormone-receptor complex then acts directly as a transcription factor, binding to hormone response elements (HREs) on DNA. Non-steroid hormones are hydrophilic and cannot cross the membrane. They bind to surface receptors (often GPCRs), triggering intracellular cascades involving G proteins, second messengers, and kinases, which ultimately phosphorylate transcription factors that then enter the nucleus to modulate gene expression.
Q: What is gene imprinting and why is it biologically significant?
A: Imprinting is the epigenetic silencing of one allele of a gene based on its parental origin, so only the maternal or the paternal copy is expressed. It is mediated primarily by DNA methylation established during gametogenesis. Imprinting is essential for normal mammalian development; disruptions cause disorders such as Prader-Willi syndrome and Angelman syndrome, depending on which parental allele is affected.
Gene silencing and DNA methylation connect to epigenetics and developmental biology, where methylation reprogramming during embryogenesis is a major area of study. The signal transduction content here is a direct bridge to cell signalling courses and to pharmacology (receptor agonists and antagonists). The GAL system connects to metabolic regulation and is widely referenced in genetics courses because the Gal4-UAS toolkit is one of the most commonly used gene expression systems in model organism research, particularly in Drosophila.
gene silencing, transcriptional repression, repressor protein, HDAC, histone deacetylase, gene imprinting, genomic imprinting, DNA methylation, CpG island, DNMT1, maintenance methylation, MeCP2, steroid hormone signalling, nuclear receptor, hormone response element, HRE, non-steroid hormone, GPCR, G protein-coupled receptor, second messenger, cAMP, PKA, CREB, GAL gene regulation, Gal4p, Gal80p, Gal3p, UAS, upstream activator sequence, Mig1, Snf1 kinase, glucose repression, catabolite repression, Tup1-Ssn6, yeast gene regulation, molecular biology II, UCF