Unit 6: Regulation of Gene Expression and Epigenetics - Subjective Questions

An operon is a cluster of functionally related genes controlled by a single promoter and regulatory region, transcribed together as a single polycistronic mRNA.

Structural components of an operon:

• Promoter (P): DNA sequence where RNA polymerase binds to initiate transcription.
• Operator (O): Regulatory DNA sequence to which a repressor protein binds, controlling transcription.
• Structural genes: The genes encoding proteins (usually enzymes) that are co-transcribed (e.g., lacZ, lacY, lacA in the lac operon).
• Regulatory gene (I): Located outside the operon; encodes a regulatory protein (repressor/activator).

Examples:

• lac operon: Involved in lactose metabolism (inducible system).
• trp operon: Involved in tryptophan biosynthesis (repressible system).

The operon model was proposed by Jacob and Monod (1961) and provides an efficient mechanism for coordinated gene regulation in prokaryotes.

The lac operon controls the metabolism of lactose in E. coli and is a classic example of an inducible (negatively regulated) system.

In the absence of lactose:

• The lac repressor (product of the lacI gene) binds tightly to the operator.
• This blocks RNA polymerase from transcribing the structural genes (lacZ, lacY, lacA).
• The operon is switched OFF.

In the presence of lactose:

• A small amount of lactose is converted to allolactose, which acts as the inducer.
• Allolactose binds to the repressor, causing an allosteric change that reduces its affinity for the operator.
• The repressor detaches, allowing RNA polymerase to transcribe the genes.
• The operon is switched ON, producing:
  • β-galactosidase (LacZ) – hydrolyses lactose
  • Permease (LacY) – transports lactose
  • Transacetylase (LacA)

Catabolite repression (positive control):

• When glucose is low, cAMP levels rise and cAMP-CAP complex binds the promoter to enhance transcription.
• This ensures the operon is maximally active only when glucose is absent and lactose is present.

Inducible operon:

• Normally switched OFF; turned ON in the presence of a specific molecule (inducer).
• Usually controls catabolic (breakdown) pathways.
• The repressor is active by default and binds the operator; the inducer inactivates it.
• Example: lac operon (inducer = allolactose).

Repressible operon:

• Normally switched ON; turned OFF in the presence of a specific molecule (corepressor).
• Usually controls anabolic (biosynthetic) pathways.
• The repressor is inactive by default; the corepressor activates it to bind the operator.
• Example: trp operon (corepressor = tryptophan).

The trp operon controls the biosynthesis of the amino acid tryptophan in E. coli and is a repressible system with two regulatory mechanisms.

1. Repression:

• When tryptophan is abundant, it acts as a corepressor.
• Tryptophan binds the inactive trp repressor, activating it.
• The activated repressor binds the operator, blocking transcription (OFF).
• When tryptophan is scarce, the repressor is inactive, allowing transcription (ON).

2. Attenuation (fine-tuning):

• Occurs in the leader region (trpL) upstream of the structural genes.
• The leader mRNA contains four regions that can form alternative secondary structures.
• Regions 3–4 form a terminator hairpin; regions 2–3 form an antiterminator.

High tryptophan:

• Ribosome translates the leader peptide quickly (rich in Trp codons).
• The 3–4 terminator forms → transcription stops early.

Low tryptophan:

• Ribosome stalls at Trp codons.
• The 2–3 antiterminator forms → transcription continues.

Attenuation allows the cell to respond to intermediate tryptophan levels, coupling transcription and translation.

Riboswitches are regulatory segments of mRNA (usually in the 5' untranslated region) that directly bind small metabolite molecules to control gene expression without the involvement of proteins.

Structural components:

• Aptamer domain: Binds the specific metabolite (ligand) with high specificity.
• Expression platform: Undergoes conformational change upon ligand binding, affecting transcription or translation.

Mechanism of action:

• When the target metabolite is present, it binds the aptamer.
• This induces a conformational change in the expression platform.
• The change can:
  • Terminate transcription by forming a terminator hairpin, OR
  • Block translation by sequestering the ribosome-binding site (Shine-Dalgarno sequence).

Examples:

• TPP riboswitch (thiamine pyrophosphate) – regulates thiamine biosynthesis.
• Riboflavin (FMN) riboswitch
• Glucosamine-6-phosphate (glmS) riboswitch – acts as a ribozyme.

Significance: Riboswitches represent a form of feedback regulation and are potential targets for novel antibiotics.

The bacteriophage lambda (λ) transcriptional switch is a genetic circuit that determines whether the phage follows the lytic or lysogenic pathway after infecting E. coli.

Key regulatory proteins:

• cI (lambda repressor): Promotes and maintains lysogeny.
• Cro protein: Promotes the lytic cycle.

Key operator regions:

• OR (right operator): Contains three binding sites — OR1, OR2, OR3.
• Two promoters: PRM (repressor maintenance) and PR (rightward, for Cro).

Lysogenic pathway (cI dominant):

• cI binds OR1 and OR2 with high affinity.
• This blocks PR (represses Cro) and stimulates PRM (enhances cI synthesis).
• Positive autoregulation maintains the lysogenic state.
• The prophage integrates into the host genome.

Lytic pathway (Cro dominant):

• Cro binds OR3 first, repressing PRM and thus cI synthesis.
• Without cI, PR remains active → lytic genes expressed.
• Phage replicates and lyses the host.

Induction:

• DNA damage activates RecA, which promotes cI autocleavage.
• This shifts the switch from lysogeny to lysis (e.g., during UV exposure).

This bistable switch is a classic model for understanding genetic decision-making.

Covalent histone modifications are chemical changes made to the amino-terminal tails of histone proteins that alter chromatin structure and regulate gene expression. These constitute the histone code.

Major types of modifications:

• Acetylation: Addition of acetyl groups to lysine residues by HATs (histone acetyltransferases).
  • Neutralizes positive charge → loosens chromatin → activates transcription.
  • Removed by HDACs (histone deacetylases) → represses transcription.
• Methylation: Addition of methyl groups to lysine/arginine by HMTs.
  • Effect depends on the residue: e.g., H3K4me = activation; H3K9me and H3K27me = repression.
• Phosphorylation: Added to serine/threonine; involved in chromatin condensation during mitosis and transcriptional activation.
• Ubiquitination and SUMOylation: Regulate chromatin dynamics and DNA repair.

Functional consequences:

• Modifications alter histone–DNA interactions and create binding sites for effector proteins (readers).
• They determine whether chromatin is euchromatin (active) or heterochromatin (silent).

These modifications are reversible and heritable, forming a core mechanism of epigenetic regulation.

Nucleosome remodelling refers to the ATP-dependent repositioning, restructuring, or ejection of nucleosomes to alter the accessibility of DNA to the transcription machinery.

Chromatin remodelling complexes:

• These are ATP-dependent protein complexes that use the energy of ATP hydrolysis to alter histone–DNA contacts.

Major families:

• SWI/SNF: Slides or ejects nucleosomes to expose promoters (activates transcription).
• ISWI: Organizes and spaces nucleosomes (often represses transcription).
• CHD: Can slide or eject nucleosomes; some contain chromodomains recognizing methylated histones.
• INO80/SWR1: Involved in histone variant exchange (e.g., H2A.Z incorporation) and DNA repair.

Mechanisms of remodelling:

• Sliding: Repositioning nucleosomes along DNA.
• Ejection: Complete removal of histone octamers.
• Histone variant exchange: Replacing canonical histones with variants.

Role in gene regulation:

• Opening chromatin exposes promoter/enhancer regions for transcription factors and RNA polymerase → activation.
• Compacting chromatin blocks access → repression.

Nucleosome remodelling works in concert with histone modifications to control gene expression dynamically.

DNA methylation is an epigenetic modification involving the addition of a methyl group (–CH₃) to the 5th carbon of cytosine residues, forming 5-methylcytosine.

Key features:

• Occurs mainly at CpG dinucleotides (cytosine followed by guanine).
• Regions rich in CpG are called CpG islands, often found near promoters.
• Catalyzed by DNA methyltransferases (DNMTs):
  • DNMT1: Maintenance methyltransferase (copies patterns during replication).
  • DNMT3a/3b: De novo methyltransferases (establish new patterns).

Role in gene regulation:

• Hypermethylation of promoter CpG islands generally causes transcriptional silencing by:
  • Blocking transcription factor binding.
  • Recruiting methyl-CpG-binding proteins (MeCP2, MBDs) that attract HDACs and remodelling complexes → condensed chromatin.
• Hypomethylation is associated with active transcription.

Biological significance:

• Genomic imprinting (parent-specific gene expression).
• X-chromosome inactivation in females.
• Silencing of transposable elements.
• Aberrant methylation is linked to cancer and other diseases.

DNA methylation patterns are heritable through cell division, making it central to epigenetic memory.

RNA interference (RNAi) is a conserved biological process in which small RNA molecules inhibit gene expression by targeting complementary mRNA for degradation or translational repression.

Key players:

• Dicer: An RNase III enzyme that cleaves double-stranded RNA (dsRNA) into small fragments.
• siRNA (small interfering RNA): ~21–23 nucleotides, derived from dsRNA.
• miRNA (microRNA): ~22 nucleotides, from endogenous hairpin precursors.
• RISC (RNA-Induced Silencing Complex): Contains Argonaute (Ago) protein.

RISC-mediated silencing mechanism:

1. dsRNA processing: Dicer cleaves long dsRNA into siRNA duplexes.
2. RISC loading: The siRNA duplex is loaded onto RISC.
3. Strand selection: The passenger strand is discarded; the guide strand is retained.
4. Target recognition: The guide strand directs RISC to complementary mRNA by base pairing.
5. Silencing:
   • Perfect complementarity (siRNA): Argonaute cleaves ('slices') the mRNA → degradation.
   • Partial complementarity (miRNA): Translation is repressed and mRNA destabilized.

Significance: RNAi is essential for gene regulation, antiviral defense, and is widely used as a research tool and in therapeutics.

Both siRNA and miRNA are small non-coding RNAs involved in RNAi, but they differ in several aspects.

Summary:

• siRNA is typically used in defense against foreign nucleic acids and in experimental gene knockdown.
• miRNA is an endogenous regulator fine-tuning expression of numerous genes during development and physiology.

Despite differences, both converge on the RISC complex and Argonaute proteins to silence gene expression.

RNA interference (RNAi) operates through several interconnected mechanisms to regulate gene expression.

1. mRNA Cleavage (Post-transcriptional gene silencing):

• Guide strand within RISC pairs perfectly with target mRNA.
• Argonaute (slicer) cleaves the mRNA, leading to degradation.

2. Translational Repression:

• With partial complementarity (typical of miRNA), RISC blocks translation initiation or elongation without cleaving the mRNA.

3. mRNA Destabilization:

• miRNA binding recruits factors that promote deadenylation (poly-A tail removal) and decapping, leading to mRNA decay.

4. Transcriptional Gene Silencing (TGS):

• Small RNAs guide chromatin-modifying complexes to specific loci.
• Promotes DNA methylation and heterochromatin formation (RITS complex in fission yeast).

Biological significance:

• Defense against viruses and transposable elements.
• Developmental regulation through miRNAs.
• Genome stability and heterochromatin maintenance.
• Applications: Gene knockdown in research, RNAi-based therapeutics (e.g., patisiran).

These mechanisms collectively provide precise, sequence-specific control over gene expression.

Heterochromatin is a tightly packed, transcriptionally inactive form of chromatin that plays a central role in gene silencing.

Role in gene silencing:

• Dense compaction restricts access of transcription factors and RNA polymerase to DNA.
• Marked by specific histone modifications, notably H3K9me2/3 and H3K27me3.
• HP1 (Heterochromatin Protein 1) binds H3K9me3 and spreads the silenced state.
• Associated with DNA methylation and hypoacetylated histones.
• Maintains genome stability by silencing repetitive and transposable elements.

Constitutive heterochromatin:

• Permanently condensed in all cell types.
• Found at centromeres and telomeres.
• Contains repetitive DNA (satellite sequences); rarely transcribed.

Facultative heterochromatin:

• Can switch between condensed (silent) and decondensed (active) states.
• Formation is regulated and cell-type/developmental-stage specific.
• Example: Inactive X chromosome (Barr body) in female mammals.

Heterochromatin is a key structural basis of epigenetic gene regulation.

Epigenetics refers to heritable changes in gene expression that occur without changes to the underlying DNA sequence. These changes are mediated by DNA methylation, histone modifications, and non-coding RNAs.

Key characteristics:

• Reversible yet heritable through cell divisions.
• Regulate gene activity in response to internal and external cues.

Environmental influences on epigenetics:

• Diet and nutrition:
  • Availability of methyl donors (folate, vitamin B12, choline) affects DNA methylation.
  • Example: The Agouti mouse model — maternal diet alters coat color and disease risk via methylation.
• Stress: Chronic stress alters methylation of genes such as the glucocorticoid receptor.
• Toxins and pollutants: Heavy metals, tobacco smoke, and endocrine disruptors modify epigenetic marks.
• Prenatal environment: The Dutch Hunger Winter studies showed famine exposure in utero caused lasting methylation changes.
• Lifestyle: Physical activity, alcohol, and smoking influence histone modifications and methylation.

Significance:

• Environmental epigenetic changes may be transmitted across generations (transgenerational inheritance).
• Explains gene–environment interactions in disease susceptibility.

Thus, epigenetics bridges the gap between genes and the environment.

Cancer arises not only from genetic mutations but also from epigenetic dysregulation that alters gene expression patterns.

Major epigenetic alterations in cancer:

1. Aberrant DNA methylation:

• Global hypomethylation: Causes genomic instability, activation of oncogenes, and reactivation of transposons.
• Promoter hypermethylation: Silences tumor suppressor genes (e.g., p16, BRCA1, MLH1), promoting uncontrolled cell growth.

2. Histone modification changes:

• Altered activity of HATs, HDACs, and HMTs disrupts normal chromatin states.
• Loss of activating marks or gain of repressive marks silences protective genes.

3. Chromatin remodelling defects:

• Mutations in remodelling complexes (e.g., SWI/SNF subunits) are common in many cancers.

4. Dysregulated non-coding RNAs:

• Abnormal miRNA expression can silence tumor suppressors or upregulate oncogenes.

Consequences:

• Silencing of tumor suppressors + activation of oncogenes → uncontrolled proliferation, evasion of apoptosis, and metastasis.

Therapeutic implications:

• Epigenetic changes are reversible, making them attractive drug targets.
• Epigenetic drugs: DNMT inhibitors (azacitidine, decitabine) and HDAC inhibitors (vorinostat, romidepsin).

Epigenetic therapy represents a promising frontier in cancer treatment.

Catabolite repression is a regulatory mechanism ensuring that bacteria preferentially use glucose (the most efficient energy source) over other sugars like lactose, even when both are available.

Role of glucose:

• When glucose is abundant, intracellular cAMP levels are low.
• When glucose is scarce, adenylate cyclase is active and cAMP levels rise.

CAP-cAMP complex (positive regulation):

• CAP (Catabolite Activator Protein), also called CRP, is a DNA-binding protein.
• CAP is inactive alone; it must bind cAMP to become functional.
• The CAP-cAMP complex binds a site upstream of the lac promoter.
• This enhances RNA polymerase binding, boosting transcription up to 50-fold.

Combined regulation of lac operon:

This dual control (negative by repressor, positive by CAP) ensures efficient energy utilization.

Transcriptional regulation differs significantly between prokaryotes and eukaryotes due to differences in cellular organization and genome structure.

Key points:

• Prokaryotic regulation is rapid and efficient, mainly at the level of transcription initiation via operons.
• Eukaryotic regulation is complex and multilayered, involving chromatin remodelling, epigenetic marks, and combinatorial transcription factor control.

Both systems ultimately aim to express the right genes at the right time in response to cellular needs.

X-chromosome inactivation (XCI) is a process by which one of the two X chromosomes in female mammals is transcriptionally silenced to achieve dosage compensation between males (XY) and females (XX).

Mechanism:

1. Counting and choice: The cell counts X chromosomes and randomly selects one to inactivate.
2. Xist RNA expression: The X-inactivation center (XIC) contains the Xist gene.
   • Xist (X-inactive specific transcript) is a long non-coding RNA expressed only from the inactive X.
3. Coating: Xist RNA coats the inactive X chromosome in cis.
4. Recruitment of silencing machinery:
   • Polycomb complexes add H3K27me3.
   • Histone deacetylation and DNA methylation occur.
   • Histone variant macroH2A is incorporated.
5. Heterochromatin formation: The X condenses into a Barr body (facultative heterochromatin).

Significance:

• Ensures equal gene dosage of X-linked genes between sexes.
• Results in mosaic expression in females (e.g., calico cat coat color).
• A model system for understanding facultative heterochromatin and epigenetic silencing.
• Once established, the inactive state is stably inherited through mitosis.

XCI beautifully illustrates coordinated epigenetic mechanisms working together.

Genomic imprinting is an epigenetic phenomenon in which certain genes are expressed in a parent-of-origin-specific manner — only the maternal or only the paternal allele is active, while the other is silenced.

Epigenetic basis:

• Imprinting is established primarily through DNA methylation at imprinting control regions (ICRs).
• Methylation marks are set during gametogenesis:
  • Different patterns in sperm vs. eggs.
• These marks are erased and reset in the germline of each generation.
• Histone modifications also contribute to the imprinted state.

Examples:

• IGF2 gene: Expressed only from the paternal allele.
• H19 gene: Expressed only from the maternal allele.

Biological importance:

• Regulates fetal growth and development (parental conflict hypothesis).
• Essential for normal placental function.

Associated disorders (imprinting defects):

• Prader-Willi syndrome: Loss of paternal gene expression on chromosome 15.
• Angelman syndrome: Loss of maternal gene expression on chromosome 15.
• Beckwith-Wiedemann syndrome: Involves IGF2/H19 region.

Imprinting demonstrates that both parental genomes are essential and functionally non-equivalent.

Histone acetylation and deacetylation are dynamic, reversible modifications that regulate chromatin structure and gene expression.

Histone acetylation:

• Catalyzed by HATs (Histone Acetyltransferases).
• Adds acetyl groups to lysine residues on histone tails.
• Neutralizes the positive charge of lysine, weakening histone–DNA interaction.
• Results in open, relaxed chromatin (euchromatin) → transcriptional activation.
• Acetyl-lysine marks are recognized by bromodomain-containing proteins.

Histone deacetylation:

• Catalyzed by HDACs (Histone Deacetylases).
• Removes acetyl groups, restoring the positive charge.
• Leads to compact chromatin (heterochromatin) → transcriptional repression.

Balance: The dynamic equilibrium between HATs and HDACs determines gene activity.

Therapeutic potential of HDAC inhibitors:

• HDACs are often overactive in cancers, silencing tumor suppressor genes.
• HDAC inhibitors (e.g., vorinostat/SAHA, romidepsin) restore acetylation and reactivate silenced tumor suppressors.
• Effects include: cell cycle arrest, apoptosis, and differentiation of cancer cells.
• Approved for cancers such as cutaneous T-cell lymphoma.
• Also being explored for neurodegenerative and inflammatory diseases.

Thus, targeting the acetylation machinery is a promising epigenetic therapy strategy.