What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

How Transcription Is Turned Off: Mechanisms and Regulation

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Key Takeaways

Transcriptional shut-off is a critical regulatory process, as vital as initiation, to prevent wasteful gene expression, aberrant protein production, and disease states like cancer. Mechanisms range from local repressor protein binding to global chromatin compaction and DNA methylation.
Repressor proteins employ diverse strategies to inhibit transcription, including physically blocking RNA polymerase binding to promoters, interfering with activator protein function, or recruiting co-repressor complexes that modify chromatin structure.
Chromatin structure plays a pivotal role in gene silencing, with heterochromatin (compacted DNA) associated with transcriptional repression and euchromatin (open DNA) with active transcription. Histone modifications like deacetylation and methylation (e.g., H3K9me, H3K27me3) and DNA methylation at CpG islands are key epigenetic mechanisms that establish and maintain repressive chromatin states.
Transcription termination, the process of releasing RNA polymerase and the nascent RNA transcript at the end of a gene, is distinct from gene silencing, which prevents transcription initiation. Termination is a routine step in the transcription cycle, essential for proper gene expression, while silencing is a regulatory state of gene inactivity.
Post-initiation regulatory mechanisms, such as promoter-proximal pausing in metazoans enforced by NELF and DSIF, and attenuation in bacteria, provide rapid and reversible control by halting transcription shortly after initiation.
Small RNAs, including siRNAs and piRNAs, can mediate transcriptional gene silencing (TGS) by guiding chromatin-modifying enzymes to homologous DNA sequences, leading to heterochromatin formation and stable gene repression, particularly important for silencing repetitive elements and transposons.

Introduction to Transcription Regulation

Transcription is the process by which RNA polymerase (RNAP) reads a DNA template to synthesize a complementary RNA molecule. In bacteria, a single RNA polymerase holoenzyme (core enzyme plus sigma factor) carries out all transcription. In eukaryotes, three nuclear RNA polymerases exist—RNA polymerase I (rRNA), RNA polymerase II (mRNA and some noncoding RNAs), and RNA polymerase III (tRNA, 5S rRNA, and other small RNAs)—each with distinct promoters and accessory factors. The rate of transcription is not a fixed property of a gene; it is dynamically controlled by the cell in response to developmental cues, metabolic state, and environmental signals.

Turning off transcription is as important as turning it on. Constitutive expression of genes that should be silent wastes energy, produces aberrant proteins, and can drive disease. For example, inappropriate expression of oncogenes such as MYC or silencing of tumor suppressors such as TP53 are hallmarks of cancer. The cell therefore employs multiple, layered mechanisms to ensure that transcription is shut off rapidly, specifically, and reversibly when required. These layers range from the local action of repressor proteins at promoter DNA to global changes in chromatin compaction and even the modification of the DNA sequence itself through methylation.

The Central Dogma and Gene Expression

The central dogma of molecular biology describes the flow of genetic information: DNA → RNA → protein. Transcription is the first and most heavily regulated step in this pathway. While post-transcriptional regulation (mRNA stability, translation efficiency, protein degradation) can fine-tune protein levels, controlling transcription is the most energy-efficient and commonly used point of regulation. A gene that is not transcribed produces no mRNA and therefore no protein, regardless of downstream regulatory mechanisms. This makes transcriptional shut-off the most decisive way to eliminate gene function.

Why Turning Off Matters

Consider a bacterium shifting from glucose to lactose as a carbon source. The lac operon must be activated to metabolize lactose, but when glucose returns, the operon must be rapidly repressed to avoid wasteful synthesis of β-galactosidase. Similarly, in multicellular organisms, cell-type-specific gene expression requires that most of the ~20,000 protein-coding genes be stably silenced in any given cell type. A liver cell and a neuron contain the same genome, but they express different subsets of genes because transcription is selectively turned off. Failure to silence genes leads to developmental abnormalities, immune dysregulation, and cancer. Thus, understanding how transcription is turned off is fundamental to understanding gene regulation, development, and disease.

The Role of Transcription Factors and Repressors

Transcription factors are proteins that bind to specific DNA sequences to activate or repress transcription. Repressors are a class of transcription factors that decrease or eliminate transcription. They achieve this by several mechanisms: blocking RNA polymerase binding, interfering with activator function, or recruiting chromatin-modifying enzymes that create a repressive environment.

DNA-Binding Repressors

The classic example of a DNA-binding repressor is the Lac repressor (LacI) in Escherichia coli. LacI binds as a tetramer to two operator sites (O1 and O3) in the lac operon, looping the intervening DNA and physically blocking RNA polymerase from initiating at the promoter. When allolactose (the inducer) binds LacI, the repressor undergoes a conformational change that reduces its affinity for the operator, allowing transcription to proceed. This is negative regulation: the default state is off, and the inducer turns it on.

In eukaryotes, sequence-specific repressors bind to DNA elements called silencers. These are often located upstream of the promoter, but can also be downstream or within introns. One well-studied example is the yeast repressor α2, which binds to a specific operator sequence in the MATa genes to repress their expression in α cells. α2 recruits the co-repressor Tup1–Ssn6 complex, which then compacts chromatin and inhibits transcription.

Repressors can also act by quenching activators. For instance, the Drosophila protein Krüppel binds to the activation domain of the activator Bicoid, preventing it from stimulating transcription of target genes. This is a direct protein–protein interaction that does not require DNA binding by Krüppel at the target promoter.

Co-Repressors and Corepressor Complexes

Some repressors do not directly contact RNA polymerase or the promoter. Instead, they recruit co-repressor complexes—multiprotein assemblies that modify chromatin structure. A canonical example is the nuclear receptor co-repressor (NCoR) and silencing mediator for retinoid and thyroid hormone receptors (SMRT). These co-repressors associate with unliganded nuclear receptors (such as thyroid hormone receptor) bound to DNA. NCoR/SMRT then recruit histone deacetylases (HDACs), enzymes that remove acetyl groups from lysine residues on histone tails. Deacetylation restores the positive charge on histones, increasing their affinity for DNA and promoting chromatin compaction, which reduces access for the transcriptional machinery.

Another important co-repressor complex is Polycomb repressive complex 2 (PRC2), which methylates histone H3 on lysine 27 (H3K27me3). This mark is recognized by Polycomb repressive complex 1 (PRC1), which ubiquitylates histone H2A and promotes chromatin compaction. Polycomb-mediated silencing is essential for maintaining homeotic (Hox) gene repression during development in animals.

Chromatin Structure and Epigenetic Silencing

In eukaryotes, DNA is wrapped around histone octamers to form nucleosomes, the basic unit of chromatin. The accessibility of DNA to RNA polymerase and transcription factors is governed by chromatin structure. Transcriptionally silent genes are typically found in regions of compact chromatin (heterochromatin), while active genes are in more open chromatin (euchromatin). Epigenetic modifications—changes to DNA or histones that do not alter the underlying sequence—are central to establishing and maintaining these states.

Histone Modifications

Histone modifications are covalent post-translational changes to the N-terminal tails of histones. They include acetylation, methylation, phosphorylation, ubiquitylation, and sumoylation. The pattern of modifications on a given nucleosome is sometimes called the "histone code," because different combinations are read by specific effector proteins to produce distinct outcomes.

Acetylation of histone lysines (e.g., H3K9ac, H3K14ac, H4K16ac) is generally associated with active transcription. Acetyl groups neutralize the positive charge of lysine, weakening histone–DNA interactions and making DNA more accessible. Conversely, deacetylation by HDACs is associated with transcriptional repression. Histone deacetylase inhibitors such as trichostatin A (TSA) are used experimentally to reactivate silenced genes; treating cells with 100–300 nM TSA for 12–24 hours is a standard protocol to globally increase histone acetylation.

Methylation of histones is more complex. Methylation of H3K4 is associated with active promoters, while methylation of H3K9 and H3K27 is associated with repression. H3K9me2/3 is recognized by heterochromatin protein 1 (HP1), which binds to the methylated mark and recruits additional methyltransferases, creating a self-propagating domain of heterochromatin. This is a key mechanism for silencing repetitive elements and maintaining centromere function.

DNA Methylation and Gene Silencing

DNA methylation in mammals occurs predominantly at cytosine residues in CpG dinucleotides. DNA methyltransferases (DNMTs) catalyze the transfer of a methyl group from S-adenosylmethionine (SAM) to the C5 position of cytosine, producing 5-methylcytosine (5mC). CpG islands—regions of high CpG density often found in promoters—are usually unmethylated in active genes. Methylation of CpG islands in promoters is strongly associated with stable transcriptional silencing.

DNA methylation silences transcription by two main mechanisms. First, methylated CpGs physically impede the binding of certain transcription factors that recognize unmethylated CpG motifs. Second, methyl-CpG-binding domain (MBD) proteins, such as MeCP2 and MBD1, bind to methylated DNA and recruit co-repressor complexes containing HDACs and histone methyltransferases. This creates a repressive chromatin environment that is heritable through cell division: DNMT1 maintains methylation patterns by copying methylation from the parental strand to the daughter strand during DNA replication.

An important example is genomic imprinting, where one allele of a gene is silenced by DNA methylation based on its parental origin. The IGF2/H19 locus is imprinted: the maternal allele expresses H19 but not IGF2, while the paternal allele expresses IGF2 but not H19. This differential expression is controlled by a differentially methylated region (DMR) that acts as a chromatin boundary. Loss of imprinting at this locus is associated with Beckwith-Wiedemann syndrome and certain cancers.

Termination of Transcription

Transcription termination is the process by which RNA polymerase stops elongation and releases the nascent RNA transcript. This is distinct from gene silencing, which prevents initiation. Termination is essential for proper gene expression: without it, RNA polymerase would read through into downstream genes, producing read-through transcripts that may be nonfunctional or toxic.

Intrinsic Termination

In bacteria, intrinsic (rho-independent) terminators are sequences in the DNA that cause RNA polymerase to pause and dissociate. The terminator consists of a GC-rich hairpin loop followed by a run of 4–8 uridine residues in the RNA transcript. As RNA polymerase transcribes the hairpin sequence, the RNA folds into a stable stem-loop structure. This hairpin disrupts the RNA–DNA hybrid in the elongation complex, and the weak A-U base pairs in the uridine tract further destabilize the complex, causing RNA polymerase to release the transcript. The efficiency of intrinsic termination depends on the stability of the hairpin and the length of the U-tract; mutations that destabilize the hairpin reduce termination efficiency.

Rho-Dependent Termination

Rho-dependent termination requires the Rho protein, a hexameric RNA helicase. Rho binds to a rut (rho utilization) site on the nascent RNA, typically a C-rich, G-poor sequence of about 40–80 nucleotides. Rho then translocates along the RNA in a 5'→3' direction, using ATP hydrolysis for energy, and catches up to the paused RNA polymerase at the termination site. Rho's helicase activity then unwinds the RNA–DNA hybrid, releasing the transcript. Rho-dependent termination is used for many bacterial genes that lack intrinsic terminators, and it also serves as a quality-control mechanism to silence aberrant transcription.

Eukaryotic Termination

Eukaryotic termination is coupled to RNA processing. For RNA polymerase II, termination is triggered by the polyadenylation signal (AAUAAA) in the nascent RNA. The cleavage and polyadenylation specificity factor (CPSF) and cleavage stimulation factor (CstF) recognize this signal and cleave the RNA 10–30 nucleotides downstream. The "torpedo" model proposes that the 5'→3' exonuclease Xrn2 degrades the remaining RNA still attached to RNA polymerase, and when it catches up to the polymerase, it triggers termination. An alternative "allosteric" model suggests that polyadenylation factors induce a conformational change in RNA polymerase that destabilizes the elongation complex. In practice, both mechanisms likely contribute.

RNA polymerase I terminates at a specific terminator sequence (Sal box in yeast) recognized by the Reb1 protein, which acts as a roadblock. RNA polymerase III terminates at a run of 4–5 thymidine residues in the DNA, which causes the polymerase to pause and release the transcript without requiring additional factors.

Post-Initiation Regulation: Pausing and Premature Termination

Transcription can be turned off not only before initiation but also shortly after it begins. These mechanisms provide rapid, reversible control and are particularly important for genes that must respond quickly to environmental signals.

Promoter-Proximal Pausing

In metazoans, RNA polymerase II often initiates transcription and then pauses after synthesizing 20–60 nucleotides, before promoter escape. This is called promoter-proximal pausing. The pause is enforced by the negative elongation factor (NELF) and DRB sensitivity-inducing factor (DSIF). Release from pausing requires the kinase P-TEFb (positive transcription elongation factor b), which phosphorylates NELF, DSIF, and the C-terminal domain (CTD) of RNA polymerase II. Phosphorylation of NELF causes it to dissociate, while phosphorylation of DSIF converts it from a negative to a positive elongation factor.

Pausing is a major regulatory checkpoint. For example, the HSP70 gene in Drosophila is paused at the promoter in unstressed cells. Upon heat shock, P-TEFb is rapidly recruited, and the paused polymerase is released, allowing very fast induction of HSP70 mRNA. Similarly, many proto-oncogenes such as MYC are regulated by pausing. Drugs that inhibit P-TEFb, such as flavopiridol, cause global transcriptional shutdown of paused genes and are being investigated as anticancer agents.

Attenuation in Bacteria

Attenuation is a mechanism of transcriptional regulation in bacteria where transcription is terminated prematurely before the full mRNA is synthesized. The classic example is the trp operon in E. coli, which encodes enzymes for tryptophan biosynthesis. The 5' region of the trp mRNA contains a leader sequence (trpL) that can form two alternative hairpin structures. When tryptophan levels are high, ribosomes translate the leader peptide (a string of tryptophan codons) rapidly and stall at the tryptophan codons. This stalling prevents formation of the antiterminator hairpin, allowing the terminator hairpin to form, which causes RNA polymerase to terminate transcription. When tryptophan is scarce, ribosomes stall at the tryptophan codons, allowing the antiterminator hairpin to form, which prevents terminator formation and allows transcription to continue.

Attenuation is a rapid response system because it couples translation and transcription directly. It is also used in amino acid biosynthetic operons other than trp, including his, phe, and leu.

RNA Interference and Transcriptional Gene Silencing

RNA interference (RNAi) is best known for degrading mRNA in the cytoplasm, but small RNAs can also direct transcriptional silencing in the nucleus. This process, called transcriptional gene silencing (TGS), involves small RNAs guiding chromatin-modifying enzymes to homologous DNA sequences.

RNAi-Directed Chromatin Remodeling

In the fission yeast Schizosaccharomyces pombe, the RNAi machinery is required for the establishment and maintenance of heterochromatin at centromeres. The RNA-induced transcriptional silencing (RITS) complex contains the Argonaute protein Ago1, which binds to small interfering RNAs (siRNAs) derived from centromeric repeat transcripts. RITS is guided by base pairing between the siRNA and the nascent noncoding RNA transcribed from the centromere. This recruits the histone methyltransferase Clr4 (a homolog of human SUV39H1), which methylates H3K9. HP1 (Swi6 in S. pombe) binds to H3K9me and recruits additional factors, including the RNA-directed RNA polymerase complex (RDRC), which generates more siRNAs, creating a positive feedback loop that spreads and maintains silencing.

In plants, RNA-directed DNA methylation (RdDM) uses 24-nucleotide siRNAs to guide the DNA methyltransferase DRM2 to methylate cytosines in homologous DNA sequences. This is important for silencing transposons and repetitive elements. In mammals, the role of RNAi in transcriptional silencing is less clear, but there is evidence that piRNAs (PIWI-interacting RNAs) direct DNA methylation at transposon loci in the germline.

piRNA and Transposon Silencing

PIWI-interacting RNAs (piRNAs) are a class of small RNAs (24–31 nucleotides) that are expressed in animal gonads. They associate with PIWI proteins, a subfamily of Argonaute proteins. piRNAs are derived from transposon sequences and guide PIWI proteins to complementary transposon transcripts, leading to their cleavage. This is the primary defense against transposon mobilization in the germline. In addition to post-transcriptional silencing, piRNAs also direct the deposition of H3K9me3 and DNA methylation at transposon loci, ensuring their stable transcriptional silencing. In mice, mutations in piRNA pathway components such as MILI or MIWI2 result in transposon derepression and male sterility.

Methods to Study Transcription Shut-Off

Studying how transcription is turned off requires methods that can distinguish between changes in transcription rate, mRNA stability, and chromatin state. Several approaches are commonly used.

ChIP and ChIP-seq

Chromatin immunoprecipitation (ChIP) is used to determine whether a specific protein (e.g., a repressor, histone modification, or RNA polymerase) is associated with a particular DNA region. Cells are treated with formaldehyde to crosslink proteins to DNA, then chromatin is sheared by sonication into fragments of ~200–600 base pairs. An antibody specific to the protein of interest is used to immunoprecipitate the protein–DNA complexes. After reversing the crosslinks, the DNA is purified and analyzed by quantitative PCR (ChIP-qPCR) or by high-throughput sequencing (ChIP-seq). For example, to test whether a gene is silenced by Polycomb, you would perform ChIP with an anti-H3K27me3 antibody and measure enrichment at the gene promoter. A typical ChIP protocol uses 1% formaldehyde for 10 minutes at room temperature for crosslinking, followed by sonication to shear DNA.

Nuclear Run-On Assay

The nuclear run-on assay measures the density of RNA polymerases on a gene at a given moment, providing a direct measure of transcription rate. Nuclei are isolated from cells and incubated with labeled nucleotides (e.g., [α-³²P]UTP or biotin-labeled UTP). RNA polymerases that were engaged in transcription at the time of nuclear isolation continue to elongate, incorporating the labeled nucleotides into nascent RNA. The labeled RNA is then hybridized to a membrane containing immobilized DNA probes for the genes of interest. The amount of labeled RNA hybridized is proportional to the transcription rate. This method distinguishes transcriptional regulation from mRNA stability changes: if a gene is silenced, the run-on signal decreases, whereas if mRNA is merely degraded, the run-on signal remains unchanged.

Reporter Constructs

Reporter genes are widely used to study transcriptional regulation. A reporter construct contains the promoter of interest fused to a gene encoding an easily measurable protein, such as firefly luciferase, green fluorescent protein (GFP), or β-galactosidase. To study repression, the promoter is cloned upstream of the reporter, and the construct is transfected into cells. After treatment with a repressor or a drug, reporter activity is measured. For example, to test whether a putative silencer element represses transcription, you would clone the element upstream of a minimal promoter driving luciferase, transfect the construct into cells, and compare luciferase activity to a construct lacking the silencer. Luciferase assays are typically performed 24–48 hours after transfection, using a luminometer to measure light output after adding luciferin and ATP.

Live-cell imaging using reporter genes fused to fluorescent proteins allows real-time monitoring of transcription. The MS2 system uses a reporter mRNA containing MS2 stem-loop sequences, which are bound by the MS2 coat protein fused to GFP. This allows visualization of individual mRNA molecules as they are transcribed, providing single-cell, single-molecule resolution of transcription dynamics.

Common Pitfalls and Misconceptions

Students frequently confuse several concepts when learning about transcriptional shut-off. The following are common errors and the correct understanding.

Termination vs. Silencing

Transcription termination and gene silencing are often conflated, but they are distinct processes. Termination is the normal, regulated end of transcription at the end of a gene; it occurs every time a gene is transcribed and is required for proper gene expression. Silencing is the prevention of transcription initiation, often through repressors, chromatin modifications, or DNA methylation. A silenced gene is not transcribed at all, so termination never occurs. Conversely, a gene that is actively transcribed undergoes termination at every round of transcription. Confusing these terms leads to errors in describing mechanisms: termination does not "turn off" a gene; it merely ends a single round of transcription.

Binary vs. Graded Control

Another misconception is that transcription is either fully on or fully off. In reality, transcription is a stochastic, probabilistic process. A gene may be transcribed at a low rate (e.g., one mRNA per hour) or a high rate (e.g., 100 mRNAs per minute), and this rate can be modulated continuously. Repressors do not necessarily abolish transcription completely; they reduce the probability of initiation. For example, a repressor may reduce transcription 10-fold, but not to zero. This graded control allows fine-tuning of gene expression levels. Single-cell studies have shown that many genes are transcribed in bursts—periods of activity followed by periods of inactivity—and repressors can alter burst frequency, burst size, or both.

Equating DNA Methylation with Permanent Silencing

While DNA methylation is associated with stable silencing, it is not irreversible. Active demethylation occurs through the TET (ten-eleven translocation) enzymes, which oxidize 5-methylcytosine to 5-hydroxymethylcytosine and further oxidation products, ultimately leading to replacement with unmethylated cytosine. This is important during development and in cellular reprogramming. Thus, DNA methylation is a stable but reversible mark, not a permanent one.

Assuming All Repressors Work the Same Way

Repressors use diverse mechanisms. Some block RNA polymerase binding directly; others recruit chromatin remodelers; others sequester activators; and some act at the level of elongation. There is no single "repressor mechanism," and the same repressor can act by different mechanisms at different target genes. For example, the yeast repressor Mig1 recruits the Tup1–Ssn6 co-repressor to glucose-repressed genes, but it also directly interferes with the binding of the activator Gal4 at some promoters.

Summary and Practical Takeaways

Transcription is turned off through multiple, interconnected mechanisms that operate at different stages of the transcription cycle. Repressor proteins block initiation by binding to DNA or interfering with activators. Chromatin modifications, including histone deacetylation, H3K9/H3K27 methylation, and DNA methylation, create a repressive environment that prevents access to the promoter. Termination ends transcription at gene boundaries, while promoter-proximal pausing and attenuation provide rapid post-initiation shut-off. Small RNAs can direct chromatin modifications to silence genes transcriptionally. Understanding these mechanisms requires distinguishing between the rate of transcription, the stability of mRNA, and the state of chromatin.

Key Points to Remember

Transcription is regulated primarily at initiation, but also at elongation and termination.
Repressors are DNA-binding proteins that block transcription by steric hindrance, quenching activators, or recruiting co-repressors.
Histone deacetylation and H3K9/H3K27 methylation promote chromatin compaction and gene silencing.
DNA methylation at CpG islands is a stable but reversible mark associated with long-term silencing.
Transcription termination is not the same as gene silencing; termination ends each round of transcription, while silencing prevents initiation.
Promoter-proximal pausing and attenuation are rapid, reversible mechanisms that halt transcription shortly after initiation.
Small RNAs (siRNA, piRNA) can direct chromatin modifications to silence transcription at the DNA level.
ChIP, nuclear run-on assays, and reporter constructs are essential tools for studying transcriptional shut-off.

Frequently Asked Questions

How is transcription turned off?

Transcription is turned off by multiple mechanisms: repressor proteins bind to DNA and block RNA polymerase; chromatin is compacted through histone deacetylation and methylation; DNA is methylated at CpG islands; RNA polymerase is paused or prematurely terminated; and small RNAs direct silencing chromatin modifications. The specific mechanism depends on the gene, the organism, and the regulatory context.

What is the difference between transcription termination and gene silencing?

Termination is the normal end of a round of transcription, releasing the RNA transcript and RNA polymerase at the end of a gene. It occurs every time a gene is transcribed. Gene silencing prevents transcription from initiating in the first place, so no transcript is produced. Silencing is a regulatory state, while termination is a routine step in the transcription cycle.

Can transcription be turned off after RNA polymerase has started?

Yes. Promoter-proximal pausing halts RNA polymerase after 20–60 nucleotides in metazoans, and release requires P-TEFb. In bacteria, attenuation terminates transcription prematurely in amino acid biosynthetic operons. Additionally, transcription can be aborted during initiation if the polymerase fails to escape the promoter.

What role do histones play in turning off transcription?

Histones package DNA into nucleosomes, and their post-translational modifications control chromatin accessibility. Deacetylation of histone lysines by HDACs increases chromatin compaction. Methylation of H3K9 and H3K27 creates binding sites for repressive proteins like HP1 and Polycomb, which spread silencing. Histone variants such as H2A.Z can also influence promoter activity.

How do repressor proteins stop transcription?

Repressors bind to specific DNA sequences (operators or silencers) and either physically block RNA polymerase binding, interfere with activators, or recruit co-repressor complexes that modify chromatin. For example, LacI blocks RNA polymerase at the lac promoter, while α2 recruits Tup1–Ssn6 to compact chromatin.

What is the role of DNA methylation in transcription off?

DNA methylation at CpG dinucleotides in promoters is associated with stable transcriptional silencing. Methylated CpGs are bound by MBD proteins that recruit HDACs and histone methyltransferases, creating repressive chromatin. DNA methylation is maintained through cell division by DNMT1, providing a heritable mechanism of gene silencing.

How do scientists measure transcription shut-off?

Scientists use nuclear run-on assays to measure the density of engaged RNA polymerases, ChIP to measure the occupancy of RNA polymerase or repressive histone marks at a promoter, and reporter genes (e.g., luciferase, GFP) to measure promoter activity. RNA-seq can measure steady-state mRNA levels, but it cannot distinguish between reduced transcription and increased mRNA degradation.