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.

Transcription Mechanism: From DNA to RNA in Prokaryotes and Eukaryotes

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

Introduction to Transcription

Transcription is the enzymatic process by which a single-stranded RNA molecule is synthesized from a double-stranded DNA template. It is the first step in gene expression, converting the genetic information stored in DNA into a form that can direct protein synthesis. The product of transcription—messenger RNA (mRNA) in protein-coding genes—carries the genetic message from the nucleus (in eukaryotes) or the nucleoid (in prokaryotes) to the ribosome, where translation converts the nucleotide sequence into an amino acid sequence.

Central Dogma and Gene Expression

The central dogma of molecular biology, first articulated by Francis Crick in 1957, describes the directional flow of genetic information: DNA → RNA → Protein. Transcription is the first arrow in this scheme. It is a highly regulated process, as cells must control which genes are expressed, at what level, and in response to what signals. Not all DNA is transcribed; only specific regions—genes—serve as templates. In a typical human cell, only about 1–2% of the genome is transcribed into mRNA at any given time, although a much larger fraction is transcribed into non-coding RNAs.

Transcription differs from DNA replication in several fundamental ways. First, it is selective: only specific genes are transcribed, not the entire genome. Second, the product is single-stranded RNA, which contains ribose instead of deoxyribose and uracil (U) instead of thymine (T). Third, transcription does not require a primer; RNA polymerase can initiate synthesis de novo. Fourth, transcription is asymmetric: only one of the two DNA strands, the template strand, is read. Finally, transcription is less processive and more error-prone than replication, with an error rate of roughly 1 in 10⁴ to 10⁵ nucleotides, compared to 1 in 10⁹ for DNA replication.

RNA Polymerases and the Transcription Bubble

All transcription is catalyzed by RNA polymerase, a large multi-subunit enzyme that reads the DNA template and polymerizes ribonucleoside triphosphates (rNTPs) into RNA. The enzyme moves along the DNA, unwinding the double helix ahead of it and rewinding it behind, creating a locally melted region of approximately 17–20 base pairs known as the transcription bubble. Within this bubble, the template strand is exposed and paired with the growing RNA transcript, forming an RNA–DNA hybrid of about 8–9 base pairs.

The reaction catalyzed by RNA polymerase is:

\[ \text{(NMP)}_n + \text{NTP} \rightarrow \text{(NMP)}_{n+1} + \text{PP}_i \]

where NMP is a nucleotide monophosphate and PP\(_i\) is inorganic pyrophosphate. The energy for phosphodiester bond formation comes from the hydrolysis of the incoming NTP, which releases pyrophosphate. The RNA chain grows in the 5′ to 3′ direction, meaning nucleotides are added to the 3′ hydroxyl group of the growing chain.

The Transcription Machinery

RNA Polymerase Structure

Prokaryotic RNA polymerase is a holoenzyme of approximately 400 kDa, composed of five core subunits (α₂ββ′ω) and a sigma (σ) factor. The core enzyme (α₂ββ′ω) is catalytically active but cannot specifically initiate transcription at promoters; it requires the σ factor for promoter recognition. The β and β′ subunits form a crab-claw-like structure with a central channel that accommodates the DNA template and the RNA product. The active site, containing a conserved Mg²⁺ ion, is located deep within this channel. The α subunits are involved in enzyme assembly and in interactions with regulatory proteins, while the ω subunit aids in enzyme folding and stability.

Eukaryotes possess three nuclear RNA polymerases, each with distinct functions and subunit compositions. RNA polymerase I (Pol I) transcribes ribosomal RNA (rRNA) genes (the 28S, 18S, and 5.8S rRNAs). RNA polymerase II (Pol II) transcribes all protein-coding genes to produce mRNA, as well as many non-coding RNAs. RNA polymerase III (Pol III) transcribes small RNAs, including transfer RNA (tRNA), 5S rRNA, and the U6 small nuclear RNA. Each polymerase contains 12–17 subunits; five of these are homologous to the prokaryotic core subunits, reflecting a common evolutionary origin. Pol II has a unique C-terminal domain (CTD) on its largest subunit, consisting of multiple repeats of the heptapeptide sequence Tyr-Ser-Pro-Thr-Ser-Pro-Ser. This domain is phosphorylated during the transcription cycle and serves as a platform for recruiting RNA processing factors.

Promoters and Regulatory Elements

A promoter is a DNA sequence, typically located immediately upstream of the transcription start site (TSS), that directs RNA polymerase to initiate transcription. Promoters are not transcribed themselves; they are recognition elements. In prokaryotes, the promoter typically contains two conserved hexameric sequences: the −10 box (consensus TATAAT) and the −35 box (consensus TTGACA), where the numbers refer to positions relative to the TSS (+1). The spacing between these boxes is critical, usually 16–18 base pairs. Some promoters also contain an UP element (upstream promoter element) located between −40 and −60, which is recognized by the α subunit of RNA polymerase.

Eukaryotic promoters are more diverse and complex. The core promoter of a Pol II gene spans approximately −40 to +40 relative to the TSS and may contain several elements, including the TATA box (consensus TATAAA), the initiator (Inr) element, the downstream promoter element (DPE), and the TFIIB recognition element (BRE). The TATA box, named for its thymine-adenine-rich sequence, is bound by the TATA-binding protein (TBP), a subunit of the general transcription factor TFIID. Not all promoters contain a TATA box; many are TATA-less and rely on other elements such as Inr or DPE. For more detail on the TATA box, see Tata Box Transcription.

In addition to core promoters, eukaryotic genes are regulated by distal elements such as enhancers and silencers, which can be located thousands of base pairs away. These elements are bound by sequence-specific transcription factors that interact with the basal transcription machinery through looping of the intervening DNA. For a broader discussion of these regulatory proteins, see Transcription Factor.

Transcription Initiation in Prokaryotes

Promoter Recognition and Binding

Transcription initiation in prokaryotes begins when the RNA polymerase holoenzyme (core + σ factor) scans the DNA for promoter sequences. The σ factor recognizes the −10 and −35 boxes, and the α subunit may contact the UP element. This initial binding forms the closed complex, in which the DNA remains double-stranded. The affinity of RNA polymerase for a promoter depends on how well the promoter sequence matches the consensus; strong promoters (e.g., the T7 phage promoter) can drive very high levels of transcription, while weak promoters deviate from consensus and require additional activators.

The σ factor is not a single protein but a family of related factors. The primary σ factor in Escherichia coli, σ⁷⁰, is responsible for most housekeeping gene transcription. Alternative σ factors, such as σ³² (heat shock), σ⁵⁴ (nitrogen starvation), and σ²⁸ (flagellar synthesis), direct RNA polymerase to specific sets of genes in response to environmental signals. This allows prokaryotes to rapidly reprogram gene expression without changing the core enzyme.

Formation of the Open Complex

After initial binding, the RNA polymerase undergoes a conformational change that melts approximately 12–14 base pairs of DNA around the −10 region, forming the open complex. This melting is facilitated by the σ factor, which contains aromatic amino acid residues that intercalate into the DNA and destabilize the base pairs. The template strand is then guided into the active site channel, positioning the TSS at the catalytic center.

The first nucleotide, almost always a purine (ATP or GTP), is brought into the active site and paired with the template base at position +1. The second nucleotide then arrives, and the first phosphodiester bond is formed. Importantly, RNA polymerase does not require a primer; it synthesizes the first dinucleotide de novo. The initial transcript is short (2–9 nucleotides) and is frequently released in a process called abortive initiation. During this phase, the enzyme remains at the promoter, synthesizing and releasing short RNA fragments. Eventually, the σ factor is released, and the enzyme transitions to the elongation phase. This transition, called promoter escape, is a key regulatory step. For a step-by-step account of this process, see Transcription Initiation.

Transcription Elongation in Prokaryotes

Nucleotide Addition and Proofreading

During elongation, RNA polymerase moves processively along the template DNA, adding nucleotides to the 3′ end of the growing RNA chain. The enzyme maintains a transcription bubble of ~17–20 base pairs, with an RNA–DNA hybrid of ~8–9 base pairs. The rate of elongation in E. coli is approximately 40–80 nucleotides per second at 37°C, though this can vary depending on the gene and the presence of regulatory factors.

The catalytic mechanism involves a two-metal-ion catalysis. Two Mg²⁺ ions are coordinated at the active site: one is permanently bound, while the second is brought in with each incoming NTP. The first Mg²⁺ activates the 3′ hydroxyl of the growing RNA chain for nucleophilic attack on the α-phosphate of the incoming NTP. The second Mg²⁺ stabilizes the transition state and facilitates pyrophosphate release.

RNA polymerase has two proofreading mechanisms. The first is pyrophosphorolytic editing: the enzyme can reverse the polymerization reaction, removing the last incorporated nucleotide by re-adding pyrophosphate. The second is hydrolytic editing: the enzyme backtracks by one or more nucleotides and cleaves the RNA chain, removing a segment that contains the error. Both mechanisms increase the fidelity of transcription, though they are less efficient than the proofreading systems of DNA polymerases. For a discussion of the error rates and their consequences, see Transcription Error.

Supercoiling and Topoisomerases

As RNA polymerase translocates along the DNA, it generates positive supercoils ahead of the transcription bubble and negative supercoils behind it. This is a consequence of the helical nature of DNA: the polymerase does not rotate freely around the DNA axis, so the unwinding ahead and rewinding behind create torsional stress. If left unchecked, positive supercoils ahead of the polymerase would make further unwinding energetically unfavorable, stalling transcription.

Topoisomerases relieve this stress. DNA gyrase (topoisomerase II) introduces negative supercoils and removes positive supercoils ahead of the polymerase, while topoisomerase I relaxes negative supercoils behind it. In E. coli, inhibition of gyrase by antibiotics such as ciprofloxacin leads to the accumulation of positive supercoils and the arrest of transcription. This is one reason why topoisomerase inhibitors are effective antibacterial agents.

Transcription Termination in Prokaryotes

Termination is the process by which RNA polymerase stops transcription and releases both the RNA transcript and the DNA template. In prokaryotes, there are two principal mechanisms: intrinsic (rho-independent) termination and rho-dependent termination. A detailed overview is available at Transcription Termination.

Intrinsic Termination

Intrinsic termination does not require any accessory proteins. It relies on two features of the nascent RNA transcript: a GC-rich hairpin loop followed by a run of 4–8 uracil residues. As RNA polymerase transcribes the terminator sequence, the RNA folds into a stable stem-loop structure. This hairpin interacts with the RNA exit channel of the polymerase, causing a conformational change that destabilizes the RNA–DNA hybrid in the active site.

The U-rich tract is critical because the rU–dA hybrid is the weakest of all RNA–DNA base pairs. The combination of the hairpin-induced conformational change and the weak hybrid in the U-rich region causes the RNA to dissociate from the template, and the polymerase falls off the DNA. Intrinsic terminators are common in bacterial genomes, particularly at the ends of operons.

Rho-Dependent Termination

Rho-dependent termination requires the hexameric Rho protein, an RNA-dependent ATPase. Rho binds to a C-rich, G-poor sequence on the nascent RNA, called the rut (Rho utilization) site, which is located upstream of the actual termination point. Rho then translocates along the RNA in the 5′ to 3′ direction, using ATP hydrolysis for energy, and catches up with the paused RNA polymerase.

When Rho reaches the transcription bubble, it interacts with the polymerase and induces a conformational change that promotes dissociation of the RNA–DNA hybrid and release of the transcript. Rho-dependent termination is common in bacteriophage genes and in some bacterial operons, and it is also used as a regulatory mechanism, for example, in attenuation of the trp operon.

Transcription in Eukaryotes: Additional Complexity

Eukaryotic transcription is fundamentally more complex than prokaryotic transcription, reflecting the larger genome, the presence of a nuclear membrane, and the need for extensive regulation. The overall steps—initiation, elongation, termination—are conserved, but the machinery and regulatory logic differ substantially.

RNA Polymerases I, II, and III

The three eukaryotic RNA polymerases are dedicated to different classes of genes. Pol I is localized in the nucleolus and transcribes a single transcription unit containing the 18S, 5.8S, and 28S rRNA genes, which are present in tandem arrays of hundreds of copies. Pol I transcription accounts for the majority of total cellular RNA synthesis (about 60%) because rRNA is so abundant.

Pol II transcribes all protein-coding genes and many non-coding RNAs. It is the most studied polymerase because its products are translated into proteins. Pol II transcription is coupled to RNA processing: capping, splicing, and polyadenylation occur co-transcriptionally, with processing factors recruited to the CTD.

Pol III transcribes small, constitutively expressed RNAs: tRNAs, 5S rRNA, U6 snRNA, and the 7SL RNA component of the signal recognition particle. Pol III promoters are unusual in that some are located entirely within the transcribed region (type 1 and type 2 promoters for 5S rRNA and tRNAs, respectively), downstream of the TSS.

General Transcription Factors and the Preinitiation Complex

Unlike prokaryotic RNA polymerase, which can bind directly to its promoter with the help of a σ factor, eukaryotic Pol II requires a set of accessory proteins called general transcription factors (GTFs) to assemble at the promoter. These factors—TFIID, TFIIB, TFIIF, TFIIE, TFIIH, and TFIIA—are required for transcription of all Pol II genes and assemble with Pol II into the preinitiation complex (PIC).

The assembly begins with TFIID binding to the core promoter. TFIID is a multi-subunit complex containing TBP and 13–14 TBP-associated factors (TAFs). TBP binds to the TATA box, inducing a sharp bend in the DNA. TFIIB then binds, followed by Pol II in complex with TFIIF. TFIIE and TFIIH join last. TFIIH has two enzymatic activities: a helicase (XPB and XPD subunits) that melts the DNA around the TSS, and a kinase (CDK7) that phosphorylates the CTD of Pol II. This phosphorylation is required for promoter escape and for the recruitment of RNA processing factors.

The PIC is a large assembly, roughly 2 MDa, and its formation is a major point of regulation. Many activators and repressors exert their effects by influencing PIC assembly or stability. The ordered assembly of the PIC is described in more detail at Transcription Steps.

Chromatin and Transcription

In eukaryotes, DNA is packaged into chromatin, with nucleosomes as the basic repeating unit. Each nucleosome consists of ~147 base pairs of DNA wrapped around an octamer of histone proteins (two each of H2A, H2B, H3, and H4). Chromatin is generally repressive to transcription because it occludes promoter sequences and impedes polymerase processivity.

Two classes of enzymes modulate chromatin structure during transcription. ATP-dependent chromatin remodelers, such as SWI/SNF and RSC, use ATP hydrolysis to slide, eject, or restructure nucleosomes, exposing promoter DNA and facilitating PIC assembly. Histone-modifying enzymes covalently modify histone tails—acetylation, methylation, phosphorylation, ubiquitination—to alter chromatin compaction and recruit downstream effectors. Histone acetylation, catalyzed by histone acetyltransferases (HATs), is generally associated with active transcription, while deacetylation by histone deacetylases (HDACs) is associated with repression.

During elongation, Pol II must transcribe through nucleosomes. The FACT (facilitates chromatin transcription) complex and other elongation factors assist in displacing histones ahead of the polymerase and reassembling them behind. This is an energy-intensive process, and elongation through chromatin is a regulated step in gene expression.

Methods to Study Transcription

In Vitro Transcription Assays

Run-off transcription assays are a classic method to study transcription in vitro. A linear DNA template containing a promoter and a defined termination point is incubated with RNA polymerase, rNTPs (one of which is radiolabeled or fluorescently labeled), and buffer. The reaction is allowed to proceed for a defined time, then stopped. The RNA products are separated by denaturing polyacrylamide gel electrophoresis and visualized. Because the template is linear, transcription terminates at the end of the DNA, producing a "run-off" transcript of a predictable length. This assay is used to measure promoter strength, test the effects of transcription factors, and map transcription start sites.

A typical reaction buffer contains 40 mM Tris-HCl (pH 8.0), 10 mM MgCl₂, 10 mM NaCl, 1 mM DTT, 0.1 mg/mL BSA, and 200 µM each NTP. Reactions are incubated at 37°C for 10–30 minutes and stopped by adding formamide loading buffer and heating to 95°C.

ChIP and ChIP-seq

Chromatin immunoprecipitation (ChIP) is used to determine where a protein of interest binds to DNA in living cells. Cells are treated with formaldehyde to cross-link proteins to DNA. The chromatin is then 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 cross-links, the DNA is purified and analyzed.

In ChIP-seq, the purified DNA is subjected to high-throughput sequencing, allowing genome-wide mapping of protein–DNA interactions. ChIP-seq is widely used to map RNA polymerase occupancy, histone modifications, and transcription factor binding sites. For example, ChIP-seq with an antibody against Pol II reveals the positions of actively transcribing polymerases across the genome, and the ratio of Pol II at promoters versus gene bodies can indicate where regulation occurs.

RNA-seq and Nascent RNA Capture

RNA sequencing (RNA-seq) measures the steady-state levels of all RNAs in a cell. Total RNA is isolated, depleted of rRNA, converted to cDNA, and sequenced. RNA-seq provides a snapshot of gene expression levels but does not distinguish between newly synthesized RNA and pre-existing RNA. To measure transcription rates directly, researchers use nascent RNA capture methods.

One approach is to label newly synthesized RNA with 4-thiouridine (4sU), a nucleotide analog that is incorporated into RNA during transcription. After a short labeling period (5–30 minutes), cells are lysed, and the 4sU-labeled RNA is biotinylated and captured on streptavidin beads. This "nascent RNA" represents transcripts synthesized during the labeling window and provides a direct measure of transcription activity. Another approach, PRO-seq (precision run-on sequencing), maps the positions of engaged RNA polymerases at nucleotide resolution by allowing them to extend a short RNA tag in the presence of biotinylated NTPs.

Common Pitfalls and Study Tips

Misconceptions to Avoid

Students frequently make several errors when learning transcription. The most common is assuming that both DNA strands are transcribed. In reality, transcription is asymmetric: only the template strand is read, and the RNA product is identical in sequence to the coding (non-template) strand, except that U replaces T. A related error is confusing the terms "template strand" and "coding strand." The template strand is read 3′ to 5′ by RNA polymerase, and the RNA is synthesized 5′ to 3′. The coding strand has the same sequence as the RNA (with T→U) and is not read.

Another frequent misconception is that RNA polymerase uses uracil as a substrate. It does not; it uses UTP (uridine triphosphate). Uracil is the base in the RNA product, but the substrate is a ribonucleotide triphosphate. Similarly, students sometimes think that transcription requires a primer, as DNA replication does. It does not; RNA polymerase initiates de novo.

A third common error is thinking that transcription and translation are coupled in eukaryotes. They are not. In eukaryotes, transcription occurs in the nucleus, and the mRNA must be processed (capped, spliced, polyadenylated) and exported to the cytoplasm before translation. In prokaryotes, transcription and translation are coupled: ribosomes can begin translating an mRNA while it is still being synthesized, because there is no nuclear membrane. For a comparison of these processes, see Transcription Translation.

Students also often confuse the roles of the σ factor in prokaryotes and general transcription factors in eukaryotes. The σ factor is a single protein that directs RNA polymerase to the promoter. The GTFs are multiple proteins that assemble the PIC; they do not simply guide Pol II to the promoter but are required for promoter melting and for regulating Pol II activity.

How to Remember Key Differences

A useful framework is to compare prokaryotic and eukaryotic transcription across five dimensions: polymerases, promoters, initiation factors, processing, and location. The table below summarizes these differences.

Feature	Prokaryotes	Eukaryotes
RNA polymerases	One (core + σ factor)	Three (Pol I, II, III)
Promoter elements	−10 box, −35 box, UP element	TATA box, Inr, DPE, BRE
Initiation factors	σ factor (one protein)	GTFs (TFIID, TFIIB, TFIIF, TFIIE, TFIIH)
RNA processing	None (mRNA used directly)	Capping, splicing, polyadenylation
Location	Cytoplasm (no nucleus)	Nucleus
Coupling with translation	Yes	No

For a visual summary of the process, see Transcription Diagram.

Frequently Asked Questions

What is the transcription mechanism diagram?

A transcription mechanism diagram typically shows RNA polymerase moving along the DNA template strand, with the transcription bubble open ahead of the enzyme and rewound behind it. The diagram illustrates the template strand being read in the 3′ to 5′ direction, the RNA transcript being synthesized in the 5′ to 3′ direction, and the growing RNA chain extending from the polymerase. Promoter and terminator sequences are usually indicated at the boundaries of the gene. For a labeled diagram, see Transcription Diagram.

How does transcription mechanism differ in prokaryotes and eukaryotes?

The core chemistry of transcription is identical in both domains: RNA polymerase reads the template strand and synthesizes RNA 5′ to 3′. The differences lie in the machinery and regulation. Prokaryotes use a single RNA polymerase with a σ factor for promoter recognition, have simple promoters (−10 and −35 boxes), and do not process their mRNA. Eukaryotes use three RNA polymerases, require multiple general transcription factors for initiation, have more complex promoters, and extensively process pre-mRNA (capping, splicing, polyadenylation) before translation. Eukaryotic transcription is also influenced by chromatin structure, which has no prokaryotic equivalent.

What are the main steps of transcription?

Transcription proceeds through three main stages: initiation, elongation, and termination. During initiation, RNA polymerase binds to the promoter, melts the DNA to form the open complex, and synthesizes the first few nucleotides. During elongation, the polymerase moves processively along the template, adding nucleotides to the 3′ end of the growing RNA. During termination, the polymerase recognizes a termination signal, releases the RNA transcript, and dissociates from the DNA. For a step-by-step breakdown, see Transcription Steps.

What is the role of the promoter in transcription?

The promoter is a DNA sequence that directs RNA polymerase to the correct transcription start site. It determines which strand is transcribed, where transcription begins, and how efficiently transcription is initiated. Promoters are recognized either directly by the σ factor (prokaryotes) or by general transcription factors such as TFIID (eukaryotes). Promoter strength—how well it matches the consensus sequence—directly influences the level of gene expression.

Why is transcription important in gene expression?

Transcription is the first and most highly regulated step in gene expression. It converts the genetic information in DNA into RNA, which can then be translated into protein. Regulation of transcription determines which genes are expressed, when they are expressed, and at what level. Many diseases, including cancer, involve dysregulation of transcription, making the transcription machinery a major target for therapeutic drugs.

What is the difference between transcription and translation?

Transcription is the synthesis of RNA from a DNA template, catalyzed by RNA polymerase. Translation is the synthesis of a polypeptide from an mRNA template, catalyzed by the ribosome. Transcription occurs in the nucleus (eukaryotes) or cytoplasm (prokaryotes) and produces RNA. Translation occurs on ribosomes in the cytoplasm and produces protein. Transcription uses DNA as the template; translation uses mRNA. The products also differ: RNA is a nucleic acid, while protein is a polypeptide chain of amino acids. For a detailed comparison, see Transcription Translation.

What are the common mistakes students make about transcription?

The most common mistakes are: (1) thinking both DNA strands are transcribed, (2) confusing the template and coding strands, (3) believing RNA polymerase requires a primer, (4) thinking transcription and translation are coupled in eukaryotes, (5) confusing the σ factor with general transcription factors, and (6) forgetting that RNA polymerase uses NTPs, not dNTPs. Understanding these distinctions is essential for exam success.

Key Takeaways

Transcription is the synthesis of RNA from a DNA template, catalyzed by RNA polymerase, and is the first step in gene expression.
Only one DNA strand (the template strand) is read; the RNA product is identical to the coding strand with U replacing T.
RNA polymerase synthesizes RNA in the 5′ to 3′ direction without a primer, using ribonucleoside triphosphates as substrates.
Prokaryotic transcription uses a single RNA polymerase with a σ factor for promoter recognition; promoters contain −10 and −35 boxes.
Eukaryotes have three RNA polymerases (I, II, III); Pol II requires general transcription factors and a complex preinitiation complex for initiation.
Prokaryotic termination occurs via intrinsic (hairpin + U-rich tract) or rho-dependent mechanisms; eukaryotic termination is coupled to RNA processing.
Eukaryotic transcription is regulated by chromatin structure, histone modifications, and chromatin remodelers, adding a layer of complexity absent in prokaryotes.
Key experimental methods include run-off transcription assays, ChIP-seq, and nascent RNA capture, each providing different information about transcription.