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 Initiated: Mechanisms and Key Steps

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

Introduction to Transcription Initiation

Transcription initiation is the first phase of gene expression, encompassing the series of molecular events that begin with RNA polymerase binding to a promoter and culminate in the synthesis of the first few nucleotides of a new RNA transcript. This process is the primary point at which cells decide whether a gene will be expressed, and it is subject to more regulatory control than any other stage of gene expression.

For a gene to be transcribed, RNA polymerase must first locate the correct DNA sequence, form a stable complex, unwind the DNA duplex, and begin RNA synthesis. Each of these steps requires specific protein–DNA and protein–protein interactions that differ substantially between prokaryotes and eukaryotes. Understanding these mechanisms is essential not only for grasping how genes are expressed but also for interpreting how mutations in promoter sequences or transcription factors contribute to human disease.

The initiation phase is distinct from elongation and termination. During elongation, RNA polymerase moves processively along the template strand, adding nucleotides to the growing RNA chain. During termination, the polymerase dissociates from the DNA and releases the completed transcript. Initiation, however, is the only phase that involves promoter recognition, DNA melting, and the assembly of a competent elongation complex. It is also the phase most frequently targeted by regulatory proteins that activate or repress gene expression.

The Promoter: DNA Sequence for Initiation

A promoter is a DNA sequence located upstream of the transcription start site (TSS) that directs RNA polymerase to the correct position for initiating RNA synthesis. Promoters are not transcribed themselves; rather, they serve as binding platforms for RNA polymerase and associated factors. The strength of a promoter—how frequently it initiates transcription—depends on how well its sequence matches the consensus recognized by the polymerase machinery.

Prokaryotic Promoters

In bacteria, the promoter typically spans about 40 base pairs and contains two conserved hexameric sequences: the −10 box (also called the Pribnow box) and the −35 box. These names refer to their positions relative to the transcription start site, which is designated +1. The −10 box has the consensus sequence TATAAT, and the −35 box has the consensus TTGACA. The spacing between these two elements is critical: 17 ± 1 base pairs is optimal. If the spacing deviates significantly from this, promoter recognition is impaired.

Some promoters also contain an upstream element (UP element), an A/T-rich sequence located between −40 and −60, which enhances promoter strength by providing additional contacts with the RNA polymerase α subunit. Promoters that lack a recognizable −35 box, such as those recognized by the alternative sigma factor σ⁵⁴, instead rely on an upstream activating sequence and a protein activator to recruit the polymerase.

Eukaryotic Promoters

Eukaryotic promoters are more diverse and complex. RNA polymerase II, which transcribes protein-coding genes, recognizes a core promoter that typically includes several sequence elements. The most well-studied is the TATA box, located approximately 25–30 base pairs upstream of the TSS, with the consensus sequence TATAAA. The TATA box is bound by the TATA-binding protein (TBP), a subunit of the general transcription factor TFIID. You can read more about this element in our article on the Tata Box Transcription.

Other core promoter elements include the initiator (Inr) sequence, which spans the TSS itself and has the consensus YYANWYY (where Y is pyrimidine, W is A or T); the downstream promoter element (DPE), located about +28 to +32; and the TFIIB recognition element (BRE), located immediately upstream of the TATA box. Not all promoters contain all elements; many contain only a subset, and some contain none of the canonical elements, relying instead on other sequences or on regulatory proteins to recruit the polymerase.

In addition to the core promoter, eukaryotic genes often have proximal promoter elements—such as GC boxes (GGGCGG) and CCAAT boxes—located within a few hundred base pairs upstream. These are binding sites for sequence-specific transcription factors that modulate the rate of initiation.

RNA Polymerase and Its Role in Initiation

RNA polymerase is the enzyme responsible for synthesizing RNA from a DNA template. It is a large, multi-subunit enzyme that must perform several tasks during initiation: recognize the promoter, melt the DNA, synthesize the first phosphodiester bond, and then transition to a processive elongation mode.

Prokaryotic RNA Polymerase

The bacterial RNA polymerase holoenzyme has a molecular mass of approximately 450 kDa and consists of five core subunits (α₂, β, β′, and ω) plus a sigma (σ) factor. The core enzyme alone can synthesize RNA but cannot initiate transcription at the correct sites. The sigma factor confers promoter specificity by recognizing the −10 and −35 sequences.

The β and β′ subunits form the catalytic cleft, with the active site containing a conserved Mg²⁺ ion essential for phosphodiester bond formation. The α subunits are involved in enzyme assembly and in contacting the UP element. The σ factor, when bound to the core enzyme, positions the enzyme over the promoter and participates in DNA melting.

Different sigma factors direct RNA polymerase to different sets of promoters. The primary sigma factor in Escherichia coli, σ⁷⁰, recognizes most housekeeping gene promoters. Alternative sigma factors, such as σ³² (heat shock), σ⁵⁴ (nitrogen starvation), and σ²⁸ (flagellar genes), redirect the polymerase to specific regulons under stress conditions.

Eukaryotic RNA Polymerases

Eukaryotes have three nuclear RNA polymerases, each transcribing a distinct class of genes. RNA polymerase I transcribes ribosomal RNA (rRNA) genes; RNA polymerase II transcribes protein-coding genes and many non-coding RNAs; and RNA polymerase III transcribes transfer RNAs (tRNAs), 5S rRNA, and other small RNAs.

RNA polymerase II is the most studied because it transcribes messenger RNA (mRNA) precursors. It contains 12 subunits, with a total mass of about 550 kDa. The largest subunit, RPB1, has a unique C-terminal domain (CTD) consisting of tandem heptapeptide repeats with the consensus sequence YSPTSPS. In humans, this repeat is present 52 times. The CTD is phosphorylated during the transition from initiation to elongation, a modification that serves as a landing pad for RNA processing factors.

Unlike bacterial RNA polymerase, eukaryotic RNA polymerases cannot recognize promoters on their own. They require the assistance of general transcription factors (GTFs) to assemble at the promoter and to melt the DNA. This is a fundamental difference between prokaryotic and eukaryotic initiation, and it is discussed in detail in the section on the preinitiation complex.

Steps of Transcription Initiation in Prokaryotes

Bacterial transcription initiation is the best-characterized example of the process and proceeds through a series of well-defined intermediates. The entire process can be reproduced in vitro using purified RNA polymerase, a DNA template containing a promoter, and ribonucleoside triphosphates (NTPs) at 37°C in a buffer containing 10 mM MgCl₂ and 50–100 mM KCl.

Closed Complex Formation

The first step is promoter recognition, in which the RNA polymerase holoenzyme binds to the promoter to form a closed complex. In this complex, the DNA remains double-stranded, and the polymerase is bound primarily through sequence-specific contacts between the σ factor and the −10 and −35 elements. The closed complex is kinetically unstable; it forms and dissociates rapidly. The equilibrium constant for this step depends on the match between the promoter sequence and the consensus.

The rate of closed complex formation is influenced by the concentration of RNA polymerase and by the presence of accessory proteins. In E. coli, the closed complex has a half-life on the order of seconds to minutes, depending on promoter strength. Strong promoters, such as the T7A1 promoter, form closed complexes more rapidly and hold the polymerase more tightly.

Open Complex Formation

The transition from the closed to the open complex involves the unwinding of approximately 12–14 base pairs of DNA around the start site, from about −11 to +3. This melting is facilitated by the σ factor, which contains an aromatic residue (tryptophan at position 433 in σ⁷⁰) that intercalates into the DNA and stabilizes the single-stranded state.

The open complex is much more stable than the closed complex, with a half-life that can exceed 30 minutes at 37°C. During this step, the template strand is positioned in the active site of the polymerase, and the first NTPs can enter. The open complex is sensitive to the DNA sequence at the start site; a purine (usually A) at position +1 is strongly preferred.

The transition from closed to open complex is temperature-dependent and is the rate-limiting step for many promoters. At temperatures below 15°C, the open complex forms very slowly, which is why in vitro transcription assays are typically performed at 37°C.

Promoter Escape

Once the open complex has formed, the polymerase begins synthesizing RNA. However, it does not immediately leave the promoter. Instead, it undergoes several rounds of abortive initiation, in which short RNA products of 2–9 nucleotides are synthesized and released while the polymerase remains bound to the promoter. This process is thought to reflect the difficulty the polymerase faces in breaking its strong contacts with the promoter.

Promoter escape occurs when the RNA product reaches a critical length (approximately 10–12 nucleotides), at which point the polymerase undergoes a conformational change, releases the σ factor, and transitions to the elongation phase. The energy for this transition comes from the hydrolysis of ATP or GTP, which is used to break the contacts between the σ factor and the promoter.

Promoter escape is a major regulatory checkpoint. Proteins such as GreA and GreB can stimulate escape by inducing cleavage of abortive products, allowing the polymerase to restart synthesis. The efficiency of escape varies widely among promoters; some promoters undergo dozens of abortive cycles before successful escape.

Transcription Initiation in Eukaryotes: The Preinitiation Complex

Eukaryotic transcription initiation by RNA polymerase II is fundamentally more complex than the bacterial process. The polymerase cannot bind to the promoter by itself; instead, it is recruited by a set of general transcription factors that assemble at the core promoter to form the preinitiation complex (PIC). The PIC is a large macromolecular assembly with a mass exceeding 2 MDa.

General Transcription Factors

The general transcription factors required for RNA polymerase II initiation are TFIID, TFIIA, TFIIB, TFIIE, TFIIF, and TFIIH. Each plays a distinct role in promoter recognition, polymerase recruitment, or DNA melting.

TFIID is a multi-subunit complex that contains the TATA-binding protein (TBP) and 13–14 TBP-associated factors (TAFs). TBP binds to the minor groove of the TATA box, inducing a sharp bend in the DNA of approximately 80°. This bend is critical for the assembly of the PIC. TAFs recognize other core promoter elements, such as the Inr and DPE, and also serve as targets for transcriptional activators.

TFIIA stabilizes the binding of TBP to the promoter and antagonizes repressors that would otherwise displace TBP. TFIIB binds to TBP and to the BRE element, and it helps recruit RNA polymerase II. TFIIF binds to the polymerase and stabilizes its interaction with TFIIB and the promoter.

TFIIE and TFIIH are required for the later steps of initiation. TFIIE recruits TFIIH and stimulates its enzymatic activities. TFIIH is a 10-subunit complex with two enzymatic activities: a helicase activity (subunits XPB and XPD) that melts the DNA around the start site, and a kinase activity (subunit CDK7) that phosphorylates the CTD of RNA polymerase II.

The assembly of the PIC proceeds in a defined order, although the exact sequence can vary depending on the promoter and the presence of activators. A simplified version of the ordered assembly is as follows:

TFIID binds to the TATA box (or other core promoter elements).
TFIIA binds to TFIID, stabilizing the complex.
TFIIB binds to TFIID and the BRE.
RNA polymerase II, in complex with TFIIF, is recruited to the promoter.
TFIIE binds to the polymerase.
TFIIH binds to TFIIE, completing the PIC.

Once the PIC is assembled, TFIIH uses its XPB helicase to melt the DNA around the start site, forming the open complex. The CDK7 subunit then phosphorylates the CTD at serine 5, which triggers promoter escape and the transition to elongation. This phosphorylation also recruits capping enzymes that add a 5′ cap to the nascent RNA.

Mediator Complex

The Mediator complex is a large multi-subunit coactivator that links sequence-specific transcription factors to the general transcription machinery. Mediator consists of approximately 26 subunits in humans and is organized into four modules: head, middle, tail, and kinase.

The head and middle modules interact with RNA polymerase II and the general transcription factors, while the tail module interacts with upstream activators. The kinase module, which contains CDK8, can phosphorylate the CTD and regulate the activity of the complex. Mediator is required for the transcription of most RNA polymerase II-dependent genes, and it plays a critical role in transmitting regulatory signals from enhancers to the promoter.

The assembly of the PIC at many promoters is stimulated by activators that recruit Mediator and chromatin remodeling complexes. This is why the PIC assembly is often described as occurring in a stepwise, activator-dependent manner, even though the core components are the same for all genes.

Regulation of Transcription Initiation

Transcription initiation is the most heavily regulated step of gene expression. Cells control which genes are transcribed, and at what rate, through a combination of sequence-specific DNA-binding proteins, chromatin structure, and non-coding RNAs.

Activators and Repressors

Sequence-specific transcription factors bind to enhancers (for activators) or silencers (for repressors) and modulate the rate of initiation. These factors are modular proteins, typically containing a DNA-binding domain and a separate activation or repression domain.

Activators stimulate initiation by recruiting coactivators such as Mediator, histone acetyltransferases (HATs), and chromatin remodeling complexes to the promoter. They can also directly contact components of the PIC, such as TFIID, to accelerate PIC assembly. For example, the yeast activator Gal4 binds to upstream activating sequences and recruits the SAGA complex, which acetylates histones and promotes PIC assembly.

Repressors inhibit initiation through several mechanisms. They may compete with activators for binding sites, recruit corepressors that deacetylate histones, or directly interfere with the PIC. For example, the yeast repressor Mig1 recruits the Tup1–Cyc8 corepressor complex, which compacts chromatin and blocks PIC assembly.

The interplay between activators and repressors at a given promoter determines the net rate of initiation. This is often described as a balance between positive and negative regulatory inputs, with the final outcome depending on the relative concentrations and affinities of the factors involved. For a deeper discussion of these regulatory proteins, see our article on the Transcription Factor.

Chromatin Remodeling

In eukaryotes, DNA is packaged into chromatin, and the accessibility of promoter sequences is a major determinant of initiation efficiency. Nucleosomes positioned over the promoter can block the binding of TFIID and RNA polymerase II, effectively silencing the gene.

Two classes of enzymes regulate chromatin accessibility: ATP-dependent chromatin remodelers and histone-modifying enzymes. Chromatin remodelers, such as SWI/SNF and RSC, use the energy of ATP hydrolysis to slide, eject, or restructure nucleosomes. Histone-modifying enzymes add or remove covalent modifications to histone tails, such as acetylation, methylation, and phosphorylation.

Histone acetylation is generally associated with active transcription. Acetyl groups neutralize the positive charge on lysine residues, weakening the interaction between histones and DNA and making the promoter more accessible. Histone deacetylases (HDACs) reverse this modification and are typically associated with repression.

Histone methylation has context-dependent effects. Methylation of histone H3 at lysine 4 (H3K4me3) is enriched at active promoters, while methylation at lysine 27 (H3K27me3) is associated with repression. These modifications are recognized by reader proteins that recruit additional regulatory complexes.

Methods to Study Transcription Initiation

Several experimental approaches are used to study transcription initiation, each providing different types of information. These methods range from biochemical reconstitution to genome-wide analysis.

In Vitro Transcription

In vitro transcription assays use purified RNA polymerase, general transcription factors, and a DNA template containing a promoter. The reaction is typically performed in a buffer containing 20 mM HEPES (pH 7.9), 50 mM KCl, 5 mM MgCl₂, 1 mM DTT, and 0.5 mM each NTP, at 30°C for 30–60 minutes. The RNA products are then analyzed by gel electrophoresis or by incorporation of radiolabeled nucleotides.

This approach allows researchers to dissect the roles of individual factors by omitting or adding components. For example, one can test whether a putative activator stimulates initiation by adding it to a reaction containing only the basal machinery. In vitro transcription was used to identify the general transcription factors and to establish the order of PIC assembly.

ChIP Assay

Chromatin immunoprecipitation (ChIP) is used to determine where proteins bind to DNA in living cells. In a typical ChIP experiment, cells are treated with formaldehyde to cross-link proteins to DNA. The chromatin is then sheared by sonication into fragments of approximately 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 associated DNA is purified and analyzed by quantitative PCR (ChIP-qPCR) or by high-throughput sequencing (ChIP-seq).

ChIP can be used to measure the occupancy of RNA polymerase II, TFIID, or specific transcription factors at a promoter. It can also be used to assess histone modifications, such as H3K4me3, that mark active promoters. The temporal resolution of ChIP is limited by the cross-linking step, but it remains the gold standard for mapping protein–DNA interactions in vivo.

Reporter Assays

Reporter gene assays measure the activity of a promoter by linking it to a gene whose product is easily quantifiable. Common reporters include firefly luciferase, β-galactosidase, and green fluorescent protein (GFP). The promoter of interest is cloned upstream of the reporter gene, and the construct is introduced into cells. After a defined incubation period, the reporter activity is measured.

For example, in a luciferase assay, cells are lysed in a buffer containing 25 mM Tris-phosphate (pH 7.8), 2 mM DTT, 2 mM 1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid, 10% glycerol, and 1% Triton X-100. The lysate is mixed with luciferin and ATP, and the resulting luminescence is measured with a luminometer. The amount of light emitted is proportional to the activity of the promoter.

Reporter assays are widely used to map promoter elements, to test the effects of mutations, and to screen for drugs that modulate transcription. They can be performed in a high-throughput format, making them suitable for large-scale studies.

Common Misconceptions and Pitfalls

Students frequently encounter several conceptual difficulties when learning about transcription initiation. Being aware of these pitfalls can help you avoid them on exams and in the laboratory.

Confusing initiation with promoter binding. Promoter binding is only the first step of initiation. The full process includes DNA melting, abortive initiation, and promoter escape. A polymerase can be bound to a promoter without initiating transcription. In fact, many repressors work by trapping the polymerase in a non-productive bound state.

Overlooking the role of sigma factors. In prokaryotes, the sigma factor is absolutely required for promoter recognition. The core enzyme alone cannot initiate transcription at the correct sites. Students sometimes forget that sigma factors are not permanent subunits; they dissociate after promoter escape and can be reused by other core enzymes.

Thinking transcription factors are only for eukaryotes. While the term "transcription factor" is often used in the context of eukaryotic regulation, prokaryotes also have numerous DNA-binding proteins that regulate initiation. Examples include the catabolite activator protein (CAP), the lac repressor, and the λ repressor. These proteins bind to specific DNA sequences and modulate the rate of initiation by RNA polymerase.

Assuming all eukaryotic promoters have a TATA box. Only about 25% of human promoters contain a TATA box. Many promoters rely on the Inr, DPE, or other elements for PIC assembly. TBP is still required for transcription of these promoters, but it is recruited through interactions with TAFs rather than by direct DNA binding.

Confusing the roles of TFIIH. TFIIH has two distinct enzymatic activities: a helicase (XPB) that melts the DNA and a kinase (CDK7) that phosphorylates the CTD. Students often conflate these activities or forget that TFIIH is also involved in nucleotide excision repair, a completely different DNA repair pathway.

Neglecting abortive initiation. Abortive initiation is not an artifact; it is a genuine and regulated step of transcription. The number of abortive cycles can vary from a few to dozens, and this variation affects the overall rate of productive initiation.

Misunderstanding the direction of transcription. Transcription proceeds 5′ to 3′ along the RNA, which means the polymerase moves 3′ to 5′ along the template strand. The promoter elements (−10, −35, TATA) are numbered relative to the +1 start site, with negative numbers indicating upstream positions.

Summary and Key Takeaways

Transcription initiation is the first and most regulated phase of gene expression. In prokaryotes, a single RNA polymerase holoenzyme recognizes promoter sequences through its sigma factor, melts the DNA to form an open complex, and escapes the promoter after a period of abortive synthesis. In eukaryotes, RNA polymerase II requires the assembly of a large preinitiation complex containing six general transcription factors and the Mediator complex, and it must contend with chromatin structure.

The key points to remember are:

Promoters are DNA sequences that direct RNA polymerase to the transcription start site. Prokaryotic promoters contain −10 and −35 boxes; eukaryotic core promoters often contain a TATA box, Inr, or DPE.
The sigma factor in prokaryotes is essential for promoter recognition and is released during promoter escape.
Eukaryotic RNA polymerase II cannot bind promoters alone; it requires TFIID, TFIIB, TFIIE, TFIIF, TFIIH, and Mediator.
The transition from closed to open complex involves DNA melting, which is performed by TFIIH in eukaryotes.
Promoter escape is a regulated step involving abortive initiation and CTD phosphorylation.
Transcription initiation is regulated by activators, repressors, and chromatin structure.
Common experimental methods include in vitro transcription, ChIP, and reporter assays.

For a broader view of the entire transcription process, including elongation and termination, see our articles on Transcription Steps and Transcription Termination. You may also find the Transcription Diagram useful for visualizing the process, and the article on Transcription Translation for how transcription connects to protein synthesis.

Frequently Asked Questions

How is transcription initiated?

Transcription is initiated when RNA polymerase binds to a promoter sequence on the DNA, melts the double helix to expose the template strand, and synthesizes the first few nucleotides of RNA. In prokaryotes, the sigma factor directs the polymerase to the promoter. In eukaryotes, general transcription factors assemble with RNA polymerase II to form a preinitiation complex, and TFIIH melts the DNA. After a period of abortive synthesis, the polymerase escapes the promoter and transitions to elongation.

What is the role of the promoter in transcription initiation?

The promoter is the DNA sequence that determines where transcription begins. It provides binding sites for RNA polymerase (in prokaryotes) or for general transcription factors (in eukaryotes). The strength of a promoter—how efficiently it initiates transcription—depends on how closely its sequence matches the consensus recognized by the polymerase machinery. Promoters also contain binding sites for regulatory proteins that activate or repress transcription.

What is the difference between transcription initiation in prokaryotes and eukaryotes?

The main differences are: (1) Prokaryotes use a single RNA polymerase with a sigma factor for promoter recognition, while eukaryotes use three RNA polymerases, none of which can recognize promoters alone. (2) Eukaryotes require six general transcription factors (TFIID, TFIIA, TFIIB, TFIIE, TFIIF, TFIIH) to assemble the preinitiation complex. (3) Eukaryotic DNA is packaged into chromatin, which must be remodeled for initiation to occur. (4) Eukaryotic initiation involves phosphorylation of the RNA polymerase II CTD, which is absent in prokaryotes.

What is the preinitiation complex?

The preinitiation complex (PIC) is the assembly of RNA polymerase II and the general transcription factors at the promoter before transcription begins. It includes TFIID, TFIIA, TFIIB, TFIIE, TFIIF, TFIIH, and the Mediator complex. The PIC is responsible for promoter recognition, DNA melting, and the initial steps of RNA synthesis. Once the polymerase escapes the promoter, the PIC disassembles, and the remaining factors can be reused for subsequent rounds of initiation.

What is promoter escape?

Promoter escape is the transition from the initiation phase to the elongation phase of transcription. During this step, the RNA polymerase breaks its contacts with the promoter, releases the sigma factor (in prokaryotes) or the general transcription factors (in eukaryotes), and begins processive RNA synthesis. Promoter escape is often preceded by abortive initiation, in which short RNA products are synthesized and released. In eukaryotes, promoter escape is triggered by phosphorylation of the CTD by TFIIH.

How do transcription factors regulate initiation?

Transcription factors regulate initiation by binding to specific DNA sequences (enhancers or silencers) and either stimulating or repressing the assembly of the initiation complex. Activators recruit coactivators such as Mediator and histone acetyltransferases, which promote PIC assembly and open chromatin. Repressors recruit corepressors that compact chromatin or directly interfere with the PIC. The net rate of initiation depends on the balance between activator and repressor activity at a given promoter.

What techniques are used to study transcription initiation?

Common techniques include in vitro transcription assays, which use purified components to reconstitute initiation; chromatin immunoprecipitation (ChIP), which maps protein–DNA interactions in living cells; and reporter gene assays, which measure promoter activity by linking it to a quantifiable product. Other methods include electrophoretic mobility shift assays (EMSA) for studying protein–DNA binding, and nuclear run-on assays for measuring transcription rates in isolated nuclei. For a discussion of errors that can occur during transcription, see our article on Transcription Error.

Key Takeaways

Transcription initiation is the first phase of gene expression and the primary target of regulatory control.
Prokaryotic initiation requires a single RNA polymerase holoenzyme and a sigma factor that recognizes promoter sequences.
Eukaryotic initiation requires the assembly of a large preinitiation complex containing RNA polymerase II, six general transcription factors, and Mediator.
The promoter determines where and how efficiently transcription begins; promoter strength depends on sequence conservation and spacing.
DNA melting (open complex formation) is a critical step, performed by the polymerase itself in prokaryotes and by TFIIH in eukaryotes.
Promoter escape is a regulated transition that involves abortive initiation and, in eukaryotes, CTD phosphorylation.
Transcription initiation is regulated by activators, repressors, and chromatin structure, making it the most important control point in gene expression.