Promoter of a Gene: Basic Structure and Function in Eukaryotic Cells

General Concept of Promoter

The promoter of a gene is a specialized DNA sequence that controls the initiation of transcription and plays a central role in regulating gene expression in eukaryotic cells. DNA is a linear molecule of polynucleotides (i.e., A, C, G, T). A region of DNA, consisting of specific sequence motifs and proper organization, that provides binding site for transcription factors and enzyme (such as RNA polymerases), to initiate synthesis of RNAs (such as rRNAs, tRNAs, mRNA, snRNA, miRNA etc.) is known as Promoter. Promoter keeps under control the transcriptions process according to the need of a cell. Promoters of cells (i.e., eukaryotic cells and prokaryotic cells) have structural similarities with similar kind of cells. In simple words, promoters of eukaryotic organisms are similar. Similarly, promoters of prokaryotic organisms are similar with each other.

Based on promoter knowledge, sequential data of a genome can be used to predict total number of genes as well as new genes can be identified in the genome of an organism using bioinformatics tools. The promoter of a gene plays a central role in regulating gene expression by determining where and when transcription begins in eukaryotic cells.

Location and size of Promoter

Location

Promoter of a gene can be found upstream of the coding region on the coding or sense strand of DNA. Upstream means toward 5’ of DNA coding strand. The position of the promoter of a gene upstream of the transcription start site allows precise regulation of gene expression.

Size

The exact size of a promoter can only be measured through experimental approaches. However, mostly structural studies conducted so far reveal that the size of promoter is between 100-1000 bps.

Parts of a Promoter

Eukaryotic promoter is structurally divided into three parts or sections based on the binding of general transcription factors (abbreviated as GTFs), specific transcription factors, transcription enzymes and other supporting factors, commonly known as co-factors. These regions are 1). Core region of promoter, 2). Proximal region of promoter, 3). Distal region of promoter. See figure for the schematic representation of the promoter regions.

Core region of promoter

Core region of a promoter is located close to the start codon of the coding region. It is a short region of approximately 100 bps, located on both sides (upstream (50bps) and downstream (50bps)) of Transcription start site (TSS). It consists of TSS (transcription start site) and TATA box, also known as Goldberg–Hogness box in eukaryotic cells. This region is binding site for RNA polymerases and general transcription factors, also known as basal transcription factors. General transcription factors are those proteins that are commonly required for transcription by all most all genes. These general transcription factors work in a complex known as pri-initiation complex (PIC).

Pri-initiation complex consists of RNA polymerase II as well as general transcription factors. Hence, these general transcription factors work in association with RNA Polymerase II. Therefore, they are denoted as TFII including TFIIA, TFIIB, TFIID, TFIIE, TFIIF and TFIIH. These general transcription factors help in binding of RNA polymerases, activating of RNA polymerases, unwinding of double stranded DNA, and denaturation of double stranded DNA to make sure coding strand accessible for transcription. RNA polymerases bind to a specific TATA box (5’-TATAAA-3’ box) via sigma factor, part of RNA polymerases.

Note: In prokaryotic cells TATA box is known as Pribnow box and has six nucleotides (5’-TATAAT-3’).

Function of Basic Transcription Factors

Basic or general transcription factors are mandatory for the initiation of transcription process. Among these transcription factors

Promoter of a Gene and Pre-Initiation Complex Formation

1. TFIIA recognize promoter sequences and recruits TFIID to the promoter region. TFIID, is a multiprotein, consists of subunits such as TBP (TATA box Binding Protein) and TAFs (TBP-Associated Factors). TBP as the abbreviation reveals that this subunit recognizes TATA box and binds to the TATA box sequence with the help of TFIIA. There are approximately 14 TBP-associated factors, these are involved in the rearrangement of promoter topology to prepare promoter region for transcription initiation.
2. TBP after binding to TATA box interact with TFIIB and consequently TFIIB interacts with RNA Polymerase II for recruitment to the PIC.
3. TFIIF comes along with RNA Polymerase II to the complex.
4. TFIIE binds to the PIC and make sure to confirm assessability of promoter region for the binding of TFIIH.

Role and Structure of TFIIH in Transcription Initiation

1. TFIIH plays important role in the multiple biological process such as DNA replication, transcription as well as cell cycle regulation.
2. Structure-wise, TFIIH is a complex protein, consisting of 10 subunits, including XPB, XPD, p8, p34, p44, p52, p62, MAT1, cyclin H, and CDK7.
3. XPB and XPD both involve in helicase activities in the direction of 3’-5’ and 5’-3’ respectively. Helicase activity make sure to form transcription bubble. In addition, both are also involved in the ATPase activities. P5 and p52 regulate ATPase activity of XPB, while p44 perform similar function for XPD.
4. CDK7 and Cyclin H are involved in kinase activities[6]. RNA Polymerase II starts transcription as phosphorylated.
5. MAT1 stabilizes CAK.

Note: about more than 80% genes are promoter-less, these genes are constitutively transcribed because the product of these are required to carry on housekeeping processes, essential to maintain basic cellular activities.

Proximal region of promoter

This region starts about -250 upstream of start codon. Specific transcription factors bind to this region. These transcription factors regulate specific genes in specific cells (i.e., differentiated cells). Such specific transcription factors recognize specific sequences in the proximal region of the promoter.These transcription factors bind to the recognition sequences in the form of either monomer or dimer, or trimer etc. These transcription factors functions either to stimulate or repress the transcription of these specific genes. Here are some examples of some specific transcription factors. For detail study see the given citations in the table No. 1.

Table 1: Specific Transcription Factors Regulating Gene Promoters
Specific Transcription Factor	Type	Target Gene(s)	Organs / Cell Type	Role	Function / Effect
SP-1	General	Claudin-1, 3, 4, -19	Intestine epithelial cells	Stimulator	Enhances tight junction protein expression, maintaining epithelial barrier integrity
AP-1	Tissue-specific	Metallothionein-2	Liver, pancreas, intestine, kidney	Stimulator	Induces metallothionein expression, protecting cells from oxidative stress
HNF4α	Tissue-specific	Transthyretin	Liver	Stimulator	Activates liver-specific genes involved in transport and metabolism
FOXO1	Tissue-specific	Claudin-5	Intestine epithelial cells	Repressor	Suppresses Claudin-5 expression, regulating paracellular permeability

Distal region of promoter

Distal region of promoter consists of regulatory DNA sequences, known as Enhancer and Silencer. These elements help in changing transcription rate of the target genes.  For example, activator proteins bind to enhancer elements to enhance transcription while repressor proteins help in silencing gene via silencer sequence.

Enhancers

Like promoter, Enhancers is also cis-acting elements that can be found upstream, downstream irrespective of promoter core region location and help to enhance the expression of the target gene. Although enhancer sequences was considered to have non-coding properties however, recently a review published describing that enhancer sequences are actively transcribed, known as enhancer RNA (eRNA). Enhancer associated factors, such as ELL3, help in enhancing RNA polymerase II initiation and elongation by making loop formation with PIC. Mediator subunit such as MED1, and MED2 interact with co-activators such as Cohesin (enhancer binding factor), are involved in the formation of looping in β-globin gene in embryonic stem cells.  

Silencers

Silencers, opposite to enhancers, negatively regulate (i.e., repress) transcription process, consequently RNA production is either slow down or completed abolished. Silencer elements are present far way to the upstream of the TSS. Repressor proteins are large groups that can be classified into many subclasses based on duration of repression (i.e., short time repressors or longtime repressors), or recruiting of histone deacetylases to promoter region, or binding properties (i.e., DNA-binding repressors, repressors binding to DNA-binding proteins, binding to other repressor proteins)(Gaston and Jayaraman reviewed transcription repressors in detail[16]). As the classification reveals that repressors inhibiting transcription promoting events by preventing binding of RNA polymerase, transcription factors to promoter regions, acetylation of promoter binding histones, consequently led to suppress the gene transcription.

Why the Promoter of a Gene Is Important

The promoter of a gene acts as the primary control element for gene expression in eukaryotic cells. By serving as the binding site for RNA polymerase II and general transcription factors, it determines whether transcription will occur and at what rate. Variations in promoter structure and regulatory elements allow cells to finely tune gene activity in response to developmental cues and environmental signals. Therefore, proper functioning of the promoter of a gene is essential for normal cellular processes such as growth, differentiation, and response to stress.

Conclusion

The promoter of a gene is a fundamental regulatory region that plays a decisive role in the initiation and control of transcription in eukaryotic cells. Its structure, including core elements such as the TATA box and binding sites for transcription factors, ensures accurate recognition by the transcription machinery. Understanding the promoter of a geneis essential for explaining how genes are turned on or off and how precise regulation of gene expression supports normal cellular function and organismal development.

Note: Explore more Molecular & Cellular Genetics articles to deepen your understanding of gene regulation

Frequently Asked Questions About the Promoter of a Gene

Bibliography & Further Reading

Promoter Identification and Computational Approaches

• N. Q. K. Le, E. K. Y. Yapp, N. Nagasundaram, and H. Y. Yeh, “Classifying Promoters by Interpreting the Hidden Information of DNA Sequences via Deep Learning and Combination of Continuous FastText N-Grams,” Front. Bioeng. Biotechnol., 2019.

Core Promoters and Transcription Initiation

• V. Haberle and A. Stark, “Eukaryotic core promoters and the functional basis of transcription initiation,” Nature Reviews Molecular Cell Biology. 2018.
• “Hogness (-Goldberg) Box (TATA Box),” in Encyclopedic Dictionary of Genetics, Genomics and Proteomics, 2004.

Pre-Initiation Complex and RNA Polymerase II Structure

• B. J. Greber and E. Nogales, “The Structures of Eukaryotic Transcription Pre-initiation Complexes and Their Functional Implications,” in Subcellular Biochemistry, 2019.
• S. Sainsbury, C. Bernecky, and P. Cramer, “Structural basis of transcription initiation by RNA polymerase II,” Nature Reviews Molecular Cell Biology. 2015.

TFIIH and General Transcription Factors

• J. K. Rimel and D. J. Taatjes, “The essential and multifunctional TFIIH complex,” Protein Science. 2018.
• B. J. Greber, D. B. Toso, J. Fang, and E. Nogales, “The complete structure of the human TFIIH core complex,” Elife, 2019.
• S. N. Le, C. R. Brown, S. Harvey, H. Boeger, H. Elmlund, and D. Elmlund, “The TAFs of TFIID bind and rearrange the topology of the TATA-less RPS5 promoter,” Int. J. Mol. Sci., 2019.

Specific Transcriptional Regulators and Gene Expression Control

• N. Khan and A. R. Asif, “Transcriptional Regulators of Claudins in Epithelial Tight Junctions,” Mediators Inflamm., vol. 2015, no. 1, pp. 1–6, 2015.
• P. de Francisco, F. Amaro, A. Martín-González, and J. C. Gutiérrez, “AP-1 (bZIP) Transcription Factors as Potential Regulators of Metallothionein Gene Expression in Tetrahymena thermophila,” Front. Genet., 2018.
• S. R. Davis and R. J. Cousins, “Metallothionein expression in animals: A physiological perspective on function,” J. Nutr., 2000.
• C. Walesky and U. Apte, “Role of hepatocyte nuclear factor 4α (HNF4α) in cell proliferation and cancer,” Gene Expression. 2015.

Enhancers and Long-Range Gene Regulation

• A. Panigrahi and B. W. O’Malley, “Mechanisms of enhancer action: the known and the unknown,” Genome Biology. 2021.
• P. R. Arnold, A. D. Wells, and X. C. Li, “Diversity and Emerging Roles of Enhancer RNA in Regulation of Gene Expression and Cell Fate,” Frontiers in Cell and Developmental Biology. 2020.
• L. A. Pennacchio, W. Bickmore, A. Dean, M. A. Nobrega, and G. Bejerano, “Enhancers: Five essential questions,” Nature Reviews Genetics. 2013.

Transcriptional Repression Mechanisms

1. K. Gaston and P. S. Jayaraman, “Transcriptional repression in eukaryotes: Repressors and repression mechanisms,” Cellular and Molecular Life Sciences. 2003.