Name: operon control by activators and repressors
Uploaded: 2020-11-23T11:47:37.000Z
Duration: 1 min 58 s
Channel: JoVE

Les régulateurs transcriptionnels sont des protéines qui se lient à de courtes étendues spécifiques de l'ADN non-codantes, appelées séquences cis-régulatrices. Ces composants essentiels pour la régulation des gènes sont présents dans tout le génome. Les régulateurs transcriptionnels peuvent se lier à des séquences cis-régulatrices sur l'ADN sous sa forme à double hélice.La forme hélicoïdale de l'ADN a deux rainures qui diffèrent en taille. La rainure la plus large, appelée rainure principale, est plus accessible que la rainure la plus petite, rainure mineure, de ce fait, la plupart des interactions a lieu au niveau de la rainure principale. Les bords des bases azotées sont exposés le long des rainures et sont capables de lier l'hydrogène et d'autres interactions non covalentes.Cette liaison de régulateurs transcriptionnels est spécifique à la séquence. Toutefois, des séquences étroitement liées sont également reconnues. Cette association est très stable en raison des multiples interactions non covalentes impliquées.Les séquences de régulation cis peuvent régir tous les aspects de la transcription génique, y compris l'inhibition ou l'initiation de la transcription, ainsi que le taux de transcription. Elles peuvent être classifiées de manière générale en celles qui sont à proximité du site de transcription et celles qui sont des milliers de paires de base plus loin. Ceux qui sont proches font généralement partie des séquences du promoteur qui initient la transcription des gènes.Ceux qui sont plus loin peuvent participer à l'amplification ou à l'inhibition de la transcription. Souvent, l'emballage compact de l'ADN et du chromosome provoque la proximité spatiale de ces séquences et du gène qu'elles régulent, même si elles sont loin dans la séquence. Une combinaison de plusieurs séquences cis-régulatrices contrôle l'expression de la plupart des gènes.Rarement, un gène sera sous le contrôle d'une seule séquence.

<ol>
	<li>Alberts et al., 6e édition ; pages 380-383</li>
	<li>Shaw, K. (2008) Régulation de la transcription négative chez les procaryotes. Éducation à la nature 1(1) :122</li>
</ol>

activateurs et répresseurs transcriptionnels procaryotes

Prokaryotic Transcriptional Activators and Repressors

L'organisation des gènes procaryotes dans leur génome est sensiblement différente de celle des eucaryotes. Les gènes procaryotes sont organisés de telle sorte que les gènes des protéines impliquées dans le même processus biochimique ou la même fonction sont regroupés. Ce groupe de gènes, ainsi que leurs éléments régulateurs, sont collectivement connus sous le nom d'opéron. Les gènes fonctionnels d'un opéron sont transcrits ensemble pour donner un seul brin d'ARNm appelé ARNm polycistronique.

La transcription des gènes procaryotes dans un opéron est régulée par deux types de protéines de liaison à l'ADN appelées activateurs et répresseurs. Les activateurs se lient au promoteur, site d'initiation de la transcription, et aident à la liaison de l'ARN polymérase, l'enzyme clé impliquée dans la transcription. Les répresseurs se lient aux opérateurs, courtes séquences régulatrices dans l'opéron entre le promoteur et les gènes, et inhibent la liaison de l'ARN polymérase au promoteur.

Une condition préalable structurelle pour les activateurs et les promoteurs est qu’ils doivent pouvoir exister sous deux conformations alternatives, l’une dans laquelle ils peuvent se lier à l’ADN et l’autre dans laquelle ils ne le peuvent pas. Une autre caractéristique spécifique aux activateurs est qu’ils possèdent deux surfaces de liaison pour se lier simultanément à l’ARN polymérase et à l’ADN. Ce recrutement des deux molécules rapproche la polymérase du promoteur et facilite sa liaison. Les activateurs n'ont aucun rôle catalytique à jouer dans la transcription et leur fonction se limite à faciliter la liaison de l'enzyme et de l'ADN. En l’absence d’activateur, l’ARN polymérase peut toujours se lier à l’ADN et présenter de faibles niveaux d’expression. Si un répresseur est présent dans ce système, alors l’expression basale de ce gène est empêchée.

La régulation de l'expression des gènes procaryotes dépend largement de la disponibilité en nutriments et des besoins des organismes. Ces nutriments contrôlent la liaison des activateurs et des répresseurs à l'opéron et garantissent que seul l'ensemble de gènes requis est exprimé. Par exemple, la présence de tryptophane dans une cellule conduit à sa liaison à un répresseur qui empêche la transcription de l'opéron trp et la production ultérieure de tryptophane.

Activateurs et répresseurs transcriptionnels procaryotes

L’organisation des gènes procaryotes dans leur génome est notablement différente de celle des eucaryotes. Les gènes procaryotes sont organisés, de telle sorte que les gènes ...

prokaryotic-transcriptional-activators-and-repressors

Education

Enseignement

JoVE Core

Molecular Biology

Biologie moléculaire

Unit 1

Unité 1

Gene Expression

Expression génique

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

operon control by activators and repressors

Operon control in prokaryotes depends on activators and repressors. Prokaryotic genes are often arranged in groups, not one by one like in eukaryotes. Genes that help carry out the same biochemical process are placed together with their regulatory elements. This group is called an operon, and it is transcribed into a single polycistronic mRNA strand.

Activators and repressors are DNA-binding proteins that control transcription in an operon. Activators bind near the promoter, which is the site where transcription begins. They help RNA polymerase, the enzyme that makes RNA, bind to the promoter. Repressors bind to operators, short regulatory sequences between the promoter and the genes, and they block RNA polymerase from binding to the promoter.

These proteins must switch between two shapes. One shape allows them to bind DNA, and the other does not. Activators also have two binding surfaces. One surface binds DNA, and the other binds RNA polymerase at the same time. This brings the polymerase closer to the promoter and helps transcription start. Activators do not catalyze transcription. Their role is to help the enzyme and DNA come together.

Without an activator, RNA polymerase can still bind to DNA, but only at low levels. If a repressor is present, even this basal expression is prevented. In prokaryotes, gene expression is largely controlled by nutrient availability and the needs of the cell. These nutrients affect whether activators and repressors bind to the operon, so only the needed genes are expressed. For example, when tryptophan is present in a cell, it binds to a repressor and stops transcription of the trp operon. This also prevents more tryptophan from being made.

Operon Control by Activators and Repressors

operon-control-by-activators-and-repressors

cell specific gene expression

How Cells Switch Genes On and Off

Cells switch genes on and off to make different proteins for different jobs. In multicellular organisms, many cell types are built from the same DNA, but they do not use the same genes in the same way. This difference in gene expression helps explain why cells can look and act so differently. Liver cells are a clear example. They help detoxify blood, make bile to help digest fats, and produce proteins needed for metabolism. To do these jobs, liver cells must express a specific set of genes.

regulation of expression occurs at multiple steps

regulation-of-expression-occurs-at-multiple-steps

How Cells Control Gene Expression

Cells control gene expression at several points between DNA and protein. The most common control point is transcription. At this stage, proteins bind to short regulatory DNA sequences and can either increase or reduce transcription of the linked gene. After transcription, a precursor mRNA is made. This pre-mRNA contains exons, which code for protein, and introns, which do not. It must be processed before translation, and splicing removes the introns and joins the exons. Cells can also regulate...

cis-regulatory sequences

How Cis-Regulatory DNA Controls Transcription

Cis-regulatory sequences are short, non-coding DNA segments that control gene transcription. They sit on the same chromosome as the genes they regulate and act as binding sites for transcriptional regulators. In eukaryotes, these proteins help control gene expression patterns in different cell types. These DNA sequences can be close to a gene or thousands of bases away. When they are far from the gene in the DNA sequence, they are often brought close together in space because chromosomes are...

cooperative binding of transcription regulators

cooperative-binding-of-transcription-regulators

How Transcription Factors Bind DNA

Transcriptional regulators bind to cis-regulatory DNA sequences to control gene transcription. These DNA sequences are very short, usually fewer than ten nucleotide pairs long. Because they are so short, the same sequence can appear by chance in many places across the genome. Some regulators can also bind to groups of similar sequences. That raises the chance of random binding even more. To lower this risk, transcription regulators often form dimers, which are pairs of proteins that bind a DNA...

prokaryotic transcriptional activators and repressors

Operon control in prokaryotes depends on activators and repressors. Prokaryotic genes are often arranged in groups, not one by one like in eukaryotes. Genes that help carry out the same biochemical process are placed together with their regulatory elements. This group is called an operon, and it is transcribed into a single polycistronic mRNA strand. Activators and repressors are DNA-binding proteins that control transcription in an operon. Activators bind near the promoter, which is the site...

operons

Operons in Bacterial Gene Control

Operons help bacteria control groups of genes at the same time. An operon is a set of genes that are regulated together and can be turned on or off as a unit. This shared control helps bacteria respond quickly to changes in their environment. In many operons, a promoter gives RNA polymerase a place to bind and start transcription. The operon also includes a regulatory region that helps control whether the genes are expressed. When conditions change, the cell can use this setup to manage gene...

the eukaryotic promoter region

Eukaryotic Promoter Motifs and Gene Control

The eukaryotic promoter region is a stretch of DNA located upstream of a gene. It includes an RNA polymerase binding site, a transcription start site, and several cis-regulatory sequences. These DNA signals help control when transcription begins. The promoter can be divided into proximal and distal regions. The proximal promoter lies close to the gene and includes cis-regulatory sequences and the core promoter. The core promoter is the RNA polymerase binding site, and it is usually found...

co-activators and co-repressors

Chromatin Control in Gene Transcription

Chromatin control plays a major role in gene transcription in eukaryotes. Gene expression is regulated by several proteins that work together at a gene regulatory site. Some of these proteins bind directly to specific DNA sequences, while co-regulators attach to those regulators but do not bind DNA on their own. Co-regulators can act as co-activators or co-repressors, depending on the complex they join. One co-regulator may even switch roles in different settings. For example, the...

eukaryotic transcription activators

How Transcription Activators Turn Genes On

Transcription activators are proteins that help turn genes on by promoting transcription from DNA to RNA. They often have two parts. One part binds to DNA, and the other part activates transcription. In some proteins, such as the glucocorticoid receptor and MyoD, one domain can do both jobs. The DNA-binding part recognizes specific regulatory sequences on the DNA. These binding domains are grouped into families based on their structure. Common examples include the leucine zipper, zinc finger,...

eukaryotic transcription inhibitors

How Eukaryotic Gene Repressors Work

Eukaryotic gene repressors help turn specific genes off when the cell needs to control growth, development, or other biochemical processes. These DNA-binding proteins regulate gene expression in eukaryotes. Their presence at the right time and place can be triggered by hormones and signals from other cells. Most transcription inhibitors have two main parts. One part binds DNA, and the other is a repressor domain, which helps shut down transcription. Repressor domains can connect with basal...

combinatorial gene control

Gene Expression by Transcription Factor Combinations

Gene expression can depend on combinations of transcription factors working together. In combinatorial gene control, several transcription factors act in a coordinated way to regulate a single gene. If one or more of these factors are missing, the level of gene expression or repression can change a lot. This kind of control helps explain how a relatively small number of transcription factors can regulate a very large number of genes. More than 30,000 genes are controlled by about 2,000 to...

induced pluripotent stem cells

Reprogramming Cells into Stem Cells

Induced pluripotent stem cells are created by reprogramming mature cells back into a stem-cell state. This process shows that specialized cells can be changed into cells with broader developmental potential. The topic is important in biology because it links cell identity, gene control, and stem cell research. These cells begin as ordinary body cells and are pushed to de-differentiate, or lose their specialized features. After reprogramming, they behave like pluripotent stem cells, which means...

master transcription regulators

MEF2C and Gene Control Cascades

MEF2C is a master transcription regulator that controls gene expression in coordinated cascades. These regulators are proteins that manage the activity of multiple genes that often work together to produce a complex outcome. When a master transcription regulator is activated, it can start a chain of transcriptional activation that supports that outcome. Master transcription regulators can act in two main ways. They may bind directly to the regulatory sequences of several genes involved in a...

epigenetic regulation

DNA Methylation and Histone Changes

Epigenetic regulation controls gene activity without changing the DNA sequence. It helps determine when genes are turned on or off in a cell. Two major epigenetic mechanisms are DNA methylation and histone modification. DNA methylation adds chemical groups to DNA, while histone changes affect how tightly DNA is packed around histone proteins. These changes can make genes easier or harder to use. In high school biology, epigenetic regulation is important for understanding how cells with the...

genomic imprinting and inheritance

Genomic Imprinting in Human Disease

Genomic imprinting affects how some inherited genes are turned on or off in human cells. Diploid organisms inherit chromosomes from both parents, and each gene usually has two alleles. In most cases, both alleles are expressed. If one allele is missing or mutated, the other can often compensate, but some genes do not work that way. In genomic imprinting, a gene or its regulatory sequence carries chemical tags, such as methyl groups, on the copy inherited from only one parent. These tags are...

key terms and concepts

Learn how activators and repressors control prokaryotic operons, from promoter binding to the trp operon response to tryptophan in cells.

Operon Control by Activators and Repressors | JoVE K12

molecular biology

JoVE Core Biologie Moléculaire illustre les processus moléculaires au sein de la cellule grâce à des leçons vidéos claires et concises. Les applications de ces concepts sont ensuite démontrées grâce à des expériences concrètes en laboratoire.

Operon Control by Activators and Repressors

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