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

Los procariotas contienen un genoma circular donde, en muchos casos, los genes relacionados para ciertos procesos bioquímicos se encuentran directamente adyacentes entre sí en la secuencia de ADN. 
Estos grupos de genes, conocidos como operones, tienen un solo promotor y se transcriben en un solo ARNm. La expresión de un operón está controlada por activadores y represores, proteínas que promueven o suprimen la transcripción, respectivamente. 
Un activador se une a un promotor, lo que conduce a la unión de la ARN polimerasa y la posterior transcripción de un operón.
Un represor interactúa con una secuencia de ADN diferente ubicada en las proximidades del promotor, conocida como operador. Cuando un represor se une al operador, evita que la ARN polimerasa se una al promotor, inhibiendo la transcripción.
La regulación de los genes procariotas a menudo depende de la disponibilidad de nutrientes. En algunas bacterias, cuando las concentraciones de glucosa son bajas, conduce a la acumulación de AMP cíclico. 
El AMP cíclico se une a un activador, la proteína activadora de catabolito, y juntos se unen a un promotor. Esto inicia la expresión del operón lac, un operón que contiene genes que permiten a las bacterias metabolizar azúcares distintos de la glucosa.
El triptófano es un aminoácido necesario para la síntesis de proteínas, pero cuando no está disponible en el medio ambiente, algunos organismos son capaces de producir su propio triptófano. Los genes para su síntesis están presentes en un operón que está regulado por un represor.
Cuando las concentraciones de triptófano son altas, se asocia con el represor. Este complejo puede unirse al operador, bloqueando la ARN polimerasa y suprimiendo la transcripción.

<ol>
	<li>Alberts et al., 6ª edición; páginas 380-383</li>
	<li>Shaw, K. (2008) Regulación negativa de la transcripción en procariotas. Educación para la Naturaleza 1(1):122</li>
</ol>

activadores y represores transcripcionales procarióticos

Prokaryotic Transcriptional Activators and Repressors

La organización de los genes procarióticos en su genoma es notablemente diferente a la de los eucariotas. Los genes procarióticos están organizados de manera que los genes de las proteínas involucradas en el mismo proceso o función bioquímica se ubican juntos en grupos. Este grupo de genes, junto con sus elementos reguladores, se conocen colectivamente como operón. Los genes funcionales de un operón se transcriben juntos para dar una sola cadena de ARNm conocida como ARNm policistrónico.

La transcripción de genes procarióticos en un operón está regulada por dos tipos de proteínas de unión al ADN conocidas como activadores y represores. Los activadores se unen al promotor, el sitio de inicio de la transcripción, y ayudan en la unión de la ARN polimerasa, la enzima clave involucrada en la transcripción. Los represores se unen a operadores, secuencias reguladoras cortas en el operón entre el promotor y los genes, e inhiben la unión de la ARN polimerasa al promotor.

Un prerrequisito estructural para los activadores y promotores es que puedan existir en dos conformaciones alternativas, una en la que puedan unirse al ADN y otra en la que no puedan. Otro rasgo característico específico de los activadores es que tienen dos superficies de unión para unirse simultáneamente tanto a la ARN polimerasa como al ADN. Este reclutamiento de las dos moléculas acerca la polimerasa al promotor y ayuda en su unión. Los activadores no desempeñan ningún papel catalítico en la transcripción y su función se limita a facilitar la unión de la enzima y el ADN. En ausencia de un activador, la ARN polimerasa aún puede unirse al ADN y mostrar bajos niveles de expresión. Si hay un represor presente en este sistema, se impide la expresión basal de ese gen.

La regulación de la expresión de genes procarióticos depende en gran medida de la disponibilidad de nutrientes y las necesidades de los organismos. Estos nutrientes controlan la unión de activadores y represores al operón y garantizan que sólo se exprese el conjunto de genes requerido. Por ejemplo, la presencia de triptófano en una célula conduce a su unión a un represor que impide la transcripción del operón trp y la posterior producción de triptófano.

Activadores y represores transcripcionales procarióticos

La organización de los genes procariotas en su genoma es notablemente diferente a la de los eucariotas. Los genes procariotas están organizados, de tal manera que los genes ...

prokaryotic-transcriptional-activators-and-repressors

Education

Educación

JoVE Core

Molecular Biology

Biología Molecular

Unit 1

Unidad 1

Gene Expression

Expresión génica

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

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 Biología Molecular presenta los procesos moleculares de la célula a través de lecciones claras y concisas de animaciones audiovisuales. Además, en los vídeos de científicos en acción, se muestran aplicaciones de estos conceptos mediante experimentos reales en laboratorios especializados.

Operon Control by Activators and Repressors

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