Name: how transcription factors bind dna
Uploaded: 2020-11-23T11:47:37.000Z
Duration: 2 min 13 s
Channel: JoVE

Os reguladores transcricionais são proteínas que reconhecem e ligam às sequências curtas do DNA, conhecidas como sequências reguladoras cis. Como essas sequências geralmente têm menos de 10 nucleotídeos de comprimento, as chances da mesma sequência ocorrendo aleatoriamente no genoma são muito altas. Muitos reguladores formam um par de dímeros a fim de limitar a ligação a sequências aleatórias.Um dímero pode ligar sequências mais longas que 10 nucleotídeos, tornando menos provável que a sequência esteja presente aleatoriamente no genoma e aumentando a especificidade da ligação. Estes dímeros reguladores podem ser um homodímero, feito dos mesmos tipos de monómero, ou um heterodímero com diferentes tipos de monómeros. Uma vez que os pares podem ser compostos por diferentes monómeros, as várias combinações permitem a ligação a diferentes sequências sem necessidade de novos tipos de proteínas.Quando não ligados ao DNA, os reguladores cooperativos existem como monómeros que às vezes formam dímeros através de interações fracas, não covalentes. No entanto, estas estruturas formam dímeros fortemente associados quando ligados ao DNA devido à ligação cooperativa. A ligação cooperativa é um fenômeno onde a ligação de um monômero à sequência reguladora cis aumenta a probabilidade de uma segunda ligação de regulador devido às mudanças estruturais.Isto permite que um segundo regulador se prenda firmemente ao outro lado do local da ligação e forme um dímero com o primeiro. Isto significa que na maior parte do tempo, ou todas as instâncias de sequências regulamentares cis específicas têm um limite regulador ou nenhuma delas tem. Para muitos genes, o DNA é envolvido firmemente em torno de proteínas histonas impedindo reguladores transcricionais de ter acesso as sequências de regulamentação cis.No entanto, a extremidade solta do DNA permite algum espaço para ligação. A ligação de um único regulador neste local pode ajudar a desenrolar a estrutura, permitindo assim que os outros reguladores possam se ligar.

<ol>
	<li>Alberts et al., 6ª edição; páginas 375, 378-380</li>
	<li>Morgunova, E., & Taipale, J. (2017). Perspectiva estrutural da ligação cooperativa do fator de transcrição. Opinião atual em biologia estrutural, 47, 1-8.</li>
</ol>

vinculação cooperativa de reguladores de transcrição

Cooperative Binding of Transcription Regulators

Os reguladores transcricionais ligam-se a sequências reguladoras cis específicas no DNA para regular a transcrição genética. Estas sequências reguladoras cis são muito curtas, geralmente com menos de dez pares de nucleotídeos de comprimento. O comprimento curto significa que há uma alta probabilidade de que exatamente a mesma sequência ocorra aleatoriamente em todo o genoma. Como os reguladores também podem se ligar a grupos de sequências semelhantes, isto aumenta ainda mais as chances de ligação aleatória. Os reguladores transcricionais formam dímeros que se ligam a uma sequência duas vezes mais tempo que um monômero, aumentando as sequências e reduzindo as chances de ligação aleatória. Os dímeros reguladores da transcrição podem ser homodímeros ou heterodímeros. Em solução, estes reguladores cooperativos existem como monômeros ou dímeros fracamente ligados. No entanto, quando estes monômeros se ligam a uma sequência reguladora cis estendida no DNA, formam dímeros estáveis.

A cooperatividade é um fenômeno onde a ligação de uma proteína monomérica causa alterações estruturais no DNA e aumenta a afinidade dos sítios reguladores por outros monômeros. Isto permite que os monómeros se liguem como dímeros na sequência reguladora cis. Este fenómeno também ajuda os reguladores a aceder a locais localizados no DNA que estão fortemente ligados às proteínas histonas no nucleossoma, que de outra forma seriam inacessíveis. A primeira ligação geralmente ocorre no DNA, na extremidade do nucleossomo, onde não está fortemente ligado. A ligação neste local faz com que o DNA se afaste das histonas, levando assim à descompactação do nucleossomo. Essa descompactação aumenta o acesso a outros sites regulatórios. Em eucariotos, a ligação do fator de transcrição depende predominantemente da cooperatividade. Embora a cooperatividade possa ocorrer em alguns casos, a maior parte da ligação dos reguladores transcricionais nos procariontes é não cooperativa. Nesses casos, os reguladores existem como dímeros estáveis mantidos juntos por diversas interações não covalentes.

Se um regulador desconhecido se liga cooperativamente ou não cooperativamente pode ser determinado traçando o número de sítios de ligação ocupados no DNA em relação à concentração de proteínas. Se o gráfico for uma curva em forma de S, indica que o regulador se liga cooperativamente aos sítios de ligação. Se a curva aumentar de forma constante antes de se estabilizar à medida que se aproxima de todos os sítios de ligação ocupados, isso indica que a ligação não é cooperativa.

Vinculação Cooperativa de Reguladores de Transcrição

Os reguladores transcricionais se ligam a sequências cis-regulatórias específicas no DNA para regular a transcrição gênica. Essas sequências cis-regulatórias são ...

cooperative-binding-of-transcription-regulators

Education

JoVE Core

Molecular Biology

Biologia Molecular

Unit 1

Unidade 1

Gene Expression

Expressão Gênica

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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 sequence that is twice as long as the sequence bound by a monomer. These dimers can be homodimers or heterodimers.

In solution, cooperative regulators may exist as monomers or as weakly linked dimers. When they bind to an extended cis-regulatory sequence on DNA, they form stable dimers. Cooperativity means that the binding of one monomer changes the DNA structure and increases the site’s affinity for other monomers. This helps the proteins bind together as dimers on the cis-regulatory sequence.

Cooperativity also helps regulators reach sites on DNA that are tightly wrapped around histone proteins in the nucleosome. These sites are usually hard to access. Binding often starts at the end of the nucleosome, where the DNA is less tightly packed. That first binding step can pull the DNA away from the histones and help unpack the nucleosome, which makes other regulatory sites easier to reach.

In eukaryotes, transcription factor binding depends mainly on cooperativity. In prokaryotes, most transcriptional regulator binding is non-cooperative, although cooperativity can occur in some cases. Non-cooperative regulators are usually stable dimers held together by several non-covalent interactions. The difference can be seen by plotting occupied binding sites against protein concentration. An S-shaped curve suggests cooperative binding, while a steady rise that levels off suggests non-cooperative binding.

How Transcription Factors Bind DNA

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

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

prokaryotic-transcriptional-activators-and-repressors

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...

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 transcription regulators bind cis-regulatory DNA, form dimers, unpack nucleosomes, and how S-shaped curves reveal cooperative binding.

How Transcription Factors Bind DNA | JoVE K12

molecular biology

JoVE Core Biologia Molecular mostra os processos moleculares da célula através de vídeos de animação claros e concisos. Além disso, nos vídeos de Cientistas em ação, a aplicação desses conceitos é demonstrada por meio de experimentos reais em laboratórios modernos.

How Transcription Factors Bind DNA

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