Name: operons in bacterial gene control
Uploaded: 2020-11-23T11:47:37.000Z
Duration: 2 min 9 s
Channel: JoVE

- [Narratore] Nelle cellule
procariotiche, un operone

è un gruppo di sequenze genetiche

che contiene elementi
regolatori e svariati

geni codificanti proteine,

i geni strutturali che
vengono trascritti insieme.

Uno dei casi meglio studiati

è l'operone lac nei batteri,

che contiene tre geni,

lac-Z, lac-Y e lac-A,

che codifica gli enzimi necessari

per il metabolismo del lattosio,

insieme alle sequenze promotore,

operatore e di terminazione

che regolano l'espressione dei geni lac.

Normalmente, quando la
concentrazione di glucosio è alta,

la proteina repressore
lac si lega strettamente

all'operatore e impedisce la trascrizione

dei geni lac,

impedendo all'RNA polimerasi

di legarsi con il promotore.

Il repressore lac è
costitutivamente espresso,

nel senso che il gene, nel codificarlo,

è attivato di dafault.

Quando la concentrazione
di glucosio è molto bassa,

la cellula userà il lattosio
come fonte di energia.

Una volta presente
all'interno della cellula,

parte del lattosio viene convertito

ad una versione modificata,
chiamata allolattosio,

che è noto come l'induttore
dell'operone lac,

perché si lega e inibisce al repressore,

innescando l'espressione dei geni lac.

Inoltre, con bassi livelli di glucosio,

la quantità della molecola di segnalazione

AMP ciclico aumenta e si lega

alla proteina attivatore
di catabolite, o CAP.

Insieme, si legano a una
sequenza regolatrice,

approssimativamente a monte del promotore,

e aiuta ad attivare l'RNA polimerasi

per aumentare significativamente
la trascrizione.

Durante la trascrizione, un
singolo filamento di mRNA

viene prodotto e rilasciato
quando la polimerasi

raggiunge la sequenza di terminazione.

Da questo mRNA, sono
tradotte le tre proteine

che sono necessarie per
elaborare il lattosio.

Osbourn, Anne E. e Ben Field. "Operoni". Scienze della vita cellulari e molecolari 66, n. 23 (2009): 3755-3775. [<a href="https://doi.org/10.1007/s00018-009-0114-3">Sorgente</a>]

operoni

Operons

I procati possono controllare l'espressione genica attraverso operoni, sequenze di DNA costituite da elementi regolatori e geni raggruppati e funzionalmente correlati alla codifica delle proteine. Gli operoni usano una singola sequenza promotrice per avviare la trascrizione di un cluster genico (cioè un gruppo di geni strutturali) in una singola molecola di mRNA. La sequenza di terminazione termina la trascrizione. Una sequenza di operatori, situata tra il promotore e i geni strutturali, vieta l'attività trascrizionale dell'operaio se vincolata da una proteina repressiva. Complessivamente, il promotore, l'operatore, i geni strutturali e il terminatore costituiscono il nucleo di un operone.

Gli operoni sono di solito inducibili o repressibili. Gli operoni inducibili, come l'operone lac batterico, sono normalmente "spenti" ma si accenderanno "in presenza di una piccola molecola chiamata induttore (ad esempio, allolactosi). Quando il glucosio è assente, ma il lattosio è presente, l'allolactosi lega e inattiva il repressore dell'operone, consentendo all'operone di generare enzimi responsabili del metabolismo del lattosio.

Gli operoni repressibili, come l'operone trp batterico, sono di solito "on" ma si spegneranno in presenza di una piccola molecola chiamata corepressor (ad esempio, triptofano). Quando il toptofano, un aminoacido essenziale, è abbondante, il tritofano si lega e attiva il repressore trp, impedendo all'operone di produrre enzimi necessari per la sua sintesi.

Gli operoni possono anche essere attivi in modo obbligatorio (cioè continuamente). Ad esempio, gli operoni batterici dell'RNA ribosomico (rRNA) sono sempre "on" perché gli rRNA sono costantemente necessari per la traduzione.

Altri elementi regolatori contribuiscono anche all'espressione genica coordinata di un operone. I geni regolatori codificano l'attivatore trascrizionale o le proteine reprimenti. I geni lacI e trpR, ad esempio, codificano per i repressori del rispettivo operone. Ulteriori sequenze regolatorie, come il sito di legame della proteina attivatore del catabolito di lac (CAP), forniscono siti di legame per altri attivatori o repressori. Ad esempio, quando il glucosio è basso, una molecola di segnalazione (cioè AMP ciclica) attiva CAP, permettendogli di legare il sito CAP, reclutare polimerasi di RNA e avviare la trascrizione dell'operone.

Operoni

Prokaryotes can control gene expression through operons&#8212;DNA sequences consisting of regulatory elements and clustered, functionally related ...

Education

Didattica

JoVE Core

Molecular Biology

Biologia Molecolare

Unit 1

Unità 1

Gene Expression

Espressione genica

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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 activity efficiently.

Operons are a key example of gene regulation in prokaryotes. The transcript highlights the lac operon as a model for this process. Studying operons shows how bacteria coordinate gene expression without separating the genes into different control systems.

Operons in Bacterial Gene Control

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

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 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 operons let bacteria regulate groups of genes together, with a focus on promoters, regulatory regions, and the lac operon model.

Operons in Bacterial Gene Control | JoVE K12

molecular biology

Le video animazioni di JoVE Core Molecular Biology, organizzate in video lezioni, illustrano i processi molecolari cellulari in maniera chiara e concisa. Inoltre, nei video di "scienziati in azione", i concetti scientifici vengono applicati e dimostrati da ricercatori attraverso esperimenti in laboratorio.

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Operons in Bacterial Gene Control

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