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

Prokaryoten enthalten ein zirkuläres Genom, in dem sich in vielen Fällen verwandte Gene für bestimmte biochemische Prozesse direkt nebeneinander in der DNA-Sequenz befinden. 
Diese Gencluster, die als Operone bezeichnet werden, haben einen einzigen Promotor und werden in eine einzige mRNA transkribiert. Die Expression eines Operons wird durch Aktivatoren und Repressoren gesteuert, Proteine, die die Transkription fördern bzw. unterdrücken. 
Ein Aktivator bindet an einen Promotor, was zur Bindung der RNA-Polymerase und der anschließenden Transkription eines Operons führt.
Ein Repressor interagiert mit einer anderen DNA-Sequenz, die sich in der Nähe des Promotors befindet und als Operator bezeichnet wird. Wenn ein Repressor an den Bediener bindet, verhindert er, dass die RNA-Polymerase an den Promotor bindet, und hemmt die Transkription.
Die Regulation der prokaryotischen Gene hängt oft von der Verfügbarkeit von Nährstoffen ab und führt bei einigen Bakterien bei niedrigen Glukosekonzentrationen zur Akkumulation von zyklischem AMP. 
Cyclisches AMP bindet an einen Aktivator, das Katabolit-Aktivatorprotein, und zusammen binden sie an einen Promotor. Dies initiiert die Expression des lac-Operons, eines Operons, das Gene enthält, die es den Bakterien ermöglichen, andere Zucker als Glukose zu verstoffwechseln.
Tryptophan ist eine Aminosäure, die für die Proteinsynthese benötigt wird. Wenn sie nicht aus der Umwelt verfügbar ist, sind einige Organismen in der Lage, ihr eigenes Tryptophan zu produzieren. Die Gene für seine Synthese liegen in einem Operon vor, das von einem Repressor reguliert wird.
Wenn die Tryptophankonzentrationen hoch sind, assoziiert es mit dem Repressor. Dieser Komplex kann dann an den Bediener binden, die RNA-Polymerase blockieren und die Transkription unterdrücken.

<ol>
	<li>Alberts et al., 6. Auflage; Seiten 380-383</li>
	<li>Shaw, K. (2008) Negative Transkriptionsregulation bei Prokaryoten. Naturpädagogik 1(1):122</li>
</ol>

prokaryotische transkriptionsaktivatoren und -repressoren

Prokaryotic Transcriptional Activators and Repressors

Die Organisation der prokaryotischen Gene in ihrem Genom unterscheidet sich deutlich von der der Eukaryoten. Prokaryotische Gene sind so organisiert, dass die Gene für Proteine, die am selben biochemischen Prozess oder an derselben Funktion beteiligt sind, in Gruppen zusammengefasst sind. Diese Gruppe von Genen zusammen mit ihren regulatorischen Elementen wird als Operon bezeichnet. Die funktionalen Gene eines Operons werden gemeinsam transkribiert, um einen einzigen mRNA-Strang zu erzeugen, der als polycistronische mRNA bekannt ist.

Die Transkription prokaryotischer Gene in einem Operon wird durch zwei Arten von DNA-bindenden Proteinen reguliert, die als Aktivatoren und Repressoren bekannt sind. Aktivatoren binden an den Promotor, die Stelle der Transkriptionsinitiation, und unterstützen die Bindung der RNA-Polymerase, des Schlüsselenzyms in der Transkription. Repressoren binden an Operatoren, kurze regulatorische Sequenzen im Operon zwischen dem Promotor und den Genen, und hemmen die Bindung der RNA-Polymerase an den Promotor.

Eine strukturelle Voraussetzung für Aktivatoren und Promotoren ist, dass sie in zwei alternativen Konformationen existieren können, einer, in der sie an die DNA binden können, und einer, in der sie nicht binden können. Ein weiteres spezifisches Merkmal von Aktivatoren ist, dass sie zwei Bindungsoberflächen haben, um gleichzeitig sowohl an die RNA-Polymerase als auch an die DNA zu binden. Diese Rekrutierung der beiden Moleküle bringt die Polymerase näher an den Promotor und unterstützt deren Bindung. Aktivatoren haben keine katalytische Rolle in der Transkription zu spielen; ihre Funktion ist auf die Erleichterung der Bindung des Enzyms und der DNA beschränkt. In Abwesenheit eines Aktivators kann die RNA-Polymerase immer noch an die DNA binden und zeigt niedrige Expressionsniveaus. Wenn ein Repressor in diesem System vorhanden ist, wird die basale Expression dieses Gens verhindert.

Die Regulation der Expression prokaryotischer Gene hängt weitgehend von der Verfügbarkeit und den Anforderungen der Organismen an Nährstoffe ab. Diese Nährstoffe steuern die Bindung von Aktivatoren und Repressoren an das Operon und stellen sicher, dass nur die benötigten Gene exprimiert werden. Zum Beispiel führt das Vorhandensein von Tryptophan in einer Zelle dazu, dass es an einen Repressor bindet, der die Transkription des trp-Operons und die nachfolgende Produktion von Tryptophan verhindert.

Prokaryotische Transkriptionsaktivatoren und -repressoren

Die Organisation der prokaryotischen Gene in ihrem Genom unterscheidet sich deutlich von der der Eukaryoten. Prokaryotische Gene sind so organisiert, dass die Gene ...

prokaryotic-transcriptional-activators-and-repressors

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Molecular Biology

Molekularbiologie

Unit 1

Einheit 1

Gene Expression

Genexpression

KEY TERMS AND CONCEPTS

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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

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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...

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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

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Operon Control by Activators and Repressors

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