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

פרוקריוטים מכילים גנום מעגלי שבו, במקרים רבים, גנים קשורים לתהליכים ביוכימיים מסוימים ממוקמים ישירות זה ליד זה ברצף הדנ"א. 
לצברי גנים אלה, המכונים אופרונים, יש מקדם יחיד והם משועתקים ל-mRNA יחיד. ביטוי אופרון נשלט על ידי מפעילים ומדכאים, חלבונים המקדמים או מדכאים שעתוק, בהתאמה. 
מפעיל נקשר למקדם, מה שמוביל לקשירה של RNA פולימראז ולשעתוק הבא של אופרון.
מדכא אינטראקציה עם רצף DNA אחר הממוקם בקרבת המקדם, המכונה המפעיל. כאשר מדכא נקשר למפעיל, הוא מונע מ-RNA פולימראז להיקשר למקדם ומעכב את השעתוק.
בקרת גנים פרוקריוטים תלויה לעתים קרובות בזמינות החומרים המזינים. אצל חיידקים מסוימים, כאשר ריכוזי הגלוקוז נמוכים, זה מוביל להצטברות של AMP מחזורי. 
AMP מחזורי נקשר למפעיל, חלבון מפעיל הקטבוליט, ויחד הם נקשרים למקדם. זה יוזם את הביטוי של אופרון lac, אופרון המכיל גנים המאפשרים לחיידקים לעכל סוכרים שאינם גלוקוז.
טריפטופן היא חומצת אמינו הנדרשת לייצור חלבונים. כאשר היא אינה זמינה מהסביבה, אורגניזמים מסוימים מסוגלים לייצר טריפטופן משלהם. הגנים לסינתזה שלו נמצאים באופרון המוסדר על ידי מדכא.
כאשר ריכוזי הטריפטופן גבוהים, הוא מתקשר עם המדכא. קומפלקס זה יכול להיקשר למפעיל, לחסום RNA פולימראז ולדכא שעתוק.

<ol>
	<li>אלברטס ואחרים, מהדורה שישית; עמודים 380-383</li>
	<li>שו, ק. (2008) ויסות שעתוק שלילי בפרוקריוטים. טבע חינוך 1(1):122</li>
</ol>

מפעילים ומדכאים תעתיקים פרוקריוטיים

Prokaryotic Transcriptional Activators and Repressors

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הארגון של גנים פרוקריוטים בגנום שלהם שונה במיוחד מזה של אוקריוטים. גנים פרוקריוטיים מאורגנים, כך שהגנים לחלבונים המעורבים באותו תהליך או תפקוד ביוכימי ממוקמים בקבוצות ביחד. קבוצה זו של גנים, יחד עם האלמנטים הרגולטוריים שלהם, ידועים ביחד כאופרון. הגנים הפונקציונליים באופרון מתועתקים יחד כדי לתת גדיל יחיד של mRNA המכונה mRNA polycistronic.

תעתוק של גנים פרוקריוטיים באופרון מווסת על ידי שני סוגים של חלבונים קושרים ל-DNA הידועים כמפעילים ומדכאים. מפעילים נקשרים לפרומוטור, לאתר התחלת התעתוק, ומסייעים בקשירה של RNA פולימראז, האנזים המרכזי המעורב בתעתוק. מדכאים נקשרים לאופרטורים, רצפים רגולטוריים קצרים באופרון בין הפרומוטור לגנים, ומעכבים את הקישור של RNA פולימראז לפרומוטור.

תנאי מוקדם מבני עבור מפעילים ומקדמים הוא שהם יהיו מסוגלים להתקיים בשתי קונפורמציות אלטרנטיביות, אחת שבה הם יכולים להיקשר ל-DNA ואחד במקום שהם לא יכולים להיקשר. תכונה אופיינית נוספת ספציפית למפעילים היא שיש להם שני משטחים מחייבים להיקשר בו זמנית ל-RNA פולימראז וגם ל-DNA. גיוס זה של שתי המולקולות מקרב את הפולימראז לפרומוטור ומסייע בקשירתו. למפעילים אין תפקיד קטליטי בתעתיק ותפקודם מוגבל להקלת הקישור של האנזים וה-DNA. בהיעדרות של מפעיל, RNA פולימראז עדיין יכול להיקשר ל-DNA ולהראות רמות נמוכות של ביטוי. אם קיים מדכא במערכת זו, אזי הביטוי הבסיסי של אותו גן נמנע.

ויסות הביטוי של גנים פרוקריוטיים תלוי במידה רבה בזמינות החומרים התזונתיים ובדרישותיהם של האורגניזמים. חומרים מזינים אלו שולטים בקשירה של מפעילים ומדכאים לאופרון ומבטיחים שרק סט הגנים הנדרש יבוא לידי ביטוי. לדוגמה, נוכחות של טריפטופן בתא מובילה לקישור שלו לדיכוי המונע את שעתוק האופרון trp וייצור של טריפטופן לאחר מכן.

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ארגון הגנים הפרוקריוטיים בגנום שלהם שונה במידה ניכרת מזה של האיקריוטים. גנים פרוקריוטים מאורגנים, כך שהגנים ...

prokaryotic-transcriptional-activators-and-repressors

Education

JoVE Core

Molecular Biology

ביולוגיה מולקולרית

Unit 1

יחידה 1

Gene Expression

ביטוי גנים

KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

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 Molecular Biology showcases the molecular processes of the cell through clear and concise animated video lessons. Additionally, in scientist-in-action videos, these concepts' application is demonstrated through real experiments in modern laboratories.

Operon Control by Activators and Repressors

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