Prokaryotic Gene Regulation via Operons - Online Tutor, Practice Problems & Exam Prep

Prokaryotic gene regulation is primarily achieved through operons, which are groups of related genes controlled by a single promoter. An operon consists of a promoter, operator, and structural genes. Inducible operons are typically off but can be activated by an inducer that inactivates a repressor protein, allowing transcription. Conversely, repressible operons are usually on but can be turned off by a corepressor activating a repressor protein. Understanding these mechanisms is crucial for grasping metabolic adaptability in changing environments.

Prokaryotic Gene Regulation via Operons

In this video, we're going to begin our lesson on prokaryotic gene regulation via operons. Prokaryotic organisms, or prokaryotes, must be able to survive in environments that constantly change in the availability of nutrients in the surroundings. This requires the prokaryotes to be able to rapidly change their metabolic pathways by regulating the expression of certain genes. Prokaryotes commonly control the expression of their genes using what are known as operons. We'll get to talk more about these operons and the structure and components of operons as we move forward in our course. So, I'll see you all in our next video.

Structure of an Operon

In this video, we're going to talk about the structure of an operon. An operon is defined as a set or a group of prokaryotic genes, usually of related function, that are controlled by a single promoter. Recall from our previous lesson videos when we talked about transcription that the promoter of a gene is going to be just ahead of a gene, a DNA sequence just ahead of the gene where the RNA polymerase will bind. If we take a look at our image down below, notice that our operon is being labeled from this region here over to this region over here. What you'll notice is that the operon contains a group or a set of related genes. And here in our image, the related genes are gene a, gene b, and gene c. Notice that these related genes a, b, and c, are all controlled by a single promoter region, and the promoter region is up here in green. The operon also includes this other yellow region here that's called the operator. The transcription of the operon is regulated by the operator. The operator is a region of DNA. It's a small DNA sequence where regulatory proteins will bind. These regulatory proteins will bind to the operator and affect the RNA polymerase binding to the promoter. Some regulatory proteins will repress or block the RNA polymerase from binding and other regulatory proteins will promote or stimulate the RNA polymerase binding.

Down below, what we're showing you are repressors which are regulatory proteins themselves that will block or inhibit RNA polymerase binding, preventing transcription. And then, of course, activators are going to be regulatory proteins themselves that will actually promote RNA polymerase binding, stimulating transcription. If we take a look at our image down below, again, notice that the operon itself contains an operator, and the operator is going to be the site for the binding of a regulatory protein here. The regulatory protein is going to have its own gene, and the regulatory gene is over here, and the regulatory gene has its own promoter. The promoter for the regulatory gene is here, the regulatory gene is here, the regulatory gene gets transcribed and translated into this regulatory protein. This regulatory protein, when it's active, will bind to the operator, as you see here. Depending on if the regulatory protein is a repressor or an activator, it will either block or promote the RNA polymerase binding. We have the RNA polymerase that will bind to the promoter, and it will not be able to bind if there is a repressor bound. But if there is an activator bound, then the RNA polymerase will be able to bind. Of course, RNA polymerase binding is necessary for transcription. This is really an operon. An operon is going to be, again, a group of related genes like a, b, and c here that are controlled by a single promoter, and transcription of these genes is controlled by this operator region, which will be the site for the regulatory protein binding.

As we move forward in our course, we'll be able to talk more and more about operons and specific types of operons and exactly how they function. But for now this here concludes our introduction to the structure of an operon and we'll be able to get some practice moving forward. So I'll see you all in our next video.

Altering patterns of gene expression in prokaryotes would likely increase a prokaryote's survival by _______.

• A. Organizing gene expression, so that genes are expressed in a given order.
• B. Allowing each gene to be expressed an equal number of times.
• C. Allowing a prokaryote to adjust to changes in environmental conditions.
• D. Allowing environmental changes to alter a prokaryote's genome.

Which of the following is true about operons?

• A. They allow the organism the opportunity to simultaneously regulate transcription of multiple genes.
• B. They allow the organism the opportunity to regulate transcription of a single gene.
• C. They allow many genes to be expressed at the same time, even those unrelated in function.
• D. They significantly increase the rate of DNA replication, thereby make transcription more efficient.

Inducible Operons

In this video, we're going to introduce inducible operons, and so inducible operons are operons themselves that are normally turned off, and when they are turned off the genes will not be expressed. Inducible operons are normally turned off. However, they can be turned on under very specific conditions. When they can be turned on, that means that they can be induced. The conditions where they can be turned on include the presence of what's known as an inducer, which we'll introduce here very shortly. Inducible operons are normally turned off but can be turned on. What happens is an active repressor protein is going to repress transcription under normal conditions. The inducible operon will be normally turned off when there is an active repressor protein repressing transcription. However, the active repressor protein can be inactivated by the inducer molecule. The inducer molecule will bind to the active repressor protein and inactivate the repressor, which will allow for transcription to proceed. In other words, the inducer molecule is going to inactivate the repressor protein. When it inactivates the repressor protein, transcription will be turned on. This is why it's called an inducible operon because it can be induced even though it's normally turned off.

If we take a look at our image down below, we can get a better understanding of an inducible operon in the presence of an inducer molecule. Here, we're showing you the inducible operon, which is very aligned with positive gene regulation because what you can see, from our previous lesson videos when we talked about positive gene regulation, is when you turn a gene on. Over here on the left hand side, what we're showing you is the inducible operon under normal conditions. The inducible operon is turned off. The reason that it's turned off is because an active repressor protein will bind to the operator and block or prevent transcription, block the RNA polymerase and block transcription, so none of these genes will be expressed normally. However, under very specific conditions that include the presence of an inducer molecule, in the presence of this inducer molecule, the inducer molecule will bind to the repressor and cause the repressor to change its shape, change its conformation. The inactive repressor here can no longer bind to the operator. That means that the operator here is going to be free, and the RNA polymerase is capable of binding to the promoter and proceeding with transcription. In the presence of an inducer molecule, the inducible operon can be turned on. You can see here we have our light switch being flipped into the on state. Of course, when transcription is turned on, the mRNA will be made, and, of course, the proteins will follow with translation are undergoing positive gene regulation, which means that they can be turned on, although they're normally off. This here concludes our brief introduction to inducible operons and as we move forward we'll continue to learn more and more about operons and be able to get some practice. So I'll see you all in our next video.

Repressible Operons

In this video, we're going to introduce repressible operons. A repressible operon is usually one that is normally turned on, and therefore its genes are expressed. It is termed a repressible operon because, even though it is typically on, it can be turned off or repressed under the right conditions, which includes having an active repressor protein. It's important to note that the inactive repressor protein cannot repress transcription without a co-repressor molecule. The co-repressor is a small molecule itself that will bind to the repressor, forming an active repressor protein. In other words, the co-repressor molecule activates the repressor protein so that transcription is turned off when there is an active repressor protein.

Let's look at the image below to get a better understanding of a repressible operon. A repressible operon aligns with negative gene regulation because the gene can be turned off under specific conditions. Normally, repressible operons are turned on. When they are turned on, RNA polymerase binds and transcribes, forming mRNA, which then gets translated into gene products. The genes are expressed because the repressor is in an inactive form. We have an inactive repressor protein that cannot bind to the operator since it requires a co-repressor to bind. Under specific conditions, if there is a co-repressor molecule, represented by a little red molecule, this co-repressor can bind to the inactive repressor to form an active repressor. The active repressor can bind to the operator, preventing transcription, thus turning off the gene and the operons. Repressible operons are normally on but can be turned off under the right conditions, which includes having a co-repressor molecule present. Notice the image of the switch being turned off, symbolizing the functionality of repressible operons being turned off.

This concludes our brief introduction to repressible operons, and we will be able to get some practice applying these concepts as we move forward. I'll see you all in our next video.

Review of Inducible vs. Repressible Operons

In this video, we're going to complete the table that you see down below as we review inducible versus repressible operons. And so notice that here in this row with the greenish background we're focusing on inducible operons, and then down below in this row with the pinkish background, we're focusing on repressible operons. And so recall from our previous lesson videos of inducible operons that they are normally turned off, which means that they will not be transcribed normally. However, these inducible operons can be turned on, and that is why they are called inducible operons because they can be induced. Now, the repressor protein is normally going to be active with inducible operons, and, of course, the repressor protein is going to repress transcription. And because the repressor protein is normally active, it will be normally repressing transcription to turn off transcription normally. However, under the right conditions, when the regulatory molecule is present, specifically the inducer is present, this little circle right here, the inducer will bind to the repressor protein and inactivate the repressor protein. And so we have here an inactivated repressor protein. The effect of the regulatory molecule, the effect of the inducer is to inactivate the repressor protein. And, of course, inactivating the repressor protein is going to allow for transcription to proceed, and so transcription will regulatory molecule inducer. And so, an example of an inducible operon is actually the lac operon, which we're going to talk more details about as we move forward in our course.

Now, of course, recall from our previous lesson videos of repressible operons that they are pretty much the exact opposite of inducible operons. And so they're actually normally turned on. Their gene expression is normally on, and so transcription is usually occurring with these repressible operons. However, under the right conditions, repressible operons can be turned off or they can be repressed. And that's why they're called repressible operons, because they can be turned off or repressed. Now, of course, the repressor protein is normally going to be inactive with operon. And in the inactive state, the repressor protein will not be able to repress transcription, and so transcription will be able to be on. However, in the presence of the regulatory molecule, which in this case is actually going to be a corepressor, this little red circle here. The co repressor will bind to the inactive repressor protein to activate the repressor protein. And so the effect of this regulatory molecule, the effect of the corepressor, is to lead to an activated repressor protein. And so notice that the activated repressor protein is now able to bind to the operator, and that will repress or block transcription, and so transcription will be turned off. And so notice, that an example of a repressible operon is going to be the trp operon, which we're going to talk more details about as we move forward in our course. But for now, this here concludes our review of inducible versus repressible operons, and we'll be able to get some practice as we move forward. So I'll see you all in our next video.

Which of the following molecules is a protein that stops the transcription of a gene?

• A. Operon.
• B. Inducer.
• C. Promoter.
• D. Repressor.

When this is present in the cell, it binds to the repressor and the repressor can no longer bind to the operator:

• A. Operon.
• B. Inducer.
• C. Promoter.
• D. Repressor.
• E. Corepressor.

Which of the following statements is FALSE?

• A. An inducible operon is turned on by the presence of an inducer molecule.
• B. An inactive repressor protein requires binding of a corepressor molecule to become active.
• C. A repressible operon is turned on when the repressor protein is not bound to the corepressor molecule.
• D. An active repressor protein is inactivated by the binding of an activator molecule.

Here’s what students ask on this topic:

What is an operon and how does it function in prokaryotic gene regulation?

An operon is a cluster of related genes in prokaryotes that are controlled by a single promoter. It includes structural genes, a promoter, and an operator. The promoter is a DNA sequence where RNA polymerase binds to initiate transcription. The operator is a DNA segment where regulatory proteins bind to control the transcription of the operon. Regulatory proteins can be repressors or activators. Repressors bind to the operator to block RNA polymerase, preventing transcription. Activators enhance RNA polymerase binding, promoting transcription. This coordinated regulation allows prokaryotes to efficiently respond to environmental changes by turning genes on or off as needed.

What is the difference between inducible and repressible operons?

Inducible operons are typically off but can be turned on in the presence of an inducer. The inducer inactivates a repressor protein, allowing transcription to proceed. An example is the lac operon, which is activated in the presence of lactose. Repressible operons are usually on but can be turned off when a corepressor is present. The corepressor activates a repressor protein, which then binds to the operator to block transcription. An example is the trp operon, which is repressed in the presence of tryptophan. These mechanisms allow prokaryotes to adapt their gene expression to environmental conditions.

How does the lac operon function as an inducible operon?

The lac operon is an inducible operon that controls the metabolism of lactose in E. coli. It is normally off because a repressor protein binds to the operator, blocking transcription. When lactose is present, it acts as an inducer by binding to the repressor and inactivating it. This allows RNA polymerase to bind to the promoter and transcribe the genes needed for lactose metabolism. The lac operon thus enables the bacteria to produce enzymes for lactose digestion only when lactose is available, conserving energy and resources.

What role does the trp operon play in prokaryotic gene regulation?

The trp operon is a repressible operon that regulates the synthesis of tryptophan in E. coli. It is normally on, allowing the production of enzymes for tryptophan synthesis. When tryptophan levels are high, tryptophan acts as a corepressor by binding to the repressor protein, activating it. The active repressor then binds to the operator, blocking transcription. This feedback mechanism ensures that the cell does not waste energy producing tryptophan when it is already abundant, maintaining metabolic efficiency.

What are the components of an operon and their functions?

An operon consists of several key components: structural genes, a promoter, an operator, and regulatory genes. Structural genes encode proteins with related functions. The promoter is a DNA sequence where RNA polymerase binds to initiate transcription. The operator is a DNA segment where regulatory proteins bind to control transcription. Regulatory genes encode proteins, such as repressors or activators, that bind to the operator to either inhibit or promote transcription. This arrangement allows coordinated regulation of gene expression in response to environmental changes.