Prokaryotic Gene Regulation via Operons: Videos & Practice Problems

Prokaryotic gene regulation occurs through operons, which are groups of related genes controlled by a single promoter. 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 and can be turned off by a corepressor that activates a repressor protein, inhibiting transcription. Understanding these mechanisms is crucial for grasping metabolic adaptability in prokaryotes and their responses to environmental changes.

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 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. 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, which is up here in green. You'll also notice that the operon includes another yellow region here that's called the operator. The transcription of the operon is regulated by the operator, which is a region of DNA. It's a small DNA sequence where regulatory proteins will bind, and 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, we're showing you that repressors are regulatory proteins themselves that will block or inhibit RNA polymerase binding, preventing transcription. Conversely, activators are regulatory proteins themselves that will actually promote RNA polymerase binding, stimulating transcription. 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. You can see the promoter for the regulatory gene here, the regulatory gene here, the regulatory gene gets transcribed and translated into this regulatory protein. This is the regulatory protein. When it's active, it 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. Here we have the RNA polymerase that will bind to the promoter, but it will not be able to bind if there is a repressor bound. However, if there is an activator bound, then the RNA polymerase will be able to bind. RNA polymerase binding is necessary for transcription. This is really what an operon operon is: 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 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. 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.

Answer: (C)

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.

Answer: (A)

Inducible Operons

In this video, we're going to introduce inducible operons. Inducible operons are operons that are normally turned off, and when they are turned off, the genes will not be expressed. However, they can be turned on under very specific conditions, and 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.

What happens is an active repressor protein represses 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 inactivates the repressor protein, and 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. We're showing you the inducible operon, which is aligned with positive gene regulation because, as we discussed in our previous lesson videos, positive gene regulation occurs when you turn a gene on. Here on the left-hand side, the inducible operon under normal condition is shown as turned off because an active repressor protein will bind to the operator and block or prevent transcription, and so none of the genes will be expressed normally. However, under very specific conditions that include the presence of an inducer molecule, the inducer molecule will bind to the repressor and cause the repressor to change its shape, changing its conformation. The inactive repressor here can no longer bind to the operator, meaning that the operator 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. When transcription is turned on, the mRNA will be made, and the proteins will follow with translation. The products here are being made. Inducible operons undergo positive gene regulation, which means that they can be turned on, although they're normally off.

This concludes our brief introduction to inducible operons, and as we move forward, we'll continue to learn more about operons and 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 an operon that is normally turned on, and its genes are normally expressed. However, it is repressible because 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 corepressor molecule. The corepressor, a small molecule itself, will bind to the repressor, forming an active repressor protein. In other words, the corepressor molecule activates the repressor protein so that transcription is turned off when there is an active repressor protein.

Let's take a look at the image below to get a better understanding of a repressible operon. A repressible operon aligns with negative gene regulation, as the gene can be turned off. Under normal conditions, they are typically turned on. You will note that when it is normally turned on, the RNA polymerase can bind and transcribe, forming an mRNA. The mRNA is then translated into the gene products. The genes are being expressed because the repressor is in an inactive form. This inactive repressor protein requires a corepressor to bind. Notice that under specific conditions, if there is a corepressor molecule, represented by a little red molecule, the little red corepressor can bind to the inactive repressor to form an active repressor molecule. The active repressor can then bind to the operator to block, inhibit, or prevent transcription, turning off the gene and the operons.

Repressible operons are normally turned on but can be turned off under the right conditions, which include having a corepressor molecule present. Notice the image of the light switch being turned off, illustrating what repressible operons are capable of doing. This concludes our brief introduction to repressible operons, and we'll 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 be on, and the inducible operon will be induced. It will be turned on in the presence of a regulatory molecule inducer. An example of an inducible operon is actually the lac operon, which we're going to talk more 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 a repressible 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 corepressor 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.

Answer: (D)

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.

Answer: (B)

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.

Answer: (D)

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 rapidly adapt 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 inhibit transcription. An example is the trp operon, which is repressed in the presence of tryptophan. These mechanisms allow prokaryotes to efficiently manage gene expression in response to environmental changes.

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 RNA polymerase and preventing transcription. When lactose is present, it acts as an inducer by binding to the repressor protein, causing a conformational change that inactivates the repressor. 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 utilization only when lactose is available.

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 transcription of genes involved in tryptophan production. When tryptophan levels are high, tryptophan acts as a corepressor by binding to the repressor protein. This complex then binds to the operator, blocking RNA polymerase and halting transcription. This feedback mechanism ensures that the cell does not waste resources producing tryptophan when it is already abundant, demonstrating efficient gene regulation in response to environmental conditions.

What are the components of an operon and their functions?

An operon consists of several key components: the promoter, operator, and structural genes. The promoter is a DNA sequence where RNA polymerase binds to initiate transcription. The operator is a DNA segment where regulatory proteins (repressors or activators) bind to control the transcription of the operon. Structural genes are the actual genes that code for proteins. Additionally, there may be a regulatory gene that produces the repressor or activator protein. These components work together to regulate gene expression, allowing prokaryotes to adapt to environmental changes by turning genes on or off as needed.