Lac Operon - Online Tutor, Practice Problems & Exam Prep

An operon is a group of prokaryotic genes transcribed together, with the lac operon being a key example. It regulates lactose metabolism through a promoter, operator, and genes encoding enzymes like β-galactosidase. The operon is activated when lactose binds to a repressor, allowing transcription. Additionally, glucose levels influence operon activity via the Catabolite Activator Protein (CAP) and cyclic adenosine monophosphate (cAMP). High lactose and low glucose lead to strong expression, while high glucose reduces transcription, demonstrating the operon's dual regulation by both sugars.

Lac Operon Overview

So an operon is defined as a group of genes that have a similar function and they're transcribed together. Now remember, we're talking about prokaryotic gene regulation. That's why the operon is always prokaryotic. You find a bunch of different types of operons in the prokaryotic genome, and we're going to talk about many of them. But the first one we're going to talk about is the lac operon, and that's because it was discovered first. To understand an operon, we have to understand its components, and so there are many different components in an operon. And the short-term way to remember this is "prog". So the "p" in prog stands for promoter and we've talked about promoters before when we talked about transcription, and so it's the same thing. A promoter is the region where a transcription initiator binds, and this starts transcription. So operons have promoters. They also have a repressor, which is the "r" in prog. And this is actually a protein that represses transcription of the operon and it does this by binding to the "o" region of prog called an operator. So the operator is where the repressor binds, and this is an on-off switch. If the repressor is bound, transcription is not going to happen because it's being repressed. But if the repressor is not bound and the operator is open, then that means transcription can occur. The operator really determines, you know, is this happening or not? And then finally, the "g" in prog stands for genes and these are the genes that are transcribed together. So if we look at a typical operon here, here's the lac operon in these boxes. You can see there's a promoter where transcription initiators will bind. There's an operator where the repressor will bind. You have your genes here, which there are 3, like z, y, and a. And then there's actually a thing over here that's not in prog, but it's called the terminator, and it's found in many operons, and it obviously will terminate the transcription of the operon.

Let's talk about the lac operon. Like I said, it was the first operon discovered by French scientists, Jacob and Monod. It was found in the 1960s. The lac operon encodes a group of genes that break down and process lactose. Now, do you remember what lactose is? Right, it's a sugar. And this sugar has to be broken down by the prokaryotic cells so that it can be used for energy. So, the lac operon is in charge of taking that lactose and chopping it down into its little pieces so that the cell can use that energy. What it breaks down into is actually encoded by the lac Z region. This encodes Beta-Galactosidase, and that chops down the lactose into glucose and galactose. Glucose is really, what's used by the cell for energy. So this is what it really wants when it breaks down that lactose. But there are other genes, and they're super also important for making sure that lactose can be digested. The lac Y encodes a gene called permease, which allows lactose to enter into the cell. So if lactose couldn't get in, it couldn't be broken down, could not be used for energy, so permease is super important. And then we have lac A, which encodes the transacetylase. And actually, most people don't really know what this function is, but without it, lactose doesn't get digested correctly. So it is crucial for this lactose processing, even if it's not understood exactly what it does. So, here's this operon again. Remember, we have a promoter, an operator, and then we have the Z, which is in charge of breaking it down. Breakdown, beta-galactosidase. We have lac Y, which is responsible for making sure that lactose can enter. And we have lac A, which is kind of unknown, but it does have an important function, and it is required for transcription of this operon. So that is the overview of the lac operon.

Now, let's turn the page.

Lac Operon Regulation

Okay. So now let's talk about lac operon regulation. Right? This is a chapter on gene regulation. So, let's spend the majority of this time talking about how prokaryotic genes, like the lac operon, are regulated. So the lac operon, remember, digests lactose, that's its function. As they encode those genes that do that. And so, it responds differently or it's regulated differently depending on the lactose concentration. So if there is a very high lactose concentration, what happens is lactose will actually bind to a repressor, and when lactose is bound to that repressor, the repressor is then removed from the operator. So remember "prog, r" is the repressor and "o" is the operator. And normally, in the lac operon, the repressor is bound to the operator and that completely stops transcription of those genes. But when lactose concentration is high, that means that the cell needs those genes in order to break it down. So lactose binds the repressor, it's removed from the operator, and that means transcription of the operon can take place. So that means the lac operon genes are made, and then they act to break down lactose. Now, if lactose concentration is low, that means it doesn't bind to the repressor. So it stays on the operator and continues to repress transcription, and so transcription does not take place.

So if we look here, so we'll say that this is the lac operon. Right? We have 123, and this here is going to be the repressor. So you can see if there is low lactose, there's really nothing over here around, it's going to remain bound and transcription will not take place. If the lactose concentration is high, so here is lactose, see there's a ton of it around, it will then bind to the repressor. That repressor will be removed from the operator, and then transcription will take place. So that is how the lac operon responds to lactose. Now, you probably have heard this before in another biology class. This is a very common example that's used, but now we're going to take it a step further.

And the fact is, the lac operon, even though it digests lactose, is actually sensitive to glucose concentrations as well. So we talked about how it's sensitive to lactose concentrations, and that's completely independent of what we're about to say. So the lac operon responds to glucose, and how it does this is through a protein called the Catabolite Activator Protein or CAP for short. And this is a protein that represses lac operon when glucose is present. So this is another type of repressor protein. And so how this works, so this is kind of the summary. Right? The CAP, it's a protein, it's a repressor, it will repress operon, glucose is present. So that is the summary. Right? Like, that's what you need to know. If glucose is present, it'll repress it. But let me explain how, and it gets a little detailed with a lot of new vocab for it, so we're going to take it slow.

So, when glucose is in the cell, so there's a high amount of glucose, it's actually going to inhibit the activity of this enzyme called Adenylyl Cyclase. And what Adenyl Cyclase does is it creates another molecule called cAMP (cAMP). So when glucose is present, that means there's going to be low amounts of cAMP. Now, if the CAP concentration is high, CAP binds to cAMP. If it's low, it doesn’t bind to CAP. So we have this CAP protein, and cAMP can come in and bind, but it's only going to do it if it's present. And remember, if glucose is present, then the cAMP concentration will be very low. So if glucose is present, cAMP is not going to bind, but if there is a low amount of glucose, it will bind. Now what happens when this complex binds? The CAP bound to the cAMP will bind to a specific site called the CAP site, and it's upstream of the promoter. So remember we said "prog," this binding site is actually here. It's upstream and it's not present in every operon, but it is present in this one. So this complex, the CAP and the cAMP, will bind here to this CAP site and activate transcription. So this means that if glucose is high, it inhibits cAMP and represses transcription, it doesn't activate it. When glucose is low, there'll be high amounts of cAMP, high amounts of this complex, and it activates transcription. So this is here the summary. So if you ever get confused on how this is working, this is the summary. So if there's a lot of glucose around, there's not a lot of cAMP, and no transcription occurs. So that means it will repress when glucose is present. Right? Which is what we said above. But if glucose isn't present, it's at low concentration, there's going to be a lot of cAMP, which means it'll bind to CAP, create this molecule which will bind here, and when it's bound, it activates transcription.

Let's look at what this looks like. Let me just appear for a second. So here we have our operon, we have our promoter, our operator, there's a repressor here, but it's not shown because we're not talking about that now. This is going to be the lactose repressor. And then you have your genes. So we have "prog." Right? But we also have this extra special region here, which right here is the CAP binding site. So, in a situation of low glucose and lactose is available, what happens is because the lactose is available, the Lactose Repressor allows transcription. But the low glucose means that cAMP will be produced, It will bind to the CAP protein that will be recruited to this region upstream of the promoter, and that means that these genes will be strongly expressed. Transcription will happen because of the lactose, and also because of the glucose. So the lac operon can be regulated by lactose and glucose, but it does so through 2 completely different ways. Lactose is through the repressor, and glucose is through the CAP and cAMP complex. So with that, let’s now turn the page.

Lac Operon Summary

Okay. So now let's talk about the summary of the lac operon expression and regulation. Right? Because now we understand, well, we know how the lac operon is regulated with lactose. We know how it is regulated with glucose, but in a cell, you don't just have one or the other. You have combinations. You can have high lactose and low glucose, or high glucose and low lactose. Let's talk about how the lac operon responds differently to different concentrations of glucose and lactose. So, if glucose and lactose are both present in high amounts in the cell, the cell is going to utilize glucose first, and the reason is because it's simpler. It doesn't need to be broken down. It doesn't need all these genes, so glucose is going to be used first. But if you only have lactose and you don't have glucose, that means the cell has to break down the lactose to generate glucose. So, with that, let's talk about the different concentrations.

Let's first focus on lactose because that's what we talked about first, the lactose repressor. So remember, if lactose is high, it's going to bind to that repressor, remove it from the operator, and allow transcription. Anytime lactose is high, transcription can occur because the repressor is removed, and transcription will occur. But, if lactose concentration is low, that means the repressor is going to be on it. Under here, you have the repressor bound. If that repressor is bound, there's going to be no transcription taking place. Whether glucose is present in the cell or not, if you have low levels of lactose, that repressor is going to be bound and there's going to be no transcription. So what happens if lactose concentration is high and glucose concentration is different? If lactose concentration is low, it doesn't matter whether glucose is high or low, it's still not going to be expressed because the lactose repressor is going to repress it.

But these two are the ones we're going to focus on. If lactose concentration is high and glucose concentration is low, when we talk about glucose, we have to refer to the CAP protein. Now remember, if glucose concentration is low, you're going to have CAP bound to cAMP, and that is going to bind upstream of the promoter, and that's going to activate it as well. So now you have high lactose concentration activating it, and you also have a low glucose concentration, which activates it more. These are going to be very strongly expressed. It's not that they are just being expressed; you actually have both of these conditions activating transcription, so it's going to be almost double activated - super activated. Whereas, if you have high concentrations of lactose and high concentrations of glucose, what you get is lactose still activating this, but glucose isn't. Because the CAP protein doesn't have any cAMP on it, meaning that it will not bind here and will not activate. So you still get expression, right? Because lactose is still causing this expression, but it's not going to be strongly expressed because you're only getting one sort of push for transcription and not two.

So, let's look at this. Here we have our lac operon, we have our promoter, we have our CAP Binding Site, and let's look at how the different concentrations affect this operon. So if lactose is available, does that mean the repressor is going to be there or not? Right, lactose is going to bind the repressor, so there's going to be no repressor. So that's going to activate. And if there's low glucose, that means the CAP protein is going to be bound to cAMP. It's going to bind, and that binding is going to activate. So you're going to get strong expression. If you have low glucose and low lactose, that means that the repressor will be bound. Right? And you have low glucose, so that means it's also not activated because you have your cAMP but no CAP. Right? And so you have your repressor here. When that repressor is bound, you're going to have no expression. And so, if you have lactose available and high glucose, what you get is the repressor is removed, so you have no repressor. You have activation. But when high glucose, that means the CAP protein will not be bound to the cAMP, with no binding. So this binding site here is left open, and that means there's going to be no activation. So no activation, and that means it will be expressed. There's a low basal level of expression, but you don't get the strong expression that you would if you had this combination of low glucose and lactose available. So that is the summary of the lac operon and how glucose and lactose concentrations affect the expression of these prokaryotic systems.

Which of the following is not a part of an operon?

• A. Promoter
• B. Repressor
• C. Operator
• D. Enhancer

The lac operon encodes genes that are responsible for what?

• A. Synthesizing more lactose
• B. Breaking down lactose
• C. Carrying lactose to the mitochondria
• D. Converting lactose into cellulose

What happens to the lac operon when lactose concentration is high?

• A. The lac operon is activated, and lactose is synthesized
• B. The lac operon is activated, and lactose is broken down
• C. The lac operon is repressed, and lactose is synthesized
• D. The lac operon is repressed, and lactose is broken down

What happens to the lac operon when the CAP/cAMP complex binds to the CAP binding site?

• A. The lac operon genes are transcribed
• B. The lac operon genes are suppressed
• C. Lactose is synthesized

Glucose concentration can regulate the lac operon?

• A. True
• B. False

Here’s what students ask on this topic:

What is the lac operon and how does it function in prokaryotic cells?

The lac operon is a group of genes in prokaryotic cells that are transcribed together and are responsible for the metabolism of lactose. It consists of a promoter, an operator, and three structural genes: lacZ, lacY, and lacA. The promoter is where RNA polymerase binds to initiate transcription. The operator is a regulatory sequence where a repressor protein can bind to inhibit transcription. When lactose is present, it binds to the repressor, causing it to release from the operator, allowing transcription to proceed. The lacZ gene encodes β-galactosidase, which breaks down lactose into glucose and galactose. The lacY gene encodes permease, which facilitates lactose entry into the cell, and the lacA gene encodes transacetylase, whose function is less understood but essential for lactose metabolism.

How is the lac operon regulated by lactose and glucose concentrations?

The lac operon is regulated by both lactose and glucose concentrations. When lactose is present, it binds to the repressor protein, causing it to release from the operator, allowing transcription to occur. If lactose is absent, the repressor remains bound to the operator, preventing transcription. Additionally, glucose levels influence the lac operon through the Catabolite Activator Protein (CAP) and cyclic adenosine monophosphate (cAMP). High glucose levels inhibit adenylyl cyclase, reducing cAMP levels, which prevents CAP from binding to the promoter, thus repressing transcription. Low glucose levels result in high cAMP, which binds to CAP, allowing it to bind to the promoter and enhance transcription. Therefore, the lac operon is strongly expressed when lactose is high and glucose is low.

What are the roles of the genes lacZ, lacY, and lacA in the lac operon?

The lac operon includes three structural genes: lacZ, lacY, and lacA, each with a specific role in lactose metabolism. The lacZ gene encodes β-galactosidase, an enzyme that breaks down lactose into glucose and galactose, which can be used for energy. The lacY gene encodes permease, a protein that facilitates the entry of lactose into the cell. The lacA gene encodes transacetylase, an enzyme whose exact function is not fully understood but is essential for the proper metabolism of lactose. Together, these genes enable the cell to utilize lactose as an energy source when glucose is not available.

How does the presence of glucose affect the expression of the lac operon?

The presence of glucose affects the expression of the lac operon through a mechanism involving the Catabolite Activator Protein (CAP) and cyclic adenosine monophosphate (cAMP). When glucose levels are high, adenylyl cyclase activity is inhibited, leading to low cAMP levels. Without sufficient cAMP, CAP cannot bind to the promoter region of the lac operon, resulting in reduced transcription. Conversely, when glucose levels are low, cAMP levels increase, allowing cAMP to bind to CAP. The cAMP-CAP complex then binds to the promoter, enhancing the transcription of the lac operon. Thus, high glucose levels repress the lac operon, while low glucose levels activate it.

What is the role of the repressor protein in the lac operon?

The repressor protein in the lac operon plays a crucial role in regulating gene expression. It binds to the operator region of the operon, preventing RNA polymerase from transcribing the structural genes lacZ, lacY, and lacA. When lactose is absent, the repressor remains bound to the operator, effectively blocking transcription. However, when lactose is present, it binds to the repressor, causing a conformational change that releases the repressor from the operator. This removal allows RNA polymerase to access the promoter and initiate transcription of the genes necessary for lactose metabolism. Thus, the repressor protein acts as an on/off switch for the lac operon based on lactose availability.