The Lac Operon - Online Tutor, Practice Problems & Exam Prep

The lac operon is an inducible operon in E. coli, consisting of three genes: lacZ, lacY, and lacA, which encode enzymes for lactose metabolism. In the absence of lactose, the active repressor protein lacI binds to the operator, blocking transcription. When lactose is present, it converts to allolactose, inactivating lacI, allowing RNA polymerase to initiate transcription. This regulatory mechanism ensures energy-efficient gene expression, activating the operon only when lactose is available and glucose is absent, highlighting the operon's role in cellular metabolism and energy management.

The Lac Operon

In this video, we're going to begin our lesson on the lac operon. The lac operon is actually an inducible operon, which means that it is normally turned off but can be turned on or induced, hence the term "inducible operon." It has three different genes that encode enzymes responsible for metabolizing lactose for energy. Lactose is a sugar, a carbohydrate that can serve as an energy source. Before lactose can serve as an energy source, it requires these specific enzymes to be expressed, which are going to metabolize lactose for energy. The lac operon gets its name because it's important for metabolizing lactose, hence the "lac" in "lactose" and "lac operon." These three genes encoded within the lac operon are lacZ, lacY, and lacA. Each of these genes encodes an enzyme, and these enzymes are all related to one another because they are all important for metabolizing lactose for energy.

If we take a look at our image down below, you'll notice the lac operon on the right. Recall that operons are a group or a set of related genes, all controlled by a single promoter. The lac promoter is over here, and also notice that there is the lac operator right here. The three genes, lacZ, lacY, and lacA, are in the lac operon and are all related to lactose metabolism. These three genes need to be transcribed into an mRNA and then translated into their appropriate enzymes, their appropriate proteins, before they can be used to metabolize lactose. However, it's important to remember that transcription and translation require a lot of energy, and therefore cells only want to express lac operon genes when needed. If these genes are not needed, then the cell does not want to be performing transcription and translation because it requires a lot of energy.

This means that the lac operon needs to be under regulatory control where it can be turned off and turned on under the right conditions when these genes are needed and when they are not. There is a repressor protein with this lac operon, and the active repressor protein is referred to as lacI. Normally, lacI is in its active state, and the active repressor protein lacI represses transcription, as a repressor protein should. Under normal conditions, the active repressor lacI represses transcription, which is why the lac operon is normally turned off. The active repressor protein lacI binds to the lac operator. Over on the left-hand side of our image down below, we show the lac regulatory gene. Here we have the lacI gene, which has its own promoter and will be transcribed and translated into an active lac repressor, a repressor protein. This active lac repressor binds to the lac operator and, as repressor proteins should, represses transcription. They block transcription and prevent RNA polymerase from proceeding forward and transcribing these genes.

Normally, the active lac repressor is made, and it blocks the transcription of these genes. However, under the right conditions, the lac operon can be turned on, it can be induced. That is why we call it an inducible operon because it's normally off, but it can be turned on, it can be induced. The lac operon will only be transcribed in the presence of lactose and in the absence of glucose. We will discuss why this is as we move forward in our course. The lac operons, like operons in general, are associated with prokaryotic organisms, and Escherichia coli is a bacterium. This is the context in which we will be looking at the lac operon. The lac operon in E. coli contains a single promoter, the lac promoter, and three genes required for lactose metabolism: lacZ, lacY, and lacA. This concludes our brief introduction to the lac operon. As we move forward in our course, we will continue to talk about the lac operon and how it works. I'll see you all in our next video.

The protein that binds to the operator of the lac operon to prevent transcription is encoded by which gene?

• A. lac I.
• B. lac Y.
• C. lac A.
• D. lac Z.

The lac operon is a(n) _______________ operon that is typically ______________.

• A. inducible; induced.
• B. repressible; repressed.
• C. inducible; repressed.
• D. repressed; inducible.

In the Absence of Lactose

In this video, we're going to continue to talk about the lac operon, specifically in the absence of lactose. And so before we even move on here recall from our previous lesson videos that the lac operon is only going to be on or expressed when those genes are needed. And recall that those genes in the lac operon, lac z, lac y, and lac a, they're needed to metabolize lactose and break it down in order to get energy from it. However, if lactose is absent, if there is no lactose in the environment, then that means that those genes in the lac operon are not really needed because there's no lactose for those genes to break down. And so we can imagine that this means that the lac operon would be off or inactive in the absence of lactose and that's exactly what we're going to see here. And so when lactose is not available to metabolize because it's not available in the environment, then the repressor protein lac I is going to repress the expression of the genes in the lac operon. And so lac I, this repressor protein, it's going to bind to the lac operator and block RNA polymerase, which recall is the enzyme that is responsible for transcription. And so it's going to block RNA polymerase and prevent RNA polymerase from initiating transcription. And so, there will be no transcription because, it is being blocked and inhibited. And so if we take a look at our image down below, we can get a better understanding of the lac operon in the absence of lactose. And so in the absence of lactose, the repressor protein lac I is going to be active, and because it's active it's going to bind to the operator and block transcription, preventing transcription and turning off the operon in the absence of lactose. And so here taking a look at this image, again, we're focusing on the lac operon in the absence in the absence of lactose. So there's no lactose available in the environment. And so taking a look here at our image, notice we've got our lac operon over here, and we've got the lac regulatory gene over here, which has the lac I gene with its own promoter. And of course the lac I gene with its own promoter, it is going to be transcribed and translated. And, in the absence of lactose, the lac I repressor protein is going to be active. So here we have an active lac repressor, and so because it is a repressor and it is active it is going to bind to the operator here. And so you can see that the active lac I repressor is binding to the operator, and when it binds to the lac operator, as we mentioned up above in the text, it's going to block RNA polymerase and prevent RNA polymerase from binding and initiating transcription. And so transcription of these genes is not going to occur, because, the lac I repressor is bound and blocking the RNA polymerase in the absence of lactose. And this is a good thing for the cell because when there is no lactose available, when lactose is absent, then the cell has no need to express these genes because these genes are all involved with breaking down lactose. But there is no lactose so it's not going to waste energy in producing the products of these genes. And so this is why in the absence of lactose the lac operon is off or inactive and will not be expressed. And so this here concludes our introduction to what happens when, the lac operon is in the absence of lactose. And as we move forward, we'll be able to talk about the lac operon in the presence of lactose. So I'll see you all in that video.

In the Presence of Lactose

So now that we know from our last lesson video that in the absence of lactose the lac operon is off or inactive, in this video, we're going to talk about the lac operon but in the presence of lactose. And so when lactose is readily available to metabolize in the environment, lactose can actually act as an inducer molecule in the lac operon. And so really it turns out that a derivative of lactose, that's called allolactose, you may or may not be responsible for knowing allolactose, but a derivative of lactose is going to bind and inactivate the repressor protein, lac I. And so if lac I is inactivated, then that means that it will not, it cannot bind to the operator. And if it cannot bind to the operator, that means that RNA polymerase will be allowed to initiate transcription of the lac operon, and so that will turn on the lac operon.

And so if we take a look at our image down below, notice we're saying in the presence of lactose, when lactose is readily available to metabolize, then lac I, the repressor protein, is going to be inactive. And if it's inactive, that's going to allow for lac operon transcription, turning on the lac operon. And so here again in the title, we're saying that the lac operon, we're focusing on the lac operon in the presence of lactose, and so this is our molecule to represent lactose. And so notice over here we have again our lac regulatory gene, lac I, with its own promoter here. And lac I is going to be transcribed and translated into the lac repressor. However, notice that in the presence of lactose, a derivative of lactose will be able to bind to the lac repressor and inactivate the LAC repressor. And so notice that the LAC repressor here has changed its conformation, and now it is no longer able to bind to the LAC operator and block transcription. And so because the inactive lac repressor cannot bind to the operator, this allows RNA polymerase to bind to the lac promoter and initiate transcription of these lac operon genes, lac Z, lac Y, and lac A. And so of course it's going to create an mRNA strand and the mRNA strand is going to have a start codon here for each of the genes within it. And, ultimately, this mRNA is going to be translated. And when it's translated, it's going to be converted into these enzymes, and these enzymes will be able to break down lactose. And so they can go over here and break down lactose and use lactose as an energy source. And so ultimately what we're seeing here is that the lac operon is really only going to be on or active in the presence of lactose when lactose is available, and that allows the genes to be turned on when they are needed.

This here concludes our brief introduction to the lac operon in the presence of lactose, and we'll be able to get some practice applying the concepts that we've learned as we move forward in our course. And then we'll also get to talk about how glucose will also impact the expression of the lac operon. So we'll talk more about that as we move forward. So I'll see you all in our next video.

In the lac operon, which of the following functions does the lactose molecule serve:

• A. It is the corepressor molecule.
• B. It is the repressor molecule.
• C. It is the inducer molecule.
• D. It serves no function in regulating the lac operon.

If E. coli bacteria are grown in the presence of lactose:

• A. The repressor will bind the operator allowing transcription of the lac operon genes.
• B. The repressor will not bind the operator preventing transcription of the lac operon genes.
• C. The repressor will not bind the operator allowing transcription of the lac operon genes.
• D. The repressor will bind the operator preventing transcription of the lac operon genes.

Here’s what students ask on this topic:

What is the lac operon and how does it function in E. coli?

The lac operon is an inducible operon in E. coli that consists of three genes: lacZ, lacY, and lacA. These genes encode enzymes necessary for lactose metabolism. In the absence of lactose, the active repressor protein lacI binds to the operator, blocking transcription. When lactose is present, it is converted to allolactose, which inactivates lacI. This inactivation allows RNA polymerase to bind to the promoter and initiate transcription of the lac operon genes. This regulatory mechanism ensures that the operon is only active when lactose is available and glucose is absent, optimizing energy use in the cell.

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

The lac operon in E. coli includes three genes: lacZ, lacY, and lacA. The lacZ gene encodes β-galactosidase, an enzyme that breaks down lactose into glucose and galactose. The lacY gene encodes lactose permease, a protein that facilitates the entry of lactose into the cell. The lacA gene encodes thiogalactoside transacetylase, which is involved in the detoxification of by-products of lactose metabolism. Together, these genes enable the cell to utilize lactose as an energy source when it is available.

How does the presence of lactose affect the lac operon?

In the presence of lactose, a derivative called allolactose binds to the repressor protein lacI, inactivating it. This prevents lacI from binding to the operator region of the lac operon. As a result, RNA polymerase can bind to the promoter and initiate transcription of the lac operon genes (lacZ, lacY, and lacA). These genes are then translated into enzymes that metabolize lactose, allowing the cell to use lactose as an energy source. This mechanism ensures that the lac operon is only active when lactose is present.

What happens to the lac operon in the absence of lactose?

In the absence of lactose, the repressor protein lacI remains active and binds to the operator region of the lac operon. This binding blocks RNA polymerase from transcribing the lac operon genes (lacZ, lacY, and lacA). As a result, the enzymes required for lactose metabolism are not produced, conserving the cell's energy. This regulatory mechanism ensures that the lac operon is turned off when lactose is not available, preventing unnecessary energy expenditure.

Why is the lac operon considered an inducible operon?

The lac operon is considered an inducible operon because it is normally turned off but can be turned on (induced) in the presence of a specific inducer molecule, which in this case is lactose. When lactose is present, it is converted to allolactose, which inactivates the repressor protein lacI. This allows RNA polymerase to initiate transcription of the lac operon genes. The inducible nature of the lac operon ensures that the genes for lactose metabolism are only expressed when lactose is available, optimizing the cell's energy use.