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 it can be turned on, or it can be induced—hence the term 'inducible operon'. It actually 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. However, 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 'lac' because it's essential for metabolizing lactose; hence the 'lac' in lactose and lac operon. These three genes encoded within the lac operon are lac z, lac y, and lac a. Each of these genes, lac z, lac y, and lac a, encodes an enzyme, and those enzymes are all related because they're crucial for metabolizing lactose for energy.

As we look at the image here on the right, we're showing the lac operon. Recall that operons are a group or set of related genes, all controlled by a single promoter. You can see the lac promoter here, and also notice the lac operator right there. Lac z, lac y, and lac a are the three genes in the lac operon related to lactose metabolism. These three genes, lac z, lac y, and lac a, would need to be transcribed into an mRNA and then translated into their appropriate enzymes, or proteins, before they can be used to metabolize lactose. However, it's important to remember that transcription and translation actually require a lot of energy, so the cell does not want to waste energy. Therefore, cells only want to express lac operon genes when those genes are needed. If those genes are not needed, then the cell does not want to perform transcription and translation because it requires a lot of energy. This means that the lac operon needs to be under regulatory control to be able to be turned off and turned on under the right conditions when these genes are needed and when they're not needed.

Importantly, there is a repressor protein with this lac operon, referred to as lac I. Normally, lac I is going to be in its active state, and as an active repressor protein, lac I will repress transcription as a repressor protein should. Normally, the active repressor lac I will repress transcription, and that is why the lac operon is usually turned off. Of course, the active repressor protein lac I will bind, or be bound, to the lac operator. Here on the left-hand side, we're showing the lac regulatory gene. Here we have the lac I gene, which has its own promoter. The lac I gene will be transcribed and translated into an active lac repressor, a repressor protein. This active lac repressor will bind to the lac operator right here, and when it's bound to the lac operator, as repressor proteins should, they repress transcription, block transcription, and prevent the RNA polymerase from proceeding forward and transcribing these genes. Under normal conditions, this is what we tend to see: the active lac repressor is made and blocks the transcription of these genes. This operon, the lac operon, is normally turned off. However, under the right conditions, the lac operon can be turned on; it can be induced. That's why we call it an inducible operon—it's normally off but 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, and these are two critical factors that impact lac operon transcription.

It's important to note that lac operons, and operons in general, are associated with prokaryotic organisms, and Escherichia coli is the bacteria where the lac operon was first discovered. Thus, that is the context in which we will be looking at the lac operon. The lac operon in Escherichia coli contains a single promoter, the lac promoter, and three genes required for lactose metabolism: lacz, lac y, and lac a. 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. Before we move on, recall from our previous lesson videos that the lac operon is only going to be on or expressed when those genes are needed. Recall that the genes in the lac operon, lacZ, lacY, and lacA, are 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 those genes in the lac operon are not really needed because there's no lactose for those genes to break down. 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.

When lactose is not available to metabolize because it's not available in the environment, then the repressor protein, lacI, is going to repress the expression of the genes in the lac operon. LacI, this repressor protein, is going to bind to the lac operator and block RNA polymerase, which, recall, is the enzyme responsible for transcription. It's going to block RNA polymerase and prevent RNA polymerase from initiating transcription. There will be no transcription because it is being blocked and inhibited.

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. In the absence of lactose, the repressor protein lacI 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. Here, taking a look at this image, again, we're focusing on the lac operon in the absence of lactose. There's no lactose available in the environment.

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 lacI gene with its own promoter. And of course, the lacI gene with its own promoter is going to be transcribed and translated. In the absence of lactose, the lacI repressor protein is going to be active. Here we have an active lac repressor, and because it is a repressor and it is active, it is going to bind to the operator. You can see that the active lacI repressor is binding to the operator, and when it binds to the lac operator, as mentioned above in the text, it's going to block RNA polymerase and prevent RNA polymerase from binding and initiating transcription.

Transcription of these genes is not going to occur because the lacI repressor is bound and blocking the RNA polymerase in the absence of lactose. This is good 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. There is no lactose, so it's not going to waste energy in producing the products of these genes. This is why, in the absence of lactose, the lac operon is off or inactive and will not be expressed. This conclusion introduces what happens when the lac operon is in the absence of lactose. 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 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 LacI. And so if LacI 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 LacI, 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 LacI with its own promoter here. And LacI 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, LacZ, LacY, and LacA. 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. And so 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 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. The operon is regulated by the lacI repressor protein, which binds to the operator region and blocks transcription in the absence of lactose. When lactose is present, it is converted to allolactose, which inactivates the lacI repressor. This 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, conserving cellular energy.

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 transport of lactose into the cell. The lacA gene encodes thiogalactoside transacetylase, which is involved in the detoxification of certain 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 lacI repressor protein, inactivating it. This prevents the repressor 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 available.

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

In the absence of lactose, the lacI repressor protein remains active and binds to the operator region of the lac operon. This binding blocks RNA polymerase from attaching to the promoter, thereby preventing the transcription of 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 only expressed when lactose is present and needed for metabolism.

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 lacI repressor protein. This allows RNA polymerase to bind to the promoter and initiate transcription of the lac operon genes. This inducible nature ensures that the operon is only active when lactose is available, making it an efficient system for energy conservation.