Understanding the Lac Operon Mechanism

I hope that introduction helps. I find that students feel really challenged learning about the Lac Operon. I think partly it's because in order to explain it we need to use these diagrams like on this slide. So let's break this diagram down and see if we can make it seem a little bit simpler. I'm even going to switch to my drawing tool here so we'll make some notes on this and see if that helps as well. Let's see. Learn how to pick a nice color. Okay, so what do we have here in the lac operon? Let's break this down. So we have structural genes. That's the lacZ, the lacY, and the lacA. And they encode three different proteins. So lacZ encodes something called beta galactosidase. lacY encodes a molecule called a permease and lacA encodes a molecule called the transacetylase. And what's really important is that you understand that all three of these enzymes, these are the genes for the enzymes. So remember, these genes have to get transcribed. See how well I can do this. They have to get transcribed. And then they have to get translated, right? So that's the next step. In order for them to become proteins, these are the steps that have to occur. And I'm sorry, I'm not very good at... typing with my mouse, but you get the idea here. Okay, so you have to be transcribed and translated into proteins, right? And so you're gonna have these three proteins that get made from lacZ, lacY, and lacA. Okay, so maybe we'll put a Z and a Y and an A here just so you kind of get the idea. All right, so this is the gene. We need to make an RNA with transcription, and then we need to translate it into these proteins. Those are the structural genes. Now, let's switch color and go over here. What's over here? Well, this is the regulatory region of the DNA. So in order to control whether or not these three genes are translated and then ultimately transcribed into protein, we have all of this DNA that's involved in just controlling whether these get turned on and off. And there are three main... places that we're going to need to know about. And I'm going to leave the CAP site for later. But right now, what I want to remind you is, we've talked about this before, is that RNA polymerase binds to the promoter. So this is what this is showing us, the promoter there, and this is our RNA polymerase. And then there's another site before the promoter called the operator. I'm going to say it's after the promoter because that's the more correct way to speak of it. So it comes after. the promoter. It's called the operator. And you see this molecule here called a repressor, right? So the repressor represses this pathway. When there's no lactose, the repressor sits on this operator like a stop sign and says, nope, RNA polymerase, you're trapped, you can't go. I blocked you so you can't transcribe these three genes. And if they're never transcribed, they'll never be translated. All right, so let's take a look. I'm going to clear my drawings and let's go to the next slide. And look at the possibilities here. All right, there we go. All right, so here's our operon again. And here, again, remember, these are the structural genes, right? And this is the regulatory region of the operon. And the promoter is where RNA polymerase binds. And the operator is where the repressor binds. And if the repressor is bound to the operator, you can think of it as being sort of a big roadblock. blocking the road to prevent RNA polymerase from moving down the genes and transcribing them into mRNA. Now, what happens when lactose is present? We said LAC, Z, Y, and A are all proteins that the cell needs in order to use lactose as a carbon source, in order to use lactose for food. Well, when lactose is present, it actually is rearranged to become something called allolactose, and it will bind to the repressor. So remember we said the repressor was bound to the operator. It was preventing RNA polymerase from transcribing these genes. Well, when lactose is present, it binds to the repressor and the repressor falls off. And now RNA polymerase says, ah, the roadblock is gone. All I have to do is go ahead and synthesize these genes. So we call lactose an inducer of the lac operon because when it's present, it induces the operon to become active. All right, so it's not a repressor. It doesn't turn the operon off. It actually causes the operon to turn on. So it's an inducer. Now, the thing about the lac operon that they didn't talk about in that video, but that's really important to understand, is that, and it comes back to our understanding of cells not wanting to waste energy. Cells always make the enzymes they need to use glucose. So we always have our glycolysis and Krebs and electron transport chain if it's undergoing aerobic respiration. Those pathways are always turned on. So if... you have lots of glucose, the cell doesn't really need to use lactose. Even if lactose is present, there's no need to break it down if it's already got glucose, all right? It would be a waste of energy to break down lactose if there's already so much glucose that the cell doesn't need anymore than it's already got, all right? So here we have, so how does this work? So we have high glucose inside a cell. There's an enzyme called adenyl cyclase that actually is inactive in the presence of high glucose. And adenyl cyclase produces a molecule called cyclic AMP. And cyclic AMP is a messenger in the cell. It basically tells the cell if it's hungry or not. So you can almost think of it as a little hormone. It's not, but it has that effect. It's basically a way for the cell to sense what its energy needs are. And if cyclic AMP is low, the cell says, I've got tons of food. Everything's great. What happens when glucose is in short supply? Well, then this enzyme, agnil cyclase, becomes active. And when it becomes active, it produces lots of this molecule called cyclic AMP, and that tells the cell that the cell is starving. And that's a signal to the cell that it absolutely needs more energy. So low cyclic AMP means that the cell has plenty of energy, and high cyclic AMP means the cell is starving. And when lactose is also present, If the cell is well fed, it doesn't need to make a lot of these enzymes to break down lactose. So we'll see very low transcription of the lac operon. But if lactose is present and the cell was starving, it didn't have glucose, then it really needs those enzymes, the beta-galactosidase and permease and transacetylase. It needs those enzymes in order to use the lactose that's coming into the cell. And so then you'll see frequent transcription of the lac operon. All right, how does this work? So remember that we said in this particular case, we're assuming that we have lactose present, okay, that there's no repressor on the operator. When there's low glucose, when there's low glucose, remember that our cyclic AMP levels are high. And that results in binding to a protein called the catabolite activating protein. And when these two molecules are bound together, they bind to the cap site, which is even further upstream of the promoter and the operator. It's also part of the regulatory region. And they activate this pathway. So if these molecules are bound but there's no lactose, the repressor will still prevent transcription. But if these molecules are bound and there's no repressor, they basically tell the RNA polymerase, go very fast. Make lots of these transcripts so we can end up with lots of this protein. When glucose levels are high though, the cell doesn't need these proteins, right? It doesn't need the proteins that come from those genes. And so basically what happens is cap does not bind. And even though there's no repressor on the operator, RNA polymerase will bind, but it's going to transcribe very inefficiently, very slowly, because we don't need these proteins to be made. We already have plenty of glucose. So there's no sense wasting ATP making enzymes that the cell doesn't actually need to survive. So this is a complicated process and I'm expecting that you'll probably replay this a couple of times until it really makes sense. And don't be surprised if you find that that's what's needed in order to really get a good understanding of this. This is another sort of a rehash of the last slide, but maybe these diagrams will help. I've got a couple of the scenarios. There's actually four different scenarios. You can have glucose present and lactose absent. You can have glucose present and lactose present. You can have glucose absent and lactose absent, and you can have glucose absent and lactose present. So this shows you all of the different possibilities and what happens. Let's look at a couple of them. So this is what happens with glucose present and lactose present. When glucose is present and lactose is present, remember that the repressor will have bound to allolactose and fallen off the operator. So the RNA polymerase can bind and transcribe the genes, but it doesn't do it very quickly because... the catabolite activating protein is not bound, and so it doesn't have the push. I always think of catabolite activating protein kind of shoving RNA polymerase to make it. transcribe the genes more quickly. So whatever visually helps you is useful. All right, so what do we have? Well in this particular case we have glucose present, lactose present, we can kind of come to our chart here, glucose present, lactose present, CAP is not binding to the CAP site, the repressor is not binding, so we end up with low-level transcription. Okay, so it's going to make a little bit of these enzymes but not a lot. What happens when glucose is absent and lactose is absent? Well, in this particular case, we have our cyclic AMP binding to CAP, and it's going to sit here on the CAP site. But nothing happens anyway, because the repressor, without lactose, the repressor is bound to the operator. All right? And so the repressor is going to prevent RNA polymerase from synthesizing these genes. And that's really important, because the cell is already starving. It doesn't have any energy to make proteins that would... only be used to digest a sugar that's not there anyway. Okay, so this makes a lot of sense from a survival of the fittest point of view. When do you make a lot of these three proteins? You only make a lot of these three proteins when glucose is absent and lactose is present. This means the cell's starving. It's going to be happy that lactose is there. It can use it as a carbon source. The repressor will have fallen off and cap will be binding and that's going to really cause there to be really strong, efficient transcription to make these three enzymes, the beta-galactosidase, the permease, and the transacetylase, which are needed for the cell to digest lactose. All right, so this is, I'm not going to read you the slide because you can read, but these are sort of the key points I hope you will take away after you've watched this entire video, okay? The lac operon is just a system by which E. coli can break down lactose and use it as a carbon source, but it only uses lactose as a carbon source when glucose levels are low. So the cell has a way to both control for the presence of lactose, that's required in order for the operon to be functional, that's what the lactose is what binds to the repressor, allowing it to fall off of the operator and providing a way for RNA polymerase to transcribe the gene. But in order to transcribe efficiently, we also have to have a lack of glucose, because only when the cell is starving and we have lots of cyclic AMP present will that catabolite activating protein bind to the cap site and really cause there to be efficient transcription of the lac operon. You would expect on an exam or a quiz to see questions where I ask you the different scenarios for the lac operon and have you be able to explain back to me. you know, when the lac operon is going to transcribe efficiently and when it might not be and why. All right, so that's my explanation. Because students do find this concept hard, I'm going to also put a video in here from the Khan Academy. And I know a lot of students don't love the Khan Academy. I think he does a great job explaining things, but it's, you know, it's your preference. It's not required that you watch this, but I am putting it here because I think enough students struggle with this concept that Hearing another person explain it in their own words may be helpful for you, and I want to make sure that this is an easy resource for you to have.

Let's see. Learn how to pick a nice color. Okay, so what do we have here in the lac operon? Let's break this down. So we have structural genes.

That's the lacZ, the lacY, and the lacA. And they encode three different proteins. So lacZ encodes something called beta galactosidase.

lacY encodes a molecule called a permease and lacA encodes a molecule called the transacetylase. And what's really important is that you understand that all three of these enzymes, these are the genes for the enzymes. So remember, these genes have to get transcribed.

See how well I can do this. They have to get transcribed. And then they have to get translated, right?

So that's the next step. In order for them to become proteins, these are the steps that have to occur. And I'm sorry, I'm not very good at...

typing with my mouse, but you get the idea here. Okay, so you have to be transcribed and translated into proteins, right? And so you're gonna have these three proteins that get made from lacZ, lacY, and lacA.

Okay, so maybe we'll put a Z and a Y and an A here just so you kind of get the idea. All right, so this is the gene. We need to make an RNA with transcription, and then we need to translate it into these proteins. Those are the structural genes.

Now, let's switch color and go over here. What's over here? Well, this is the regulatory region of the DNA. So in order to control whether or not these three genes are translated and then ultimately transcribed into protein, we have all of this DNA that's involved in just controlling whether these get turned on and off.

And there are three main... places that we're going to need to know about. And I'm going to leave the CAP site for later. But right now, what I want to remind you is, we've talked about this before, is that RNA polymerase binds to the promoter.

So this is what this is showing us, the promoter there, and this is our RNA polymerase. And then there's another site before the promoter called the operator. I'm going to say it's after the promoter because that's the more correct way to speak of it.

So it comes after. the promoter. It's called the operator. And you see this molecule here called a repressor, right? So the repressor represses this pathway.

When there's no lactose, the repressor sits on this operator like a stop sign and says, nope, RNA polymerase, you're trapped, you can't go. I blocked you so you can't transcribe these three genes. And if they're never transcribed, they'll never be translated.

All right, so let's take a look. I'm going to clear my drawings and let's go to the next slide. And look at the possibilities here. All right, there we go.

All right, so here's our operon again. And here, again, remember, these are the structural genes, right? And this is the regulatory region of the operon. And the promoter is where RNA polymerase binds.

And the operator is where the repressor binds. And if the repressor is bound to the operator, you can think of it as being sort of a big roadblock. blocking the road to prevent RNA polymerase from moving down the genes and transcribing them into mRNA.

Now, what happens when lactose is present? We said LAC, Z, Y, and A are all proteins that the cell needs in order to use lactose as a carbon source, in order to use lactose for food. Well, when lactose is present, it actually is rearranged to become something called allolactose, and it will bind to the repressor.

So remember we said the repressor was bound to the operator. It was preventing RNA polymerase from transcribing these genes. Well, when lactose is present, it binds to the repressor and the repressor falls off.

And now RNA polymerase says, ah, the roadblock is gone. All I have to do is go ahead and synthesize these genes. So we call lactose an inducer of the lac operon because when it's present, it induces the operon to become active. All right, so it's not a repressor. It doesn't turn the operon off.

It actually causes the operon to turn on. So it's an inducer. Now, the thing about the lac operon that they didn't talk about in that video, but that's really important to understand, is that, and it comes back to our understanding of cells not wanting to waste energy. Cells always make the enzymes they need to use glucose. So we always have our glycolysis and Krebs and electron transport chain if it's undergoing aerobic respiration.

Those pathways are always turned on. So if... you have lots of glucose, the cell doesn't really need to use lactose. Even if lactose is present, there's no need to break it down if it's already got glucose, all right?

It would be a waste of energy to break down lactose if there's already so much glucose that the cell doesn't need anymore than it's already got, all right? So here we have, so how does this work? So we have high glucose inside a cell.

There's an enzyme called adenyl cyclase that actually is inactive in the presence of high glucose. And adenyl cyclase produces a molecule called cyclic AMP. And cyclic AMP is a messenger in the cell.

It basically tells the cell if it's hungry or not. So you can almost think of it as a little hormone. It's not, but it has that effect.

It's basically a way for the cell to sense what its energy needs are. And if cyclic AMP is low, the cell says, I've got tons of food. Everything's great. What happens when glucose is in short supply? Well, then this enzyme, agnil cyclase, becomes active.

And when it becomes active, it produces lots of this molecule called cyclic AMP, and that tells the cell that the cell is starving. And that's a signal to the cell that it absolutely needs more energy. So low cyclic AMP means that the cell has plenty of energy, and high cyclic AMP means the cell is starving.

And when lactose is also present, If the cell is well fed, it doesn't need to make a lot of these enzymes to break down lactose. So we'll see very low transcription of the lac operon. But if lactose is present and the cell was starving, it didn't have glucose, then it really needs those enzymes, the beta-galactosidase and permease and transacetylase.

It needs those enzymes in order to use the lactose that's coming into the cell. And so then you'll see frequent transcription of the lac operon. All right, how does this work?

So remember that we said in this particular case, we're assuming that we have lactose present, okay, that there's no repressor on the operator. When there's low glucose, when there's low glucose, remember that our cyclic AMP levels are high. And that results in binding to a protein called the catabolite activating protein. And when these two molecules are bound together, they bind to the cap site, which is even further upstream of the promoter and the operator.

It's also part of the regulatory region. And they activate this pathway. So if these molecules are bound but there's no lactose, the repressor will still prevent transcription.

But if these molecules are bound and there's no repressor, they basically tell the RNA polymerase, go very fast. Make lots of these transcripts so we can end up with lots of this protein. When glucose levels are high though, the cell doesn't need these proteins, right?

It doesn't need the proteins that come from those genes. And so basically what happens is cap does not bind. And even though there's no repressor on the operator, RNA polymerase will bind, but it's going to transcribe very inefficiently, very slowly, because we don't need these proteins to be made.

We already have plenty of glucose. So there's no sense wasting ATP making enzymes that the cell doesn't actually need to survive. So this is a complicated process and I'm expecting that you'll probably replay this a couple of times until it really makes sense.

And don't be surprised if you find that that's what's needed in order to really get a good understanding of this. This is another sort of a rehash of the last slide, but maybe these diagrams will help. I've got a couple of the scenarios.

There's actually four different scenarios. You can have glucose present and lactose absent. You can have glucose present and lactose present.

You can have glucose absent and lactose absent, and you can have glucose absent and lactose present. So this shows you all of the different possibilities and what happens. Let's look at a couple of them.

So this is what happens with glucose present and lactose present. When glucose is present and lactose is present, remember that the repressor will have bound to allolactose and fallen off the operator. So the RNA polymerase can bind and transcribe the genes, but it doesn't do it very quickly because... the catabolite activating protein is not bound, and so it doesn't have the push.

I always think of catabolite activating protein kind of shoving RNA polymerase to make it. transcribe the genes more quickly. So whatever visually helps you is useful. All right, so what do we have? Well in this particular case we have glucose present, lactose present, we can kind of come to our chart here, glucose present, lactose present, CAP is not binding to the CAP site, the repressor is not binding, so we end up with low-level transcription.

Okay, so it's going to make a little bit of these enzymes but not a lot. What happens when glucose is absent and lactose is absent? Well, in this particular case, we have our cyclic AMP binding to CAP, and it's going to sit here on the CAP site. But nothing happens anyway, because the repressor, without lactose, the repressor is bound to the operator. All right?

And so the repressor is going to prevent RNA polymerase from synthesizing these genes. And that's really important, because the cell is already starving. It doesn't have any energy to make proteins that would...

only be used to digest a sugar that's not there anyway. Okay, so this makes a lot of sense from a survival of the fittest point of view. When do you make a lot of these three proteins? You only make a lot of these three proteins when glucose is absent and lactose is present. This means the cell's starving.

It's going to be happy that lactose is there. It can use it as a carbon source. The repressor will have fallen off and cap will be binding and that's going to really cause there to be really strong, efficient transcription to make these three enzymes, the beta-galactosidase, the permease, and the transacetylase, which are needed for the cell to digest lactose.

All right, so this is, I'm not going to read you the slide because you can read, but these are sort of the key points I hope you will take away after you've watched this entire video, okay? The lac operon is just a system by which E. coli can break down lactose and use it as a carbon source, but it only uses lactose as a carbon source when glucose levels are low. So the cell has a way to both control for the presence of lactose, that's required in order for the operon to be functional, that's what the lactose is what binds to the repressor, allowing it to fall off of the operator and providing a way for RNA polymerase to transcribe the gene.

But in order to transcribe efficiently, we also have to have a lack of glucose, because only when the cell is starving and we have lots of cyclic AMP present will that catabolite activating protein bind to the cap site and really cause there to be efficient transcription of the lac operon. You would expect on an exam or a quiz to see questions where I ask you the different scenarios for the lac operon and have you be able to explain back to me. you know, when the lac operon is going to transcribe efficiently and when it might not be and why. All right, so that's my explanation. Because students do find this concept hard, I'm going to also put a video in here from the Khan Academy. And I know a lot of students don't love the Khan Academy.

I think he does a great job explaining things, but it's, you know, it's your preference. It's not required that you watch this, but I am putting it here because I think enough students struggle with this concept that Hearing another person explain it in their own words may be helpful for you, and I want to make sure that this is an easy resource for you to have.

Transcript for:Understanding the Lac Operon Mechanism

Transcript for:
Understanding the Lac Operon Mechanism