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.
