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.
