So from our previous lesson videos, we know that cyclic AMP or cAMP will increase the rate of lac operon transcription. But in this video, we're going to talk about exactly how that works by focusing on positive control by cAMP and a molecule that we're going to introduce called CRP. And so CRP is really just an abbreviation for cyclic AMP receptor protein or just CRP for short. And CRP is actually an activator protein, which recall from our previous lesson videos, activator proteins are regulatory proteins that will stimulate transcription. And so CRP is an activator protein that will stimulate the lac operon transcription when CRP is bound to cAMP. And so you can see that even embedded in CRP's full name here, it has cyclic AMP. And so it does rely on cAMP. And so recall from our previous lesson videos that low glucose levels are going to translate to high cellular cAMP levels. And high cellular cAMP levels are going to allow cAMP to bind and activate CRP. And an active CRP is going to act as an activator protein, and it is going to bind to a region of DNA upstream of the lac promoter, and it is going to stimulate transcription by helping to recruit RNA polymerase. And when the RNA polymerase is recruited and bound to the promoter, then transcription is stimulated and can proceed. And so essentially what we're saying here is that when glucose levels are low, if we have low glucose, that's going to translate to having high cAMP. And having high cAMP is going to allow cAMP to bind to and activate CRP. So we have an active CRP and having an active CRP is going to increase the rate of lac operon transcription. And so, really, this here this line here shows you the takeaway, that low glucose levels lead to high cAMP levels, high cAMP levels lead to an active CRP, and an active CRP increases the rate of lac operon transcription. And so let's take a look at our image down below to get a better understanding of how cAMP and CRP positively control the expression of the lac operon. And it's positive control because it is increasing and turning on the lac operon. And so that's what we're focusing on in this image is positive control of the lac operon mainly by an active CRP protein. And so if we take a look at our image down below, notice that we have it broken up into 2 halves. We have the top half here and then we have the bottom half down below. We'll focus on the top half first, And what you'll notice is that we're showing you the lac operon over here, but this time notice that the lac operon has a new region that we have not yet introduced, and we're showing it here for the first time, and that is the CRP binding site. And so the CRP binding site is the site that is upstream of the promoter, that is going to be where the active CRP will bind. And so notice that, in the top half of this image, we have some specific conditions indicated by this box over here. And so notice here you can see that there's glucose here, And so notice here you can see that there's glucose here in the cell, these little, green hexagons, and you can also see that there is lactose within the cell. And we know already that lactose, a derivative of lactose, will bind to the LAC repressor, LAC I, and inactivate the LAC repressor. But also when glucose is present glucose is, going to lead to having low levels of cAMP. And low levels of cAMP means that the inactive, we're going to have an inactive CRP protein. And the inactive CRP protein will not bind to the CRP binding site. And so all of these conditions mean that the RNA polymerase, even though it is not being blocked by the inactive lac repressor, it will not be able to bind to the promoter very effectively without the active CRP here. And so the inactive CRP is not going to bind, and we need this active CRP to bind in order for RNA polymerase to also bind. And so what we're seeing here is that transcription of the lac operon is off in this state here. So the lac operon is in an off position here, under these conditions. And again, this makes sense because we know that glucose is the preferred energy source. And so if there are high levels of glucose, then that means that the cell is going to be using glucose as the energy source and it should not waste energy transcribing the lac operon because even though lactose is high, it is not going to be using lactose as the energy source. It is going to be using glucose as the energy source here. Now notice down below in this image, we have some slightly different conditions. Notice that the lactose concentration levels are still high just as they were before. Okay. So lactose levels are still high. But this time, notice that the glucose concentration levels are low. And again, we know that cAMP has an inverse relationship with glucose levels. And so if glucose levels are low, that means that cAMP levels are going to be high. And so you can see here in this image that cAMP, this little green molecule, its concentration is high. And when it is high, cAMP is going to bind to CRP. And when cAMP binds to CRP, it activates CRP, so we have an active CRP. And the active CRP will bind to this CRP binding site. And the active CRP bound to the CRP binding site is going to help to recruit the RNA polymerase so that the RNA polymerase will actually bind. And if the RNA polymerase is bound, then it can proceed forward with transcription and activate all of these genes. And so what we have in here is a lac operon that is on and transcription and gene expression are on not only does it require lactose levels to be high to inactivate the lac repressor, but it also requires glucose levels to be low, so that cAMP can activate CRP and CRP can bind and recruit RNA polymerase and RNA polymerase can stimulate transcription. And so this here concludes our brief introduction to positive control by cAMP and CRP, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.
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- Glucose's Impact on Lac Operon25m
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16. Regulation of Expression
Glucose's Impact on Lac Operon
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