Glucose's Impact on Lac Operon - Video Tutorials & Practice Problems
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1
concept
GlucoseLevels, cAMP, & the Lac Operon
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5m
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Video transcript
In this video, we're going to begin our lesson on glucose's impact on the lac operon. And so we're going to focus on glucose levels, a molecule called cAMP, and how they affect the LAC operon. And so what's important to note is that in most prokaryotes, glucose is actually the preferred energy source even in the presence of lactose. And so what this means is that if glucose is available to cells, then the cells are going to be using glucose as their primary energy source, not lactose, and therefore the lac operon should be turned off in the presence of glucose. Now, it turns out that glucose levels are linked to cellular levels of a molecule that's called cyclic AMP or just cAMP for short, through a process that we're not going to get into in this video. However, what you should know is that when glucose concentrations are really really low or when glucose is just absent and glucose is not available for metabolism, then cellular levels of this molecule called c amp are going to increase. And so high cellular cAMP levels will also actually increase the rate of transcription of the lac operon. And so basically what we're seeing here is when glucose is low or absent cAMP levels increase absent. But if glucose is available then the lac operon will be turned off. And so this allows glucose to be used as the preferred and the primary energy source. And lactose will only be used as a secondary energy source when glucose is not available. Now cAMP, levels, they actually do not affect the repressor proteins activity. So they do not affect black eyes activity. They only increase transcription of the lac operon when glucose is absent. And as we move forward in our course, we'll be we'll be able to talk about exactly how it is that cAMP affects the lac operon's transcription. But for now, let's take a look at this image down below. And so what it's saying here in this example is that glucose levels control cAMP levels in the cell. And the cAMP levels in the cell are gonna control the rate of the lac operon transcription. And so over here on the left hand side, notice that we have a scenario. We're showing you a cell here. So here is our cell, and notice that there is a high glucose concentration, this little green hexagon represents glucose, and there is low lactose concentration. And so notice that on the inside of our cell because there is high glucose that actually translates to having low cAMP within the cell. And so notice that within our cell the cAMP molecule, which is represented by this little green circle that we see here, The the cAMP levels inside the cell are relatively low when glucose concentrations are high. So there's an inverse relationship there between glucose concentrations and cAMP concentrations. When one is high, the other is low. And the low c amp is actually going to translate to having low lac operon transcription. And so the lac operon will not be transcribed at a high rate. It will be a low lac operon transcription when glucose is high. And again, this allows glucose to be used as the primary energy source. However, over here on the right hand side, notice that we have a different scenario. Again, we still have our cell here in the middle, but notice that this time there's a low glucose concentration and a high lactose concentration. And so the low glucose concentration is going to have an inverse relationship with the amount of cAMP. And so there's going to be a high levels of cAMP within the cell. And so you can see that within our cell down below here. All of these little green circles that you see are representing the cAMP molecules. And there are a lot of cAMP on over here on this side as compared to over here on this side. And so the high c amp is going to translate to having high lactose operon, lac operon transcription. And so, what this allows for is the lactose is going to be used as an energy source when glucose is not available. And so again, this level of regulation allows glucose to be the primary energy source and lactose to be used only as a secondary energy source when glucose is not available. Now as we move forward in our course, we're going to talk more details about exactly how is it that cAMP increases the transcription of the oper operon, and we'll be able to learn more as we move forward in our course. So I'll see you all in our next video.
2
example
Glucose's Impact on Lac Operon Example 1
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6m
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So here we have an example problem that wants us to complete the table that's down below right here, and notice that this table is showing us the environmental levels of glucose and lactose concentrations, cellular levels of glucose c amp and lactose concentrations, And then in this last column over here, it wants us to determine if the lac operon is being expressed or not based on the indicated conditions. And so we'll focus on one row at a time here, filling out all of the blanks that we see throughout. And so notice that the first row here has glucose environmental levels being high and lactose environmental levels being high. So of course cellular levels of glucose are also going to be relatively high when environmental levels are high as well. And recall that glucose has an inverse relationship with c amp levels. And so if glucose concentrations are high, then c amp levels are going to be low. Now of course if lactose concentration levels on the environment, the outside environment are high, that's also going to translate to lactose levels being high within the cell as well. Cellular levels of Lactose will also be high. And so, recall that the lac operon is only going to be expressed when glucose is not available because glucose is going to be the preferred energy source, and the lac operon should be off when glucose is available. And so notice that glucose levels are pretty high here, and so glucose is is available for the cell. And so that means that it will be using glucose as the primary source even though lactose is available. And so the lac operon is going to be off here, or we can say that no the lac operon will not be expressed. Now looking over here we can see again that glucose concentrations are going to be high. This time lactose levels are going to be low. And so of course having a high environmental glucose levels is going to lead to having high cellular glucose levels. And so here we can put in high. And of course, recall that cAMP has an inverse relationship with glucose, and so if glucose is high then cAMP levels are going to be low. And, of course, having a low lactose, environmental levels is going to translate to having low cellular lactose. And again, under these conditions, the lac operon is only going to be expressed when those genes are needed to break down lactose. And, again, because glucose is the preferred source, we know that the lac operon is not going to be expressed in the presence of glucose. It's also not going to be expressed in the absence of lactose. And so once again, here we have some conditions that say the lac operon will not be on. It will be off under these conditions. Now next, we have the 3rd row here, which is highlighted in yellow, but, we're actually going to skip this row and go to the last row here before we actually go to the yellow row. So let's skip, that row and we'll come back to it here shortly. Let's focus on the bottom row here. And so notice here that glucose concentrations are low on the environment, and so that's gonna translate to glucose cellular levels also being low. And we know that cAMP has an inverse relationship with glucose, and so if glucose levels are low cAMP levels are going to be high. Now of course having low lactose in the environment is also going to translate to having low lactose intracellular levels or cellular levels. And so, should the lac operon be expressed or should it not be expressed? Well, notice that glucose levels are low, so glucose is not going to be available as the primary energy source. But also notice that lactose levels are also low, and so the lac operon should only be turned on if lactose is available as well. And so because lactose is low here, the lac operon should not be expressed. And so once again, we have a scenario that is no. And so now we focus on this third column here that's highlighted in yellow. And what you'll notice is that glucose environmental levels are low, but lactose environmental levels are high. Now having low environmental glucose levels is gonna translate to having low cellular glucose levels. And cAMP once again has an inverse relationship with glucose levels. And so if glucose levels are low, cAMP levels are going to be high. And, again, if lactose is high on the environment, then lactose is also going to be relatively high in the cellular levels, and so we can say high here as well. And so under these conditions, should the lac operon be expressed? And here we can say, yes. The lac operon should be expressed under these conditions because glucose is not available to be the primary energy source and lactose is available. And And so these are the conditions that are needed to allow for, lac operon to be expressed. And also notice that the high cellular c amp levels are going to lead to the lac operon being expressed. And, again, we're going to talk about exactly how cAMP impacts the lac operon in our next lesson video. But for now this here concludes this example problem completing this entire table down below. And so, we'll be able to get some more practice as we move forward. So I'll see you all in our next video.
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Problem
Problem
How does extracellular glucose inhibit transcription of the lac operon?
A
By strengthening the binding of the repressor to the operator.
B
By weakening the binding of the repressor to the operator.
C
By inhibiting RNA polymerase from opening the strands of DNA to initiate transcription.
D
By reducing the levels of intracellular cAMP.
4
concept
Positive Control by cAMP & CRP
Video duration:
8m
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Video transcript
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 c AMP. And so recall from our previous lesson videos that low glucose levels is going to translate to high cellular c amp levels. And high cellular c amp levels is going to allow c amp to bind and activate CRP. And an active CRP is going to act as an activator protein, and it's 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 leads to high cAMP levels, high cAMP levels leads 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 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 that 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 means that the RNA polymerase, even though it's not being, blocked by the inactive lac repressor, it will not be able to bind to the promoter very effectively without the, the active CRP here. And so, the inactive CRP, is not going to bind, and we need this act this 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 gonna be using glucose as the energy source and it should not waste energy transcribing the lac operon because even though lactose is high, it's not going to be using lactose as the energy source. It's gonna 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 c amp, 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 is 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.
5
Problem
Problem
When glucose is present:
A
cAMP is high, CRP binds to the activator binding site, and transcription of the lac operon is turned off.
B
cAMP is low, CRP binds to the site activator binding site, and transcription of the lac operon is turned on.
C
cAMP is high, CRP does not bind to the activator binding site, and transcription of the lac operon is turned on.
D
cAMP is low, CRP does not bind to the activator binding site, and transcription of the lac operon is turned off.