Glucose's Impact on Lac Operon - Online Tutor, Practice Problems & Exam Prep

Glucose is the preferred energy source for prokaryotes, inhibiting the lac operon when present. Low glucose levels increase cyclic AMP (cAMP), which activates cyclic AMP receptor protein (CRP), enhancing lac operon transcription. This positive control mechanism allows cells to utilize lactose as a secondary energy source when glucose is scarce. The relationship between glucose and cAMP is inversely proportional, influencing the operon's activity based on nutrient availability, thus regulating energy metabolism efficiently.

Glucose Levels, cAMP, & the Lac Operon

In this video, we're going to begin our lesson on glucose's impact on the lac operon. We'll focus on glucose levels, a molecule called cAMP, and how they affect the LAC operon. It's important to note that in most prokaryotes, glucose is the preferred energy source, even in the presence of lactose. This means that if glucose is available to cells, then the cells will use glucose as their primary energy source, not lactose, and therefore the lac operon should be turned off in the presence of glucose. It turns out that glucose levels are linked to cellular levels of a molecule 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 low or when glucose is absent and not available for metabolism, then cellular levels of this molecule called cAMP are going to increase. High cellular cAMP levels will also actually increase the rate of transcription of the lac operon. Essentially, when glucose is low or absent, cAMP levels increase, but if glucose is available, then the lac operon will be turned off. 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.

cAMP levels do not affect the repressor protein's activity. They only increase transcription of the lac operon when glucose is absent. As we move forward in our course, we'll be able to talk about exactly how cAMP affects the lac operon's transcription. Let's take a look at this image down below. In this example, glucose levels control cAMP levels in the cell, and the cAMP levels in the cell are going to control the rate of the lac operon transcription. On the left hand side, we're showing a cell with a high glucose concentration, represented by a little green hexagon, and a low lactose concentration. This translates to having low cAMP within the cell. The cAMP molecule is represented by a little green circle, and the cAMP levels inside the cell are relatively low when glucose concentrations are high. There's an inverse relationship between glucose concentrations and cAMP concentrations. When one is high, the other is low. The low cAMP is actually going to translate to having low lac operon transcription. The lac operon will not be transcribed at a high rate, and again, this allows glucose to be used as the primary energy source.

However, on the right hand side, we have a different scenario. 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. The low glucose concentration is going to have an inverse relationship with the amount of cAMP, leading to high levels of cAMP within the cell. All of these little green circles represent the cAMP molecules. There are a lot of cAMP on this side as compared to the other side. The high cAMP is going to translate to having high lac operon transcription. This allows lactose to be used as an energy source when glucose is not available. 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. As we move forward in our course, we're going to talk more about exactly how cAMP increases the transcription of the lac operon, and we'll be able to learn more as we progress. I'll see you all in our next video.

Glucose's Impact on Lac Operon Example 1

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, cAMP, and lactose concentrations. 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. We'll focus on one row at a time here, filling out all of the blanks that we see throughout. 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. Recall that glucose has an inverse relationship with cAMP levels. If glucose concentrations are high, then cAMP levels are going to be low. Now, of course, if lactose concentration levels in 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. 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. Notice that glucose levels are pretty high here, and so glucose is available for the cell. This means that it will be using glucose as the primary source even though lactose is available. So the lac operon is going to be off here, or we can say that no the lac operon will not be expressed.

Looking over here we can see again that glucose concentrations are going to be high. This time lactose levels are going to be low. Having high environmental glucose levels is going to lead to having high cellular glucose levels. Recall that cAMP has an inverse relationship with glucose, and so if glucose is high then cAMP levels are going to be low. Having low lactose environmental levels is going to translate to having low cellular lactose. Under these conditions, the lac operon is only going to be expressed when those genes are needed to break down lactose. 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. 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 third 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. Let's skip that row and we'll come back to it here shortly. Let's focus on the bottom row here. Notice here that glucose concentrations are low in the environment, and so that's going to translate to glucose cellular levels also being low. We know that cAMP has an inverse relationship with glucose, and so if glucose levels are low cAMP levels are going to be high. Having low lactose in the environment is also going to translate to having low lactose intracellular levels. Should the lac operon be expressed or should it not be expressed? 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. Because lactose is low here, the lac operon should not be expressed. Once again, we have a scenario that is no.

Now we focus on this third column here that's highlighted in yellow. What you'll notice is that glucose environmental levels are low, but lactose environmental levels are high. Having low environmental glucose levels is going to translate to having low cellular glucose levels. cAMP once again has an inverse relationship with glucose levels. If glucose levels are low, cAMP levels are going to be high. If lactose is high in the environment, then lactose is also going to be relatively high in the cellular levels, and so we can say high here as well. 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. These are the conditions that are needed to allow for the lac operon to be expressed. Notice that the high cellular cAMP levels are going to lead to the lac operon being expressed. We're going to talk about exactly how cAMP impacts the lac operon in our next lesson video. This here concludes this example problem completing this entire table down below. We'll be able to get some more practice as we move forward. So, I'll see you all in our next video.

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.

Positive Control by cAMP & CRP

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 is going to translate to high cellular cAMP levels. And high cellular cAMP levels is going to allow cAMP to bind and activate CRP. And an active CRP is going to act as an activator protein and 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 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 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 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's not going to be using lactose as the energy source. It's 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 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 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.

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.

Here’s what students ask on this topic:

How does glucose affect the lac operon in prokaryotes?

In prokaryotes, glucose is the preferred energy source. When glucose is present, it inhibits the lac operon, preventing the transcription of genes involved in lactose metabolism. This is because high glucose levels lead to low levels of cyclic AMP (cAMP). Low cAMP levels mean that the cyclic AMP receptor protein (CRP) remains inactive. Without active CRP, RNA polymerase cannot effectively bind to the promoter region of the lac operon, thus reducing transcription. This regulatory mechanism ensures that the cell prioritizes glucose metabolism over lactose metabolism when both sugars are available.

What role does cAMP play in the regulation of the lac operon?

cAMP (cyclic AMP) plays a crucial role in the positive regulation of the lac operon. When glucose levels are low, cAMP levels increase. High cAMP levels allow cAMP to bind to the cyclic AMP receptor protein (CRP), activating it. The active CRP then binds to a specific site upstream of the lac operon promoter. This binding helps recruit RNA polymerase to the promoter, enhancing the transcription of the lac operon. Thus, cAMP acts as a signal that indicates low glucose levels, enabling the cell to switch to lactose metabolism by increasing the transcription of the lac operon genes.

Why is glucose considered the preferred energy source over lactose in prokaryotes?

Glucose is considered the preferred energy source over lactose in prokaryotes because it can be metabolized more efficiently and quickly. The metabolic pathways for glucose are more direct and require fewer enzymatic steps compared to lactose metabolism. Additionally, the presence of glucose inhibits the lac operon, preventing the unnecessary production of enzymes required for lactose metabolism. This regulatory mechanism ensures that the cell conserves energy and resources by using the most readily available and easily metabolizable sugar first, which is glucose.

How does the CRP-cAMP complex influence lac operon transcription?

The CRP-cAMP complex plays a vital role in enhancing lac operon transcription. When glucose levels are low, cAMP levels rise, leading to the formation of the CRP-cAMP complex. This complex binds to a specific site upstream of the lac operon promoter. The binding of the CRP-cAMP complex facilitates the recruitment of RNA polymerase to the promoter region, thereby increasing the rate of transcription of the lac operon. This positive control mechanism ensures that the genes required for lactose metabolism are transcribed only when glucose is scarce, allowing the cell to utilize lactose as an alternative energy source.

What is the relationship between glucose levels and cAMP levels in prokaryotic cells?

In prokaryotic cells, there is an inverse relationship between glucose levels and cAMP levels. When glucose levels are high, cAMP levels are low. Conversely, when glucose levels are low, cAMP levels increase. This inverse relationship is crucial for the regulation of the lac operon. High glucose levels result in low cAMP, leading to an inactive CRP and reduced transcription of the lac operon. Low glucose levels result in high cAMP, which activates CRP, enhancing the transcription of the lac operon. This regulatory mechanism ensures efficient energy metabolism based on nutrient availability.