The Trp Operon - Online Tutor, Practice Problems & Exam Prep

Tryptophan (TRP) is an essential amino acid and a key component of the trp operon, which is a repressible operon containing five genes that encode enzymes for synthesizing tryptophan. In the presence of high tryptophan levels, it acts as a corepressor, activating the TRP repressor protein to inhibit transcription. Conversely, low tryptophan levels keep the TRP repressor inactive, allowing transcription to proceed and enabling the cell to synthesize its own tryptophan. This regulatory mechanism is crucial for maintaining amino acid homeostasis in cells.

The Trp Operon

In this video, we're going to begin our lesson on tryptophan. Tryptophan is an amino acid. Recall that amino acids are the monomers or the building blocks of proteins. Tryptophan is an amino acid whose three-letter code or abbreviation is TRP. Tryptophan can either be absorbed by the cell from the surrounding environment or synthesized from scratch by the cell.

The TRP operon is a repressible operon, which, as we recall from our previous lesson videos means that the TRP operon is usually on or active, but it can be turned off, meaning that it can be repressed, and that's why it's called a repressible operon. The TRP operon is a repressible operon with 5 genes, and these 5 genes are all related to each other because they encode enzymes required for synthesizing tryptophan from scratch.

Now, if we take a look at our image down below, notice that we're showing you an image of the TRP operon, which has these five genes, TRP E, D, C, B, and A. These genes are important for encoding enzymes for synthesizing tryptophan from scratch. The TRP operon has a TRP operator where the regulatory protein will bind, and it also has a TRP promoter as well where the RNA polymerase will bind.

On the left-hand side, you will see a TRP regulatory gene, which is TRP R. TRP R is the regulatory gene that will encode the TRP repressor protein. The TRP repressor protein is a repressor, meaning that it will bind to the operator and inactivate transcription. It will repress transcription. However, the TRP repressor protein is expressed in the inactive form, which means that when it is initially expressed, it is inactive and will not bind to the operator. The inactive TRP R repressor protein requires a corepressor in order for it to become active and bind to the operator to repress transcription. The corepressor for the inactive TRP R is usually going to be tryptophan itself.

When tryptophan, this corepressor, is bound to the inactive TRP R, it activates TRP R and allows TRP R to bind to the operator. When it is bound to the operator, it can repress transcription. So, the TRP operon contains 5 genes required for synthesizing tryptophan and is regulated by the TRP repressor. When tryptophan is available and present, it can serve as a corepressor to activate the TRP repressor, and that will block transcription. This here concludes our brief introduction to the TRP operon. As we move forward in our course, we will take a closer look at the TRP operon in the presence and in the absence of tryptophan itself. I'll see you all in our next video.

In the Presence of Tryptophan

In this video, we're going to talk about the operon specifically in the presence of tryptophan. And so when tryptophan, this amino acid, is really abundant and prevalent and found in high levels in the environment, then the cell does not really have a need to synthesize its own tryptophan. And so the operon, which has genes for synthesizing its own tryptophan, it's going to be inactive when there is plenty of tryptophan that is abundant in the environment. And so what happens is tryptophan is going to act as a co-repressor to the repressor protein. And so the co-repressor, tryptophan, is going to bind to and activate the TRP repressor protein. And so if we take a look at our image down below, we can see that cellular tryptophan is going to corepress the trp operon when it's readily abundant for the cell. And so we're specifically looking at the trp operon in the presence of tryptophan. And so when tryptophan is really abundant and prevalent, then the cell has no need to synthesize its own tryptophan and so it needs to shut down the TRP operon. And so this is how it works. The TRP regulatory gene, which encodes TRP R, which is our suppressor protein, which has its own promoter, it's going to be transcribed and translated into an inactive TRP repressor. And the inactive TRP repressor is only going to be activated in the presence of tryptophan. And so when tryptophan is really, really abundant, then there's going to be tryptophan available to bind to the inactive tryptophan is going to act as the co-repressor here. So the tryptophan is acting as the co-repressor. And so the tryptophan co-repressor, and, that is going to form an active TRP repressor. And notice the active TRP repressor is capable of binding to the TRP operator, and when it's bound to the TRP operator, it's going to block and inhibit transcription, turning off the TRP operon. And so it's going to prevent the RNA polymerase from binding to the promoter and proceeding with transcription. And so, basically, what we're seeing here is, again, when the TRP operon is in the presence of tryptophan, when tryptophan is readily available, then the cell has no need to create its own tryptophan, and so it's going to shut down the trp operon through this mechanism where tryptophan acts as a corepressor. And so this here concludes our brief introduction to what happens to the operon in the presence of tryptophan. And so moving forward, we'll be able to talk about the operon in the absence of tryptophan. So I'll see you all in that video.

In the Absence of Tryptophan

In this video, we're going to continue to talk about the trp operon, but specifically in the absence of tryptophan. And so in the absence of tryptophan, when tryptophan levels are really, really low and tryptophan is not readily available in the environment for the cell to absorb, then the cell must be able to synthesize or build its own tryptophan. And so, of course, the cell is going to synthesize its own tryptophan by using the enzymes from the trp operon. And so in the absence of tryptophan, when tryptophan levels are really, really low and tryptophan is not readily available in the environment, then the trp operon is going to be active and turned on so that the cell can make its own tryptophan. And so when cellular tryptophan levels are really, really, really low, what happens is the Trp repressor protein is going to remain inactive. And the reason that TRP repressor remains inactive is because it's not going to have tryptophan, because the tryptophan is really really low, to serve as a co-repressor. And so if tryptophan cannot serve as a co-repressor because its levels are really really low, then Trp repressor is going to remain in its inactive state. That means that it will not be able to bind to the operator and block transcription. And so if it's not able to block transcription, that is going to allow for transcription to proceed and that's going to turn on the trp operon and allow the cell to synthesize its own tryptophan. Let's take a look at this image down below to get a better understanding of how low cellular tryptophan levels results in increased transcription of the trp operon. We're focusing specifically on the trp operon in the absence of tryptophan when no tryptophan is available. And so notice that in this image, again, we've got our TRP operon over here on the right, and over here we have our TRP regulatory gene, which encodes for TRP repressor, this repressor protein. It has its own promoter. And of course, the TRP repressor is going to be transcribed and translated, but when it's originally transcribed and translated, it's going to form the inactive Trp repressor. And the inactive Trp repressor is not capable of binding to the trp operator without the co-repressor, which is tryptophan itself. But if tryptophan is absent, then there's no co-repressor to bind to the Trp repressor. And so that means that the TRP repressor is going to remain inactive. And because it remains inactive, this allows for RNA polymerase to bind to the TRP promoter and initiate transcription, transcribing these genes here into the messenger RNA, and then the messenger RNA will be translated into the proteins. And, again, these proteins, these enzymes, are going to help create tryptophan. And so the cell will be able to make its own tryptophan when tryptophan is absent from the environment. This here concludes our brief introduction to what happens to the trp operon in the absence of tryptophan, and we'll be able to get some practice applying the concepts that we've learned here as we move forward in our course. So I'll see you all in our next video.

The trp operon consists of ________ genes that encode tryptophan biosynthesis enzymes.

• A. One.
• B. Two.
• C. Three.
• D. Four.
• E. Five.

Under what conditions does the trp repressor block transcription of the trp operon?

• A. When the repressor binds to the inducer.
• B. When the repressor binds to tryptophan.
• C. When the repressor is not bound to tryptophan.
• D. When the repressor is not bound to the operator.

If the trp regulatory gene mutates so that the repressor protein can no longer bind to tryptophan what will be the result?

• A. The trp operon will not be expressed.
• B. The trp operon will be continuously expressed.
• C. The trp operon will be expressed in the presence of tryptophan only.
• D. The trp operon will be expressed in the absence of tryptophan only.
• E. There will be no effect on the trp operon.

In the absence of tryptophan, ______:

• A. The inducer cannot bind to the operator, so trp operon transcription occurs.
• B. The active repressor cannot bind to the operator, so trp operon transcription is reduced.
• C. The inactive repressor cannot bind to the operator, so trp operon transcription occurs.
• D. The repressor binds to the corepressor, and trp operon transcription occurs.
• E. The active repressor binds to the operator, so trp operon transcription is repressed.

Based on the information you know about the trp operon, is the creation of tryptophan expensive to the cell?

• A. Yes, this is why tryptophan is the co-repressor of the trp operon.
• B. No, this is why tryptophan is the inducer of the trp operon.
• C. Yes, this is why tryptophan is the repressor of the trp operon.

Here’s what students ask on this topic:

What is the trp operon and how does it function?

The trp operon is a repressible operon in bacteria that contains five genes (trpE, trpD, trpC, trpB, and trpA) responsible for synthesizing the amino acid tryptophan. It functions by regulating the production of tryptophan based on its availability in the environment. When tryptophan levels are high, tryptophan acts as a corepressor, binding to the inactive trp repressor protein (TrpR) and activating it. The active repressor then binds to the operator region of the trp operon, blocking RNA polymerase from transcribing the genes. Conversely, when tryptophan levels are low, the repressor remains inactive, allowing transcription to proceed and enabling the cell to synthesize its own tryptophan.

How does tryptophan act as a corepressor in the trp operon?

Tryptophan acts as a corepressor in the trp operon by binding to the inactive trp repressor protein (TrpR). When tryptophan levels are high, tryptophan molecules bind to the TrpR protein, causing a conformational change that activates the repressor. The active repressor then binds to the operator region of the trp operon, preventing RNA polymerase from transcribing the operon's genes. This inhibition of transcription stops the synthesis of tryptophan, as the cell no longer needs to produce it when it is abundant in the environment.

What happens to the trp operon in the absence of tryptophan?

In the absence of tryptophan, the trp repressor protein (TrpR) remains inactive because there is no tryptophan to act as a corepressor. Without the corepressor, the inactive TrpR cannot bind to the operator region of the trp operon. This allows RNA polymerase to bind to the promoter and transcribe the operon's genes, leading to the production of enzymes that synthesize tryptophan. This mechanism ensures that the cell can produce its own tryptophan when it is not available in the environment.

What are the five genes in the trp operon and their functions?

The trp operon contains five genes: trpE, trpD, trpC, trpB, and trpA. These genes encode enzymes that are involved in the biosynthesis of tryptophan. Specifically, TrpE and TrpD form the anthranilate synthase enzyme, which catalyzes the first step in the tryptophan biosynthesis pathway. TrpC encodes the enzyme indole-3-glycerol phosphate synthase, which is involved in the intermediate steps. TrpB and TrpA encode the subunits of tryptophan synthase, the enzyme that catalyzes the final step in the synthesis of tryptophan. Together, these enzymes enable the cell to produce tryptophan from precursor molecules.

How does the trp operon maintain amino acid homeostasis in cells?

The trp operon maintains amino acid homeostasis by regulating the synthesis of tryptophan based on its availability. When tryptophan levels are high, it acts as a corepressor, activating the trp repressor protein (TrpR) to inhibit transcription of the operon's genes, thus stopping tryptophan synthesis. Conversely, when tryptophan levels are low, the repressor remains inactive, allowing transcription to proceed and enabling the cell to synthesize its own tryptophan. This feedback mechanism ensures that the cell produces tryptophan only when it is needed, preventing wasteful overproduction and maintaining a balance of amino acids.