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

Tryptophan (TRP) is an essential amino acid and a key component in protein synthesis. The trp operon, a repressible operon, contains five genes that encode enzymes for synthesizing tryptophan. In the presence of high tryptophan levels, it acts as a corepressor, activating the TRP repressor 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 illustrates the balance between nutrient availability and metabolic needs.

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 has a three-letter code abbreviation, TRP. It can be absorbed by the cell from the surrounding environment or synthesized from scratch by the cell.

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

Our image below shows the trp operon with these five genes: trpE, trpD, trpC, trpB, and trpA. These genes are crucial for encoding enzymes for synthesizing tryptophan from scratch. The trp operon has a trp operator where the regulatory protein will bind and a trp promoter where the RNA polymerase will bind. On the left-hand side, notice that there is a trp regulatory gene, trpR. TRP R is the regulatory gene encoding the trp repressor protein. The trp repressor protein will bind to the operator and inactivate transcription; it will repress transcription.

However, the trp repressor protein is actually expressed in the inactive form, which means it is inactive when initially expressed and will not bind to the operator. The inactive trpR repressor protein requires a corepressor to become active and bind to the operator to repress transcription. The corepressor for the inactive trpR is usually tryptophan itself. When tryptophan, this corepressor, is bound to the inactive trpR, it activates trpR, allowing it to bind to the operator and repress transcription.

If we take a closer look at our image, we can get a better understanding. The trp operon contains 5 genes required for synthesizing tryptophan, and it is regulated by the trp repressor. Over here, we have our trp regulatory gene, where you can see the trp repressor gene, the trpR gene, which has its own promoter. So it will be transcribed and translated into the repressor protein. However, when the trpR is expressed, it's expressed in the inactive form. Notice we have the inactive trp repressor, which on its own, cannot bind to the operator. First, the inactive trp repressor needs to be bound by a corepressor. The corepressor is usually tryptophan itself, so tryptophan can bind to the inactive trp repressor to form an active trp repressor. The active trp repressor with tryptophan bound can bind to the operator and repress and block transcription of the trp operon genes. Thus, the trp operon is repressed in the presence of tryptophan. When tryptophan is available and present, it serves as a corepressor to activate the trp repressor, which will then block transcription.

This concludes our brief introduction to the trp operon. As we move forward in our course, we'll take a closer look at the trp operon in the presence and absence of tryptophan. I'll see you all in our next video.

In the Presence of Tryptophan

In this video, we're going to talk about the tryptophan 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 tryptophan operon, which has genes for synthesizing its own tryptophan, is 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 co-repress 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 the TRP repressor, has its own promoter, it's going to be transcribed and translated into an inactive TRP repressor. 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 TRP repressor, and tryptophan is going to act as the co-repressor here. 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 co-repressor. And so this here concludes our brief introduction to what happens to the tryptophan operon in the presence of tryptophan. And so moving forward, we'll be able to talk about the tryptophan 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 TrpR repressor protein is going to remain inactive. And the reason that TrpR remains inactive is because it's not going to have tryptophan, because the tryptophan is really really low, to serve as a corepressor. And so if tryptophan cannot serve as a corepressor because its levels are really really low, then TrpR 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.

And so 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. And so 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 TrpR, this repressor protein. It has its own promoter. And of course, the TrpR 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 corepressor, which is tryptophan itself. But if tryptophan is absent, then there's no corepressor 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 cluster of genes in bacteria responsible for the synthesis of the amino acid tryptophan. It consists of five structural genes (trpE, trpD, trpC, trpB, and trpA) that encode enzymes necessary for tryptophan production. The operon is regulated by the trp repressor protein, which is encoded by the trpR gene. In the presence of high levels of tryptophan, tryptophan acts as a corepressor, binding to the trp repressor protein and activating it. The active repressor then binds to the operator region of the operon, blocking RNA polymerase from transcribing the genes. In low tryptophan conditions, the repressor remains inactive, allowing transcription to proceed and enabling the cell to synthesize tryptophan.

How does the trp operon respond to high levels of tryptophan?

When tryptophan levels are high, the trp operon is repressed to prevent the unnecessary synthesis of tryptophan. Tryptophan acts as a corepressor by binding to the trp repressor protein, activating it. The active trp repressor then binds to the operator region of the trp operon, blocking RNA polymerase from transcribing the operon's genes. This inhibition of transcription ensures that the cell does not waste energy and resources producing tryptophan when it is already abundant in the environment.

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

In the absence of tryptophan, the trp repressor protein remains inactive because it lacks the corepressor (tryptophan) needed for activation. As a result, the inactive repressor 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 required for synthesizing tryptophan. This mechanism ensures that the cell can produce tryptophan when it is not available in the environment.

What role does the trp repressor protein play in the regulation of the trp operon?

The trp repressor protein, encoded by the trpR gene, plays a crucial role in regulating the trp operon. In its inactive form, the repressor cannot bind to the operator region of the operon, allowing transcription to proceed. However, when tryptophan levels are high, tryptophan acts as a corepressor by binding to the trp repressor protein, activating it. The active repressor then binds to the operator, blocking RNA polymerase from transcribing the operon's genes. This regulation ensures that the cell only synthesizes tryptophan when it is needed.

How does feedback inhibition work in the trp operon?

Feedback inhibition in the trp operon involves the regulation of tryptophan synthesis based on its levels in the cell. When tryptophan is abundant, it acts as a corepressor by binding to the trp repressor protein, activating it. The active repressor binds to the operator region of the trp operon, preventing transcription of the genes responsible for tryptophan synthesis. This mechanism ensures that the cell does not produce excess tryptophan, conserving energy and resources. Conversely, when tryptophan levels are low, the repressor remains inactive, allowing transcription to proceed and enabling the cell to synthesize tryptophan.