Review of the Lac Operon & Trp Operon - Online Tutor, Practice Problems & Exam Prep

The lac operon is an inducible operon, typically off but can be activated by a lactose derivative, allolactose, which inactivates the lac I repressor. In contrast, the trp operon is repressible, usually on, and is inhibited by tryptophan, which activates the trp R repressor. The lac operon consists of three genes for lactose metabolism, while the trp operon has five genes for tryptophan synthesis. Understanding these operons illustrates key concepts in gene regulation and metabolic pathways.

Review of the Lac Operon & Trp Operon

In this video, we're going to review the lac and trp operons as we complete the table that you see down below. Notice in this table, this column right here is focusing on the lac operon and this column over here is focusing on the trp operon. In terms of the operon type, recall that the lac operon is an inducible operon, and that is because it is normally turned off, but it can be turned on. It can be induced. So, it's normally or initially off, but it can be induced. Now, the trp operon over here, recall, is a repressible operon, which means that it is normally turned on, but it can be turned off. It can be repressed, and so, that is why the trp operon is a repressible operon.

In terms of the number of genes, recall that the lac operon has 3 genes, which are lacZ, lacY, and lacA. The trp operon, in terms of the number of genes, has 5 genes, which are trpA, trpB, trpC, trpD, and trpE. In terms of the functions of the genes within the operon, the functions of the 3 genes within the lac operon are to metabolize lactose, or in other words, to break down lactose to get energy out of it. The functions of the 5 genes in the trp operon are to synthesize the amino acid tryptophan, to build tryptophan.

The repressor gene in the lac operon is lacI, and the repressor gene in the trp operon is trpR. The regulatory molecule in the lac operon is a derivative of lactose, also known as allolactose, and it acts as an inducer molecule to help induce transcription of the operon. The trp operon's regulatory molecule is tryptophan itself and acts as a corepressor to help activate the trpR repressor protein.

Discussing the effect of the regulatory molecule, in the lac operon, lactose binds to the active lacI repressor and inactivates it. So we have an effect where it inactivates the repressor protein. It starts off with an active repressor that will repress transcription, and then, upon the lactose derivative binding, it inactivates the repressor. For the trp operon, tryptophan actually activates the repressor protein. Notice that the repressor protein starts off in an inactive state, but upon tryptophan binding, the repressor ends up in an active state. These are basically opposite effects, and that's why one is an inducible operon and the other is a repressible operon.

When the regulatory molecule is absent, in the lac operon, lactose or the inducer being absent means that the lacI repressor is going to be in its active state. That means it will repress transcription, and the operon will be turned off in the absence of lactose. In the absence of the regulatory molecule in the trp operon, the absence of tryptophan means the repressor protein, trpR, is going to remain inactive, and that means it will not be able to repress transcription. The operon will be turned on without tryptophan.

Conversely, when the regulatory molecule is present, lactose's presence helps turn on the trp operon by inactivating the repressor. Transcription will be able to proceed, and the operon will be turned on. In the case of the trp operon, when tryptophan is present, it is going to activate the trpR repressor, allowing it to repress transcription, and so the operon will then be turned off. You can see here that the inducible lac operon and the trp operon are practically opposites in many of these different categories. This table here helps to provide a side by side comparison of these two operons. Having completed this table, we've completed our review of the lac and trp operons, and we'll be able to get some practice applying these concepts as we move forward. So, I'll see you all in our next video.

Which of the following statements is FALSE?

• A. The lac operon is an inducible operon that is normally turned off.
• B. The trp operon is a repressible operon that is normally turned on.
• C. Lactose is the inducer molecule for the lac operon.
• D. Tryptophan is the activator molecule for the trp operon.
• E. All of the above are true.

Which of the following statements is TRUE?

• A. Lac I is the inducer molecule for the lac operon.
• B. Tryptophan is an inducer molecule for the trp operon.
• C. In the presence of lactose, the lac operon is expressed.
• D. In the presence of tryptophan, the trp operon is expressed.

Here’s what students ask on this topic:

What is the main difference between the lac operon and the trp operon?

The main difference between the lac operon and the trp operon lies in their regulation mechanisms. The lac operon is an inducible operon, meaning it is typically off but can be turned on in the presence of a lactose derivative called allolactose. Allolactose inactivates the lac I repressor, allowing transcription to proceed. In contrast, the trp operon is a repressible operon, usually on, but can be turned off when tryptophan is present. Tryptophan activates the trp R repressor, which then inhibits transcription. This fundamental difference highlights how cells can regulate gene expression in response to environmental changes.

How does the presence of lactose affect the lac operon?

In the presence of lactose, a derivative called allolactose binds to the lac I repressor protein, inactivating it. This inactivation prevents the repressor from binding to the operator region of the lac operon, allowing RNA polymerase to access the promoter and initiate transcription of the operon's genes. As a result, the genes responsible for lactose metabolism (lacZ, lacY, and lacA) are expressed, enabling the cell to break down lactose for energy. Thus, lactose effectively induces the lac operon, turning it on.

What role does tryptophan play in the regulation of the trp operon?

Tryptophan acts as a corepressor in the regulation of the trp operon. When tryptophan levels are high, it binds to the trp R repressor protein, activating it. The activated repressor then binds to the operator region of the trp operon, blocking RNA polymerase from transcribing the operon's genes. This prevents the synthesis of tryptophan, conserving cellular resources. When tryptophan levels are low, the repressor remains inactive, allowing transcription to proceed and the operon to be turned on, leading to the synthesis of tryptophan.

What are the functions of the genes in the lac operon and the trp operon?

The lac operon consists of three genes: lacZ, lacY, and lacA. These genes are involved in lactose metabolism. LacZ encodes β-galactosidase, which breaks down lactose into glucose and galactose. LacY encodes permease, which facilitates the entry of lactose into the cell. LacA encodes transacetylase, which has a less clear role but is involved in the metabolism of lactose. The trp operon consists of five genes: trpE, trpD, trpC, trpB, and trpA. These genes encode enzymes that are involved in the biosynthesis of the amino acid tryptophan. Together, they convert chorismate to tryptophan through a series of enzymatic steps.

How does the absence of regulatory molecules affect the lac operon and the trp operon?

In the absence of the regulatory molecule allolactose, the lac I repressor remains active and binds to the operator region of the lac operon, preventing transcription and keeping the operon off. Conversely, in the absence of tryptophan, the trp R repressor remains inactive and cannot bind to the operator region of the trp operon. This allows RNA polymerase to transcribe the operon's genes, keeping the operon on. Thus, the absence of regulatory molecules results in the lac operon being off and the trp operon being on.