Review of the Lac Operon & Trp Operon: Videos & Practice Problems

The lac operon is an inducible operon, typically off but can be activated by a lactose derivative, allolactose, which inactivates the lac I repressor. It consists of three genes (lac z, lac y, lac a) that metabolize lactose. In contrast, the trp operon is repressible, usually active but can be turned off by tryptophan, which activates the trp r repressor. It contains five genes (trp a, b, c, d, e) responsible for synthesizing tryptophan. These operons exemplify regulatory mechanisms in gene expression, highlighting the balance between activation and repression in metabolic pathways.

Review of the Lac Operon & Trp Operon

The lac operon and the trp operon are two essential models for understanding gene regulation in prokaryotes, specifically in Escherichia coli. The lac operon is classified as an inducible operon, meaning it is typically in an off state but can be activated in the presence of an inducer. In contrast, the trp operon is a repressible operon, which is usually active but can be turned off when a corepressor is present.

In terms of gene composition, the lac operon consists of three genes: lacZ, lacY, and lacA, which are involved in the metabolism of lactose, allowing the organism to utilize lactose as an energy source. The trp operon, on the other hand, contains five genes: trpA, trpB, trpC, trpD, and trpE, which are responsible for the biosynthesis of the amino acid tryptophan.

The regulation of these operons involves specific repressor genes: lacI for the lac operon and trpR for the trp operon. In the lac operon, the presence of a lactose derivative known as allolactose serves as an inducer that binds to the active lacI repressor, inactivating it and allowing transcription to proceed. Conversely, in the trp operon, tryptophan acts as a corepressor that binds to the inactive trpR repressor, activating it and leading to the repression of transcription.

When the regulatory molecules are absent, the lac operon remains off due to the active lacI repressor, while the trp operon remains on because the inactive trpR repressor cannot inhibit transcription. However, when lactose is present, it inactivates the repressor in the lac operon, allowing transcription to occur. In contrast, when tryptophan is present, it activates the repressor in the trp operon, resulting in the repression of transcription.

This comparison highlights the contrasting mechanisms of gene regulation in the lac and trp operons, illustrating how environmental conditions can influence gene expression in bacteria. Understanding these operons provides insight into fundamental biological processes and the intricate control of metabolic pathways.

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.

Answer: (D)

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.

Answer: (C)

Here’s what students ask on this topic:

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

The lac operon is an inducible operon, meaning it is typically off but can be turned on by a lactose derivative called allolactose. It consists of three genes (lacZ, lacY, lacA) that metabolize lactose. In contrast, the trp operon is a repressible operon, usually active but can be turned off by tryptophan, which activates the trpR repressor. It contains five genes (trpA, trpB, trpC, trpD, trpE) responsible for synthesizing tryptophan. These operons exemplify regulatory mechanisms in gene expression, highlighting the balance between activation and repression in metabolic pathways.

How does allolactose regulate the lac operon?

Allolactose, a derivative of lactose, acts as an inducer molecule for the lac operon. When lactose is present, it is converted into allolactose, which then binds to the lacI repressor protein. This binding inactivates the repressor, preventing it from binding to the operator region of the operon. As a result, RNA polymerase can bind to the promoter and initiate transcription of the lac operon genes (lacZ, lacY, lacA), leading to the metabolism of lactose.

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

The trp operon contains five genes: trpA, trpB, trpC, trpD, and trpE. These genes encode enzymes that are involved in the biosynthesis of the amino acid tryptophan. Specifically, these enzymes catalyze the sequential steps in the tryptophan synthesis pathway, converting chorismate to tryptophan. This operon is typically active, but when tryptophan levels are high, tryptophan acts as a corepressor, activating the trpR repressor protein, which then binds to the operator region to inhibit transcription.

What happens to the lac operon in the absence of lactose?

In the absence of lactose, the lacI repressor protein remains active and binds to the operator region of the lac operon. This binding prevents RNA polymerase from attaching to the promoter and initiating transcription of the operon’s genes (lacZ, lacY, lacA). As a result, the lac operon remains off, and the genes responsible for lactose metabolism are not expressed.

How does tryptophan regulate the trp operon?

Tryptophan acts as a corepressor for the trp operon. When tryptophan levels are high, it binds to the trpR 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 (trpA, trpB, trpC, trpD, trpE). This prevents the synthesis of tryptophan, thereby conserving cellular resources.