Gene regulation in bacteria - Regulation of the trp operon

14 important questions on Gene regulation in bacteria - Regulation of the trp operon

How is the trp operon regulated?

- By a repressor protein;
- By a mechanism called attenuation.

Which kind of sequences does the trp operon consist of?

- trpR and trpL, which play a role in the regulation of the trp operon;
- promoter;
- operator;
- genes: trpE, trpD, trpC, trpB, and trpA.

What happens to the repressor when the tryptophan levels within the cell are very low?

- trp repressor cannot bind to the operator site;
- RNA polymerase transcribes the trp operon;
- the cell expresses the genes in the trp operon.
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What happens to the repressor when the tryptophan levels within the cell are high?

- tryptophan acts as a corepressor and binds to trp repressor;
- a conformational change in the repressor occurs;
- repressor binds to operator site;
- the ability of RNA polymerase to transcribe the trp operon is inhibited.

What is the concept of attenuation?

Transcription begins, but is turned off before the entire mRNA is made.

Where is the transcription stopped during attenuation?

At the attenuator sequence, which is downstream from the trpL gene, but upstream from the other genes. So none of the proteins required for tryptophan synthesis will be encoded.

What are the two key features of attenuation?

- Two tryptophan codons are found within the mRNA that is made from the trpL gene;
- The mRNA made from the trpL gene has four regions that are complementary to each other, so that the formation of stem loops is possible.

To what does a stem-loop in regions 3 and 4 lead?

It causes RNA polymerase to pause, and the U-rich region dissociates from the DNA (intrinsic or p-independent termination).

In which kind of operons does attenuation often occur?

In operons involved with amino acid biosynthesis.

What kind of operons are lac operon and trp operon with regard to how they are regulated?

Lac operon is an inducible operon and trp operon is a repressible operon.

What kind of genes are often regulated in an inducible manner?

Genes that encode for proteins that function in the catabolism (breakdown) of a substance. The substance to be broken down often acts as an inducer.

What kind of genes are often regulated in a repressible manner?

Genes that encode anabolic enzymes. The corepressor is commonly the molecule that is the product of the enzymes' biosynthetic activities.

Why is gene regulation in genes that code for proteins involved in the breakdown of substances useful?

The appropriate genes are only expressed when they are needed.

Why is gene regulation in genes that encode for anabolic enzymes useful?

To prevent the overproduction of the product.

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