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Ch. 17 - Transcription, RNA Processing, and Translation
Freeman - Biological Science 8th Edition
Freeman8th EditionBiological ScienceISBN: 9780138276263Not the one you use?Change textbook
All textbooksFreeman 8th EditionCh. 17 - Transcription, RNA Processing, and TranslationProblem 12
Chapter 17, Problem 12

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Eating even a single death cap mushroom (Amanita phalloides) can be fatal due to a compound called α-amanitin, a toxin that inhibits transcription.
α-Amanitin inhibits transcription by binding inside an RNA polymerase to a region other than the active site that catalyzes addition of a nucleotide to the RNA chain. Based on the model of RNA polymerase shown in Figure 17.3, predict how the toxin might function to inhibit transcription.

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1
Understand the role of RNA polymerase in transcription. RNA polymerase is the enzyme responsible for synthesizing RNA from a DNA template.
Identify the function of α-Amanitin. α-Amanitin is a toxin that inhibits RNA polymerase by binding to it.
Note that α-Amanitin binds to a region other than the active site. This suggests that the toxin affects the enzyme's function through an allosteric mechanism.
Predict how binding to a non-active site region could inhibit transcription. Binding of α-Amanitin likely induces a conformational change in RNA polymerase, which could hinder its ability to bind to DNA or add nucleotides to the growing RNA chain.
Conclude that α-Amanitin inhibits transcription by altering the structure of RNA polymerase, thereby preventing it from properly performing its function.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Transcription

Transcription is the process by which genetic information from DNA is copied into RNA. This process is essential for gene expression, allowing cells to produce proteins based on the instructions encoded in their DNA. During transcription, RNA polymerase binds to the DNA and synthesizes a complementary RNA strand, which is crucial for cellular function and regulation.
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04:16
1) Initiation of Transcription

RNA Polymerase

RNA polymerase is the enzyme responsible for synthesizing RNA from a DNA template during transcription. It unwinds the DNA double helix and catalyzes the addition of ribonucleotides to form an RNA strand. The enzyme has distinct regions, including an active site for nucleotide addition and other regulatory sites that can be targeted by inhibitors like α-amanitin.
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DNA Polymerase Requirements

Inhibition Mechanism

Inhibition mechanisms refer to the ways in which certain substances, like toxins, interfere with enzyme activity. In the case of α-amanitin, it binds to RNA polymerase at a site other than the active site, altering the enzyme's conformation and preventing it from effectively synthesizing RNA. This type of inhibition can lead to a complete halt in transcription, resulting in cell death if critical proteins are not produced.
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Enzyme Inhibition
Related Practice
Textbook Question

The nucleotide shown here is called cordycepin triphosphate. It is a natural product of a fungus that is used in traditional medicines.

If cordycepin triphosphate is added to a cell-free transcription reaction, the nucleotide is added onto the growing RNA chain but then no more nucleotides can be added. Examine the structure of cordycepin and explain why it ends transcription.

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Textbook Question

Controlling the rates of transcription and translation is important in bacteria to avoid collisions between ribosomes and RNA polymerases. Calculate what the maximum rate of translation by a ribosome in a bacterial cell would have to be, in units of amino acids per second, so as not to overtake an RNA polymerase that is synthesizing mRNA at a rate of 60 nucleotides per second. How long would it take for this bacterial cell to translate an mRNA containing 1800 codons?

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Textbook Question

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Eating even a single death cap mushroom (Amanita phalloides) can be fatal due to a compound called α-amanitin, a toxin that inhibits transcription.

What would you predict to be the immediate outcome of adding α-amanitin to a cell?

a. Reduced DNA synthesis

b. Reduced production of one or more types of RNA

c. Reduced binding of tRNAs to anticodons

d. Reduced rate of translocation of ribosomes translating mRNA

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Textbook Question

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Eating even a single death cap mushroom (Amanita phalloides) can be fatal due to a compound called α-amanitin, a toxin that inhibits transcription.

Toxins like α-amanitin are used for research in much the same way as null mutants (Chapter 16)—to disrupt a process and see what happens when it no longer works. Researchers examined the ability of α-amanitin to inhibit different RNA polymerases. They purified RNA polymerases I, II, and III from rat liver, incubated the enzymes with different concentrations of α-amanitin, and then tested their activity. The results of this experiment are shown here. These findings suggest that cells treated with α-amanitin will have a reduced level of:

a. tRNAs

b. rRNAs

c. snRNAs

d. mRNAs

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Textbook Question

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Eating even a single death cap mushroom (Amanita phalloides) can be fatal due to a compound called α-amanitin, a toxin that inhibits transcription.

If you wanted to use α-amanitin to shut down 95 percent of transcription by RNA polymerase II, roughly what concentration of α-amanitin would you use? Note that the scale on the x-axis of the graph in Question 13 is logarithmic rather than linear, so that each tick mark shows a tenfold higher concentration.

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Textbook Question

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Eating even a single death cap mushroom (Amanita phalloides) can be fatal due to a compound called α-amanitin, a toxin that inhibits transcription.

Biologists have investigated how fast pre-mRNA splicing occurs by treating cells with a toxin that blocks the production of new pre-mRNAs, then following the rate of splicing of the pre-mRNAs that were transcribed before adding the toxin. Why is addition of a toxin important in this study?

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