Ch. 17 - Transcription, RNA Processing, and Translation

Freeman - Biological Science 8th Edition

Freeman8th EditionBiological ScienceISBN: 9780138276263Not the one you use?Change textbook

Freeman 8th Edition

Ch. 17 - Transcription, RNA Processing, and Translation

Problem 14

Chapter 17, Problem 14

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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.

Verified step by step guidance

Step 1: Analyze the graph provided. The x-axis represents the concentration of α-amanitin (in μM) on a logarithmic scale, and the y-axis represents the percent polymerase activity. The graph shows a decreasing trend in polymerase activity as the concentration of α-amanitin increases.

Step 2: Identify the target polymerase activity. Since the question asks to shut down 95% of transcription, the remaining polymerase activity would be 5%. Locate the point on the y-axis corresponding to 5% polymerase activity.

Step 3: Trace horizontally from the 5% polymerase activity mark on the y-axis to the curve. Then, trace vertically down to the x-axis to determine the corresponding concentration of α-amanitin.

Step 4: Note that the x-axis is logarithmic, meaning each tick mark represents a tenfold increase in concentration. Carefully interpret the position of the curve relative to the tick marks to estimate the concentration.

Step 5: Use the estimated concentration from the graph as the approximate value of α-amanitin required to reduce RNA polymerase II activity to 5%.

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Key Concepts

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

α-amanitin

α-amanitin is a cyclic peptide toxin derived from the Amanita phalloides mushroom, known for its ability to inhibit RNA polymerase II. This enzyme is crucial for synthesizing mRNA in eukaryotic cells. By binding to RNA polymerase II, α-amanitin effectively halts transcription, making it a valuable tool in molecular biology for studying gene expression and regulation.

Transcription Inhibition

Transcription inhibition refers to the process of blocking the synthesis of RNA from a DNA template. In the context of α-amanitin, it specifically targets RNA polymerase II, which is responsible for transcribing protein-coding genes. Understanding the concentration required to achieve a certain level of inhibition is essential for experimental design in studies involving gene expression.

Logarithmic Scale

A logarithmic scale is a nonlinear scale used for a large range of values, where each tick mark represents a tenfold increase in value. In the context of the graph provided, this means that the concentration of α-amanitin is plotted logarithmically, allowing for easier visualization of changes in transcription activity across a wide range of concentrations. This is crucial for accurately interpreting the data and determining the concentration needed to achieve 95% inhibition.

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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.

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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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.

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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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.

The primary cause of death from α-amanitin poisoning is liver failure. Suppose a physician informs you that liver cells die because their rate of protein production falls below a level needed to maintain active metabolism. Given that α-amanitin is an inhibitor of transcription, you wonder if this information is correct. Propose an experiment to determine whether the toxin also has an effect on protein synthesis.

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