attenuation

Question

Prokaryotes commonly use attenuation as a mechanism to control gene expression, but eukaryotes do not. Why do you think that attenuation is more common in prokaryotes than eukaryotes?

In eukaryotes, transcription and translation usually take place in separate physical locations.

Only prokaryotes synthesize amino acids.

Eukaryotes only regulate gene expression at the level of chromatin structure.

Eukaryotic ribosomes are faster than prokaryotic ribosomes, making attenuation impossible.

Only prokaryotes can form hairpin structures in RNA.

Correct. In prokaryotes transcription and translation are coupled. This makes attenuation possible. In eukaryotes, transcription takes place in the nucleus and most translation takes place in the cytoplasm.

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Incorrect. In prokaryotes transcription and translation are coupled. This makes attenuation possible. In eukaryotes, transcription takes place in the nucleus and most translation takes place in the cytoplasm.

Question

What effect on the expression of the trp operon might you predict in the trp operon if the Shine-Dalgarno sequence prior to the leader sequence were deleted?

The entire trp operon would be transcribed no matter what the level of tryptophan in the cell.

The leader sequence of the trp operon will be transcribed, but none of the structural genes for the trp operon will be transcribed.

The structural genes for the trp operon will only be transcribed when levels of tryptophan are low.

Transcription initiation will fail and the entire trp operon will remain untranscribed.

The entire trp operon will be transcribed but none of the structural genes for the trp operon will be translated.

Correct. In the absence of the Shine-Dalgarno sequence prior to the leader sequence, the leader sequence will not be translated. Therefore, the ribosome will never prevent the binding of region 2 with region 3 and the terminator loop no matter what the level of tryptophan is.

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Incorrect. In the absence of the Shine-Dalgarno sequence prior to the leader sequence, the leader sequence will not be translated. Therefore, the ribosome will never prevent the binding of region 2 with region 3 and the terminator loop no matter what the level of tryptophan is.

Question

What effect on the expression of the trp operon might you predict if there was a large deletion in region 3 of the trp operon?

The structural genes for the trp operon will only be transcribed when levels of tryptophan are low.

The structural genes for the trp operon will not be transcribed no matter what the intracellular levels of tryptophan are.

The leader sequence of the trp operon would be transcribed, but none of the structural genes for the trp operon would be transcribed.

The entire trp operon will be transcribed no matter what the level of tryptophan in the cell.

The entire trp operon will be transcribed but none of the structural genes will be translated.

Correct. If region 3 is deleted, the terminator loop consisting of region 3 and region 4 will not form no matter what the level of tryptophan is. This assumes that region 2 could not pair with region 4 to create an alternate termination loop.

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Incorrect. If region 3 is deleted, the terminator loop consisting of region 3 and region 4 will not form no matter what the level of tryptophan is. This assumes that region 2 could not pair with region 4 to create an alternate termination loop.

Question

What effect on the expression of the trp operon might you expect if a deletion occurred in the leader sequence that removed the trp codons in the trp operon?

The entire trp operon will be transcribed but none of the structural genes for the trp operon will be translated.

The leader sequence of the trp operon will be transcribed, but none of the structural genes for the trp operon will be transcribed.

The structural genes for the trp operon will only be transcribed when levels of tryptophan are low.

The entire trp operon will be transcribed no matter what the level of tryptophan in the cell.

The structural genes for the trp operon will not be transcribed no matter what the intracellular levels of tryptophan are.

Correct. If the trp codons were removed, there would be no stalling of the ribosome, so region 2 will always be blocked from pairing with region 3, causing constitutive formation of the termination loop consisting region 3 paired with region 4. Therefore, the structural genes of the operon will never be transcribed no matter what the intracellular levels of tryptophan are.

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Incorrect. If the trp codons were removed, there would be no stalling of the ribosome, so region 2 will always be blocked from pairing with region 3, causing constitutive formation of the termination loop consisting region 3 paired with region 4. Therefore, the structural genes of the operon will never be transcribed no matter what the intracellular levels of tryptophan are.

Question

What effect on the expression of the trp operon might you expect if the trp codons in the leader sequence of the trp operon were replaced by codons for isoleucine?

The structural genes for the trp operon will only be transcribed when isoleucine levels are low.

The structural genes for the trp operon will only be transcribed when levels of tryptophan are low.

The entire trp operon will be transcribed but none of the structural genes will be translated.

The structural genes for the trp operon will always be transcribed.

The structural genes for the trp operon will never be transcribed.

Correct. If the trp codons were replaced by isoleucine codons, the ribosomes would stall on the leader sequence only when isoleucine levels were limited. Therefore the transcription of the structural genes will depend on the intracellular level of leucine rather than tryptophan.

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Incorrect. If the trp codons were replaced by isoleucine codons, the ribosomes would stall on the leader sequence only when isoleucine levels were limited. Therefore the transcription of the structural genes will depend on the intracellular level of leucine rather than tryptophan.

Question

What effect on the expression of the trp operon might you expect if the stretch of T:A base pairs following region 4 were deleted?

The entire trp operon will be transcribed no matter what the level of tryptophan in the cell.

The structural genes for the trp operon will only be transcribed when levels of tryptophan are low.

The structural genes for the trp operon will not be transcribed no matter what the intracellular levels of tryptophan are.

The leader sequence of the trp operon will be transcribed, but none of the structural genes for the trp operon will be transcribed.

The entire trp operon would be transcribed but none of the structural genes would be translated.

Correct. If the T:A base pairs are removed following region 4, the structure will no longer resemble a rho-independent terminator (review Chapter 13), and termination will not follow the formation of the terminator loop consisting of the pairing of regions 3 and 4. This would allow transcription to continue through the structural genes of the operon.

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Incorrect. If the T:A base pairs are removed following region 4, the structure will no longer resemble a rho-independent terminator (review Chapter 13), and termination will not follow the formation of the terminator loop consisting of the pairing of regions 3 and 4. This would allow transcription to continue through the structural genes of the operon.

Question

What effect on the expression of the trp operon might you expect if there were a deletion of the Shine-Dalgarno sequence prior to the trpD structural gene?

The leader sequence of the trp operon will be transcribed, but none of the structural genes for the trp operon will be transcribed.

The entire trp operon will be transcribed but none of the structural genes will be translated.

The structural genes for the trp operon will not be transcribed no matter what the intracellular levels of tryptophan are.

The entire trp operon will always be transcribed unless the cell is able to obtain tryptophan from the external environment.

Transcription of the operon will terminate after trpD.

Correct. If the Shine-Dalgarno sequence of the trpD gene were mutated, the trpD gene would not be translated. Therefore, the bacterial cell could not produce tryptophan from chorismate and tryptophan could not be synthesized by the bacterial cell. This would cause levels of tryptophan within the cell to fall and would lead to constitutive formation of the antiterminator (pairing of regions 2 and 3) unless abundant tryptophan could be obtained from the environment.

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Incorrect. If the Shine-Dalgarno sequence of the trpD gene were mutated, the trpD gene would not be translated. Therefore, the bacterial cell could not produce tryptophan from chorismate and tryptophan could not be synthesized by the bacterial cell. This would cause levels of tryptophan within the cell to fall and would lead to constitutive formation of the antiterminator (pairing of regions 2 and 3) unless abundant tryptophan could be obtained from the environment.

Question

If the trp operon from E. coli was cloned between a ubiquitous eukaryotic promoter and a strong eukaryotic transcriptional termination sequence, how would this operon function in a eukaryotic cell?

The entire trp operon will be transcribed and all of the genes will be translated no matter what the level of tryptophan in the cell.

The structural genes for the trp operon will only be transcribed when levels of tryptophan are low.

The leader sequence of the trp operon will be transcribed, but none of the structural genes for the trp operon will be transcribed.

The entire trp operon will be transcribed but none of the structural genes of the trp operon will be translated no matter what the level of tryptophan in the cell.

The structural genes for the trp operon will not be transcribed no matter what the intracellular levels of tryptophan are.

Correct. If the bacterial trp operon were given a eukaryotic promoter and transcription termination sequence, the operon will be transcribed by RNA polymerase II in eukaryotic cells, but it would be very unlikely that any of the structural genes would be translated. Eukaryotic ribosomes do not recognize the Shine-Dalgarno sequence but scan for appropriate AUG codons from the 5’ end of the transcript; therefore while the leader peptide might initiate translation, none of the structural genes would be expected to initiate translation. In any case, because transcription and translation are physically separated in eukaryotic cells, and transcription termination in eukaryotes differs from that in prokaryotes, attenuation will not occur no matter what the levels of tryptophan in the cell.

You have one more chance to answer the question correctly. Try again.

Incorrect. If the bacterial trp operon were given a eukaryotic promoter and transcription termination sequence, the operon will be transcribed by RNA polymerase II in eukaryotic cells, but it would be very unlikely that any of the structural genes would be translated. Eukaryotic ribosomes do not recognize the Shine-Dalgarno sequence but scan for appropriate AUG codons from the 5’ end of the transcript; therefore while the leader peptide might initiate translation, none of the structural genes would be expected to initiate translation. In any case, because transcription and translation are physically separated in eukaryotic cells, and transcription termination in eukaryotes differs from that in prokaryotes, attenuation will not occur no matter what the levels of tryptophan in the cell.

Chapter 16.2. Animation Activity: Attenuation

Animation Assessment

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