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What enzymes does the trp operon encode?
Encodes enzymes that are needed for the biosynthesis of the amino acid, tryptophan.
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The trpE, trpD, trpC, trpB, and trpA genes encode enzymes involved in...
Tryptophan biosynthesis
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The trpR gene encodes the...
trp repressor protein
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When tryptophan levels within the cell are very low, the trp repressor cannot....
bind to the operator site.
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When trytophan levels in the cells are low, and the trp repressor cannot bind to the operator site, what transcribes the trp operon?
RNA polymerase
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When the RNA polymerase transcribes the trp repressor, the cells express...
the gene required for the synthesis of tryptophan.
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When the tryptophan levels within the cell become high, tryptophan acts as a....
corepressor
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When the tryptophan levels within the cell become high, tryptophan acts as a corepressor that binds to.....
the trp repressor protein.
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When tryptophan levels are high, what causes a conformational change in the trp repressor?
Tryptophan binding to the trp repressor protein
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When tryptophan levels are high, the bidning of tryptophan to the trp repressor, allows the trp repressor to bind to...
The trp operator site.
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When the trp repressor binds to the trp operator, it inhibits...
the ability of RNA polymerase
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When a high level of tryptophan is present within the cell what happens to the trp operon?
it is turned off
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Transcription of the entire trp operon occurs when...
tryptophan levels are low
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Why does transcription of the trp operon occur when trytophan levels are low?
Low levels means that tryptophan does not bind to the trp prepressor protein, preventing the repressor protein from binding to the operator site, thus causing the RNA Polymerase to transcribe the operon.
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The transcription of the trp operon by the RNA polymerase leads to the expression of...
trpE, trpD, trpC, trpB, trpA genes
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What is the structure of trp operon from left to right?
- Promoter
- Operator
- trpL
- attenuator sequence
- trpE
- trpD
- trpC
- trpB
- trpA
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When tryptophan levels are high, what occurs?
repression
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When tryptophan levels are high, the tryptophan-trp repressor complex binds to..
the operator site
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When tryptophan levels are high, the tryptophan-trp repressor complex binds to the operator site to inhibit...
transcription
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When the tryptophan levels are high the other form of regulation that can occur that is not repression is...
attenuation
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what happens in attenuation when the tryptophan levels are high?
the RNA is transcribed only to the attenuator sequence and then transcription is terminated.
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Leader peptide initiator codon
AUG
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Leader peptide termination codon
UGA
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If Regions 1 and 2 are base-paired what can form?
Transcription-termination hairpin
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If Regions 3 and 4 are base-paired, what can form?
Transcription-termination hairpin
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If Regions 2 and 3 are basepaired, can transcription-termination hairpins form?
no
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Why can attenuation occur in bacteria?
Because the processes of transcription and translation are coupled.
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How does attenuation inhibit the further production of typtophan in the cell?
because transcription ends shortly after the trpL gene, thus the short piece of RNA cannot encode the rest of the proteins required for tryptophan biosynthesis
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What are the three possible stem-loop structures formed from trpL mRNA under different conditions of trnaslation?
- No translation = (1/2 loop and 3/4 loop = rho independent terminator)
- Low tryptophan levels, 2-3 stem-loop forms
- High tryptophan levels, 3-4 stem-loop forms
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When translation is not coupled with transcription, the most stable form of the mRNA occurs when..
- region 1 hydrogen bonds to region 2 and
- region 3 hydrogen bonds to region 4.
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When translation is not coupled with transcription, what forms..
a terminator stem-loop
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when translation is not coupled with transcription, a terminator stem-loop forms, and transcription will be ...
terminated just past the trpL gene.
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When tryptophan levels are low, ribosomes pause at the...
trp codons in the trpL gene
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When tryptophan levels are low, ribosomes pause at the trp codons in the trpL gene because...
insufficent amounts of charged tRNAtrp are present.
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When tryptophan levels are low, the ribosomal pause blocks..
region 1 or the mRNA.
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When tryptophan levels are low, the ribosomal pause blocks region 1 of the mRNA, so region 2 can...
hydrogen bond only with region 3
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When tryptophan levels are low, the ribosomal pause blocks region 1 of the mRNA, so region 2 can hydrogen bond with .....
region 3
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When the 3-4 stem loop cannot form, what does not occur?
transcriptional termination
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When the 3-4 stem loop cannot form, transcriptional termination does not occur and RNA polymerase does what?
transcribes the rest of the operon
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When trytophan levels are high, translation of the trpL gene progresses to its...
stop codon
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When trytophan levels are high, translation of the trpL gene progresses to its stop codon, where the ribosome...
pauses
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When the tryptophan levels are high and the the ribosome pauses, what region is blocked?
region 2
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When the tryptophan levels are high and the the ribosome pauses, region 2 is blocked from hydrogen bonding with any region and therby enables what to happen?
region 3 to hydrogen bond with region 4
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When tryptophan levels are high and region 3 hydrogen bonds with region 4, transcription is terminated at...
the U-rich attenuator.
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low tryptophan levels = ____ terminator forms
3/4
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high tryptophan levels = _____ terminator forms
2/3
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What are the three stages of transcription initiation of the trp operon.
- polymerase pauses
- Ribosome binds to transcript
- Moving ribosome releases the paused polymerase
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what are the two stages of a trytophan-starved culture after the initiation of transcription?
- Ribosome stalls at a trp codon (Antiterminatore forms)
- Terminator cannot form, transcription continues
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what are the two stages of a growth with excess tryptophan after the initiation of transcription?
- Ribosome moves to stop codon
- Terminator forms
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The TRAP protein consits of what two codons?
GAG and UAG
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what is attenuation?
premature termination of transcription
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Region 1 is mutant, tryptophan is high, and translation is not occurring would attentuation occur?
No, because the 2/3 stem loop will form
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Region 2 is mutant, tryptophan is low, and translation is occurring would attentuation occur?
Yes, because the 3/4 loop is formed
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Region 3 is mutant, tryptophan is high, and translation is not occurring would attentuation occur?
No, because the 1/2 loop will form and the 3/4 loop cant form.
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Region 4 is mutant, tryptophan is low, and translation is not occurring would attentuation occur?
no, because the 2/3 stem loop occurs
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Would you expect attenuation to occur if the tryptophan levels were high and mutations at the end of the trpL gene changed theUUUUUUUU sequence to UGGUUGUC?
The addition of Gs and Cs into the U-rich sequence would prevent attenuation. The U-rich sequence promotes the dissociation of them RNA from the DNA, when the terminator stem-loop forms. This causes RNA polymerase to dissociate from the DNA and thereby causes transcriptional termination.
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