Trp operon Flashcards
How is transcription of the trp operon controlled?
What are the 2 ways the operon can be regulated?
What is the function of the operon?
In order of the operon, what do the following do:
Promotor
trpR
Promotor
Operator
TrpL (containing trpa)
trp genes
Tryptophan concentration - low trp high transcription (to make more tryptophan)
Aporepressor
Attenuation
Convert chorismic acid -> tryptophan
Initiates transcription of aporepressor
Aporepressor gene (constantly expressed)
Initiates transcription of genes
Controls transcription of genes
TrpL is the leader sequencing containing trpa (attenuator)
trp genes - genes expressed to synthesise more tryptophan
How does the aporepressor become an active repressor?
What is trp in this case?
What happens when [trp] is high?
What kind of control is this?
What happens when [trp] is low?
1 trp binding to 1 aporepressor & then dimerisation of 2 of these units
Co-repressor
No need for more trp so no need to transcribe operon - trp binds to aporepressor & active repressor binds to operon preventing transcription
Negative control
Aporepressor cannot form active dimer form due to lack of trp, so RNA pol binds to promotor & transcribes
What is attenuation?
When does it occur?
How does it prevent transcription?
What is transcribed in attenuation?
What does the transcript contain?
What does attenuation depend on?
Incomplete transcription of the entire operon
when [trp] is high
Only transcription of trpL and trpa occur when [trp] is high to prevent transcription of trp genes further downstream & waste resources
trpL & trp a (within trpL) to form 161 bp mRNA transcript
coding region for leader peptide (14aa) and attenuator sequence at 3’ end
- folding pattern of leader mRNA - transcription & attenuator seq
- extent to which ribosome has translated the leader mRNA - translation & leader peptide
Between what regions on the leader mRNA does the GC/attenuator stem loop form?
What process does this similarly resemble?
How does the mRNA stem loop terminate transcription of the trp operon?
Regions 3 and 4 within the attenuator sequence (regions of rich GC)
rho independent attenuator transcription termination
uracil tail forms weak DNA:RNA complex which is destabilised - DNA collapses & RNA polymerase is released
What does the leader peptide have contained?
So if [trp] is high, what happens to the peptide?
What does this mean for transcription?
What is another product of this?
if [trp] is low?
Why is this significant? (hint: location of codons)
What does this mean for RNA polymerase?
2 trp residues next to eachother
Trp residues incorporated in translation
Therefore, regions 3 and 4 can bind & form attenuator loop so transcription is inhibited as causes RNA polymerase to dissociate
leader peptide
Won’t incorporate so ribosome will stall at UGGUGG codons on mRNA
UGGUGG codons at region 1 of leader mRNA - forms stem loop between regions 2 and 3 (not 3 and 4) so no attenuator stem loop is formed
RNA polymerase is not stalled by the loop (not an attenuator loop) so carries on transcribing
The combination of repression & attenuation adds for greater control. What is the x fold calculation of control over the operon?
How did scientists discover attenuation in the trp operon?
What did they find about the 5’ end of the operon?
How did they figure out the sequences at the 5’ end?
What did this show?
70fold[repression] x 10fold[attenuation] = 700 fold
Looked at cells where different regions of the operon were missing
If the 5’ end was missing (trpR) - operon should have been 100% transcribing due to absence of repressor - but still wasn’t fully
DNA deletions & which sequences influenced transcription
5’ end sequences played role in attenuation
