The Trp Operon Flashcards
label all the parts
1 = double stop
2 = stop codon
3 = 2 region
4 = 3 region
5= intrinsic terminator
label all the parts
1= double stop
2 = stop codon
3 = region 2
4 = region 3
5 = region 4
6 = intrinsic terminator
What would happen if the intrinsic terminator was mutated?
Would not be able to terminate, act as constantly needs trp
Attenuation would never function
Always be active as if shortage
What would happen if the ribosome was mutated to stop early?
- act as if there was a shortage of trp
- intrinsic terminator would never form
what does the trp operon produce?
chorismic acid
anthranalic acid
tryptophan
describe the process of autoregulation
when trp is present, TrpR produces an aporepressor that binds with trp
- this in turn blocks the trp promoter and no transcription occurs
- when trp is absent, there is no trp to bind to the aporepressor, so transcription occurs
how is trp autoregulated?
- Trp R negatively regulates trp operon in the presence of trp and its own transcription
- when trp is present, 70x higher transcriptional regulation
what is the advantage of autoregulation?
- checks and balances
- helps to keep the system in equilibrium with not too high or low levels of trp
attenuation
- terminates transcription before transcription of first structural gene
- has a 10 fold effect
- is a special termiantor
describe the process of attentuation
- at the pause site, there is 1-2 hairpin loop that forms, which is what caused RNA polymerase to stall
- the attentuator will stop transccription and decide which hairpin to form
- if a 2-3 hairpin forms transcription continues
- if a 3-4 hairpin forms, there will be no transcription of the trp structural genes
corepressor
- trp is a corepressor because it has to bind to the aporepressor TrpR in order to stop transcription
aporepressor
- needs a corepressor to bind in order to work
- acts as a feedback loop
- once trp is present, it will bind to the aporepressor to stop the formation of trp
Why is DNA looping not needed to repress transcription for the trp Operon, in contrast to the lac and ara operons?
- Leader (trpL) mRNA can form three different hairpin loops.
- Alternative mRNA structures determine which loops form.
- One of the hairpin loops is factor independent terminator.
Why would it not be a good idea for the trp Operon to be regulated by CAP?
x - revisit after CAP review
Compared to the lac operon, would you expect the trp Operon to have a strong consensus -35 site in the promoter?
compared to the lac operon, the trp operon should have a strong -35 site because there is no CAP binding
How much does trpR contribute to repression of the trp promoter? How about attenuation?
- trp R = 70 fold
- attenuation = 10 fold
- together = 700 fold
What would be the outcomes regarding expression of the trp operon if the string of A’s in the template strand was missing?
transcription will proceed regardless of the trp levels
- the hairpin at region 3-4 is not sufficient to cause termination without a sting of T’s on the template strand close by.
- the switch from A’s to T’s on the template strand means that the instrinsic terminator will never work no matter which hairpin forms
How about if a stop codon was placed immediately after the double trp codons?
- the addition of the upstream stop codon causes the ribosome to leave the mRNA before it gets a chance to block region 2
- no matter what levels of trp exist, the ribosome will never be able to reach the second stop codon and overlap into region 2
what happens with transcription when an extra stop codon is placed upstream of the two trp codons?
transcription will proceed regardless of trp levels
- the addition of the upstream stop codon causes the ribosome to leave the mRNA before it gets a chance to block region 2
- no matter what levels of trp exist, the ribosome will never be able to reach the second stop codon and overlap into region 2
