The Trp Operon

Question 1

label all the parts

Answer 1

1 = double stop
2 = stop codon
3 = 2 region
4 = 3 region
5= intrinsic terminator

Question 2

label all the parts

Answer 2

1= double stop
2 = stop codon
3 = region 2
4 = region 3
5 = region 4
6 = intrinsic terminator

Question 3

What would happen if the intrinsic terminator was mutated?

Answer 3

Would not be able to terminate, act as constantly needs trp
Attenuation would never function
Always be active as if shortage

Question 4

What would happen if the ribosome was mutated to stop early?

Answer 4

• act as if there was a shortage of trp
• intrinsic terminator would never form

Question 5

what does the trp operon produce?

Answer 5

chorismic acid
anthranalic acid
tryptophan

Question 6

describe the process of autoregulation

Answer 6

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

Question 7

how is trp autoregulated?

Answer 7

• Trp R negatively regulates trp operon in the presence of trp and its own transcription
• when trp is present, 70x higher transcriptional regulation

Question 8

what is the advantage of autoregulation?

Answer 8

• checks and balances
• helps to keep the system in equilibrium with not too high or low levels of trp

Question 9

attenuation

Answer 9

• terminates transcription before transcription of first structural gene
• has a 10 fold effect
• is a special termiantor

Question 10

describe the process of attentuation

Answer 10

• 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

Question 11

corepressor

Answer 11

• trp is a corepressor because it has to bind to the aporepressor TrpR in order to stop transcription

Question 12

aporepressor

Answer 12

• 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

Question 13

Why is DNA looping not needed to repress transcription for the trp Operon, in contrast to the lac and ara operons?

Answer 13

• 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.

Question 14

Why would it not be a good idea for the trp Operon to be regulated by CAP?

Answer 14

x - revisit after CAP review

Question 15

Compared to the lac operon, would you expect the trp Operon to have a strong consensus -35 site in the promoter?

Answer 15

compared to the lac operon, the trp operon should have a strong -35 site because there is no CAP binding

Question 16

How much does trpR contribute to repression of the trp promoter? How about attenuation?

Answer 16

• trp R = 70 fold
• attenuation = 10 fold
• together = 700 fold

Question 17

What would be the outcomes regarding expression of the trp operon if the string of A’s in the template strand was missing?

Answer 17

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

Question 18

How about if a stop codon was placed immediately after the double trp codons?

Answer 18

• 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

Question 19

what happens with transcription when an extra stop codon is placed upstream of the two trp codons?

Answer 19

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