15. The Tryptophan (Trp) operon

Question 1

define what attentuation is? what does it depend on?

Answer 1

a mechanisms that allows premature termination of transcription.

depends on the fact that in bactera, transcription and translation proceed simultaneously

Question 2

when is the trp operon turned off?

Answer 2

when tryptophan levels are high

Question 3

what is the trpR protein? describe its structure and function

Answer 3

its a gene downstream of the operator that becomes the trpR protein - an aporepressor monomer. this binds with another trpR protein to become a dimer.

if the dimer has tryptophan binded (the co-repressor) it will block transcription

Question 4

what does the trpR protein do when there are low levels of tryptophan?

Answer 4

the trpR protein monomer makes a dimer. this dimer cant bind to the operator because theres no tryptophan to be a corepressor. RNA pol transcribes

Question 5

what are the two ways the trp operon is regulated?

Answer 5

1. protein mediated repression (trpR - making dimers to block transcription)
2. RNA mediated repression (attenuation of transcritpion)

Question 6

why is it important to have both mechanisms of regulation of the trp operon? (what are the negatives that mean there needs to be both

Answer 6

protein mediated repression is weak - transcription can still occur when the repressor is present

Question 7

how does attentuation work?

what strucutre is made

what does the suceess determine on

Answer 7

there are four leader sequences - region 1 through to 4. when mRNA is made through transcritpion, the secondary hair pin structure is made.

the sucess of the transcription depends on if region 2 and 3 (success) bind or 3 and 4 (termination)

Question 8

what regions will bind together in the trpL (trp leader) sequence if:

tryptophan is present

tryptophan is absent

Answer 8

tryptophan present: region 3 and 4 will bind - resulting in excess uracils at the end which terminates transcription of synthesising tryptophan

tryptophan absent: region 2 and 3 will bind - resulting in a sucessful hair pin helix and RNA pol can continue to transcribe

look at image closely

Question 9

why does attenuation only work in prokaryotes? 2 points

Answer 9

• because transcription and translation occur simutaneously. important as its mRNA that causes the structure in the trpL that can terminate or continue transcription
• in eukaryotes, transcription occurs in the nucelous and translation is in the cytoplasm - its a seperate process