2.The tryptophan operon in E. coli is controlled in part by attenuation. The fol
ID: 73554 • Letter: 2
Question
2.The tryptophan operon in E. coli is controlled in part by attenuation. The following figure shows the mRNA that would be made from the trpl region if transcription were to occur (a) TrpL mRNA End of trplStop coding sequence Ser Arg Trp Trp 4 Region 1 -U-rich termination sequence (3-4 stem loop only) Gly Lys Leu Met GlnThr 10 20 Met Val 50 Lys 150 Ala lle Phe 100 162 Beginning of trpE coding sequence Beginning of trpL30 coding sequence 40 For each of the following mutations, describe its effect on expression of the trp genes and explain your reasoning. a. Region 3 is deleted. Region 4 is deleted The entire trpl segment is deleted. d. The start codon of the Trpl peptide is deleted. e. Two nucleotides are inserted immediately after the UGA stop codon. f.Twenty nucleotides are inserted immediately after the UGA stop codonExplanation / Answer
Trp operon is a negative repressible feedback mechanism. The repressor for the trp operon is produced by the trpR gene situated upstream to the promoter. trpR gene is expressed continually at a low level. This produces monomer of repressor proteins, which are in inactive state. When tryptophan is present, these monomers combine to form tetramers and combine with tryptophan, this cause a conformation (in the repressor) that allows the repressor to bind the operator. So it prevents the RNA polymerase to bind and transcribe the operon, so tryptophan is not produced from its precursor. When tryptophan is absent, the repressor is in its inactive conformation and cannot bind the operator, so transcription continues to produce tryptophan.
When the tryptophan levels are low, the O site (operator site) of the trp operon is not occupied by repressor protein; repressor proteins also not occupied by tryptophan due to low levels. This will activate transcription of the Trp operon.
Attenuator RNA has four short sequences 1-4, which can form 3 distinct secondary structures: 1-2, 2-3, or 3-4. Formation of 2-3 stem loop structure works as a antiterminator form, while 3-4 stem loop works as a terminator form.
a. Region 3 is deleted – stem loop structures 2-3 or 3-4 are not formed. If these stem loop are not formed, then trp operon is continuously active and produces enzymes to metabolize tryptophan.
b. Region 4 is deleted - stem loop 3-4 is not formed, which results in the continuous synthesis of trp even the presence of high levels of tryptophan.
c. The entire trpL segment is deleted – fine tuning of the trp operon is not possible, and the operon is continuously active.
d. The start codon of the TrpL peptide is deleted – even at low level of tryptophan, the trp operon is shutdown.
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