Question

Which of the following occurs as a result of an abundance of tryptophan in E. coli?
A.The leader sequence is not transcribed.
B.The 5 trp genes (TrpA � TrpE) are not transcribed.
C.Stalling of the ribosome at trp codons in the leader sequence
D. The 5 trp genes (TrpA � TrpE) are transcribed, but not translated.

Which of the following features of the trp operon is likely least essential to the process of attenuation?
A.The order of the structural genes, E, D, C, B, A
B. Trp codons near the beginning of the leader sequence
C. The ability of sequences within the leader mRNA to pair with one another
D. Transcription and translation of the leader sequence occur simultaneously.

Answer 1

1)

An abundance of tryptophan in E. coli, inhibit the expression trp-operon genes (stops the transcription). This mechanism is called repressible negative gene regulation.

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.

Production of a protein from an mRNA sequence is known as translation. Ribosomes bind to mRNA, and recruit tRNA molecules bound with tRNA specific amino acids. These tRNA have anticodons that are complementary to codon sequence of mRNA. Ribosomes form peptides bonds between amino acids, thus forming a peptide protein sequence.

The adjoined picture below shows repression of trp-operon under high tryptophan concentration.

Trp repressor Trp High (trp): repression occurs 나 Leaderㅓ Attenuator trpE trpD trpC trpB trpA -Structural gen Regulatory region High (trp)Low (trp) Attenuated mRNA AAAAAA trp mRNA MAXAAA

Thus, the correct option is (B) The 5 trp genes (TrpA -TrpE) are not transcribed.

2)

The feature the trp operon is likely least essential to the process of attenuation is the order of the structural genes, E, D, C, B, A. Thus, the correct option is (A).