Attenuation of t r p operon transcription is controlled by the formation of stem

ID: 133587 • Letter: A

Question

Attenuation of trp operon transcription is controlled by the formation of stem-loop structures in mRNA. The attenuation function can be disrupted by mutations that alter the sequence of repeat DNA regions 1 to 4 and prevent the formation of mRNA stem loops.

Determine the likely effects on attenuation of each of the following mutations under the conditions specified.

Drag the appropriate items to their respective bins.

Mutated Region a. Region1 b. Region1 C. Region 2 d. Region2 e. Region3 f. Region3 g. Region 4 h. Region4 Tryptophan Level Low High Low High Low High Low High

Explanation / Answer

Tryptophan is an amino acid used up by bacteria to make proteins. Trp operon consists of structural genes and regulatory genes.

Attenuation refers to the mechanism by which the expression of trp operon is reduced preventing the completion of transcription. Attenuation of trp operon transcription is controlled by the formation of stem-loop structures in mRNA.

In between the operator and the first gene of the trp operon, there is a section called the leader sequence that plays an important role in attenuation.

Transcription and translation occurs simultaneously in bacteria. Once RNA polymerase has started transcribing the operon, a ribosome can attach to the still forming transcript and begin translating the leader region.

If the level of tryptophan is high, the ribosome is able to translate rapidly, since tryptophan is already available to complete the process. But if tryptophan level is low, translation process also occurs slowly.

Let us look at the different possibilities at each regions.

1. When there is low level of tryptophan at region 1, ribosome stutters at region 1, which enables region 2 to bind with region 3 forming anti-terminator loop which allows transcription to proceed. But if there is mutation in region 1 (having high level of tryptophan), it gets bound to region 2 making it unavailable to form anti-termination loop with region 3. This leads to the halt of transcription.

2. When there is low level of tryptophan at region 2, ribosome slows down at this point and region 2 binds with region 3 forming anti-terminator loop which allows transcription to proceed. But if there is mutation in region 2 due to high level of tryptophan, there will be no stopping at this point that leads to the formation of loops between regions 3 and 4 or attenuator, which stops the transcription process.

3. When there is low level of tryptophan at region 3, it is able to bind with region 2 forming the anti-terminator loop. But if there is a mutation in this region, there will be no stopping at the position of tryptophan codon it gets bound to region 4 making it available to form termination loop, thus stopping the transcription process.

4. When there is low level of tryptophan at region 4, transcription proceeds normally. But if the level is high, it gets bound to region 3 forming the termination loop. This will in turn cause the transcription to stop.

Thus, effects on attenuation of each of the mutations are as follows

Transcription does not occur

Transcription occurs

b. Region 1

a. Region 1

d. Region 2

c. Region 2

f. Region 3

e. Region 3

h. Region 4

g. Region 4