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Question

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https://en.wikipedia.org/wiki/Trp_operon

What is the effect of the following mutations on the expression of the trpD protein?

a. mutation of the trpEgene to give a nonfunctional protein,

b. mutation of attenuator region 3 that eliminates base pairing with attenuator region 4

c. mutation of the trpR ene that changes the dissociation constant of tryptophan from 40 uM (40 micromolar) to 0.4 uM

d. mutation of trpR that changes its dissociation constant at the trp operator from 0.5 nM (0.5 nanomolar) to 5.0 nM.

e. Swapping the positions of the lacl gene and trpR gene in the chromosome.

Anothor statement on the same subject:

Procaryotic transcription

. Explain in molecular terms of the proteins or enzymes that regulate gene expression what the effect would be (e.g. increase, decrease, no difference from wild type) of the following mutations on the expression of the trpA protein

: (a) Deletion of the Trp operator,

(b) Deletion of the Trp leader peptide AUG (translation start) codon,

(c) Deletion of the Trp promoter,

(d) Constitutive (always ON) expression of the TrpR gene

(E) Transfer of the TrpL gene (163 bp leader to the 3' endof the Trp operon (as shown in the figure below).

Explanation / Answer

(a) if mutation occurs in trpE region no tryptophan will be formed or it can produce a truncated amino acid which will be non-functional in nature..

(b) if mutation occurs in 3-4 attenuation region it will not form hair pin loop hence the tryptophan level will keep on increasing as its binding stop the transcription. Hence no binding means no termination.

(c) as trpR gene produces a Repressor protein which binds to the Promoter region inhibiting formation of tryptophan when tryptophan levels are high hence mutation in this will leads to keep the Promoter in ON state always resulting in high tryptophan level..

(e) if the position is being swaped among them it will lead to non formation of the tryptophan..!!

2(a) it will lead to decrease in tryptophan level from the wild type..

(b) it will not be able to synthesis tryptophan as the start codon is being removed hence the ribosome with the translation machinery will no be able to identify the start codon resulting in no translation..!!

(c) as the Promoter site is deleted the translation will not be initiated nor be signaled hence no translation hence decrease in level of tryptophan.

(d) if the Repressor gene is kept on always it will produce Repressor protein but it's binding to the Promoter will depend on the the level of tryptophan present thre as the Repressor protein is activated by the binding of tryptophan..!!

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