Question

What would be the result of a mutation that inactivates the regulatory gene of a repressible operon in an E.Coli cell? Use detailed descriptions of the molecules that may interact with the operon to explain your answer

Answer 1

• In prokaryotes operon system represents gene expression.
• Operon consist of cluster of genes that are co-transcribed from the same promoter or are expressed together.
• Operon comprises of different types of gene:

a. Structural genes

b. Controlling genes- promoter/ operator.

• All genes under the same promoter/ operator system are expressed at the same time together (on and off together).
• Operons are under regulation according to environmental conditions as: positive control with inducer or negative control with repressor.
• Thus, genes are not expressed all the time, to generate the required protein.
• The control of gene expression: under positive control, represents up-regulation and under negative control represents down regulation of gene expression to produce the proteins as per requirement of the organism.

• If mutation occurs in repressible system of an operon in E. coli, the operon will continue to express the structural genes.
• Example in case of Trp operon of E. coli.

Trp operon act as repressible system. Co-repressor activates the repressor.

Trp operon Representations:

R = Repressor gene

O= Operator

Trp L = leader sequence, Attenuator sequence present. att = Attenuator

P = Promoter

trpE- and trpC- = non-functional protein products produced.

Two types of repression:

Repression:

• In repressible system, like Trp operon, tryptophan acts as co-repressor.
• Tryptophan act as co-repressor for its own biosynthesis.
• When tryptophan is present, the cell does not require more tryptophan.
• Tryptophan binds to the repressor (encoded by R gene) to activate it and binds to operator to prevent transcription of the gene required for tryptophan biosynthesis.
• Thus, in presence of tryptophan, trp operon is switched off.

Attenuation:

• At attenuator 3-4 region leads to formation of stem-loop structure, that halts the RNA polymerase.
• This process takes comparatively more time than repressor/inducer system.

Mutation may be in controlling genes:

• Rn = Repressor cannot bind tryptophan, transcription occurs even in presence of Tryptophan.
• R- = Repressor cannot bind to the operator.

• O- = operator cannot bind repressor protein, transcription occurs even in presence of Tryptophan.

• L- or att- = deletion of the attenuator sequence, so repressor cannot bind, transcription will occur.
• P- = a deletion of the trp operon promoter

Thus, mutation in controlling gene will allow structural genes to continuously express.