Question

752. A mutation in the operator of the Trp operon prevents repressor protein binding. Will the Trp operon be expressed when tryptophan levels are high? Explain your reasoning.

753. A mutation in the promoter of the Trp operon prevents sigma factor binding. Will the Trp operon be expressed when tryptophan levels are low? Explain your reasoning.

754. A mutation in the Lac repressor protein prevents it from binding to lactose. Will the Lac operon be expressed when lactose levels are high and glucose levels are low? Explain your reasoning.

755. A mutation prevents expression of the CAP protein. Will the Lac operon be expressed when lactose levels are high and glucose levels are low? Explain your reasoning.

Answer 1

752) In Trp operon the expression is regulated by a regulatory protein which binds to the Trp operator and inhibits the expression of Trp operon in the presence of Trp, the regulatory protein is a repressor which inhibits the expression of Trp operon.Trp is a co-repressor.

A mutant that has a mutation in the operator of the Trp operon prevents repressor protein binding, cannot be inhibited by the regulatory protein in the presence of Trp.

there is another level of transcription control by transcription attenuation.

Trp operon has a leader sequence, which has four regions. Region 1, Region 2, Region 3, Region 4.

Region 2 and 3 can form stem-loop and regions 3 and 4 can form stem-loop if stem-loop forms between region 2 and 3 transcription can continue, but if stem-loop forms between region 3 and 4 transcription termination occur.

In prokaryotes, transcription and translation can occur simultaneously.

Region 1 has two consecutive Trp codons. If the Trp is abundant in the cell, as the RNA polymerase transcribes region 1 Ribosome translates region 1 and reach region 2 before region 3 is transcribed so stem-loop forms between region 3 and 4 ( region 2 and 3 cannot form stem-loop because the region 2 is bound with RNA polymerase). so transcription termination occurs.

Even if the operator of the Trp operon is mutated, the Trp operon cannot be expressed if Trp is present at higher levels in the cell, due to transcription attenuation.

753) RNA polymerase binds to the promoter and transcribes the gene. If the promoter is mutated such that it cannot bind to RNA polymerase that gene cannot be transcribed.

Sigma factor in the prokaryotes directs the RNA polymerase to the promoter.

A mutation in the promoter of the Trp operon prevents sigma factor binding, will prevent the expression of Trp operon even if Trp levels are low in the cell.