Question 4: Another operon You are studying the trp operon, which includes 5 gen
ID: 257930 • Letter: Q
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Question 4: Another operon You are studying the trp operon, which includes 5 genes (trpA, trpB, trpC, trpD and trpE) that code for all of the enzymes required to synthesize the amino acid tryptophan. You discover that a nearby gene (trpR), which encodes the Trp Repressor protein (TrpR) regulates expression of the genes in this operon by binding to the Operator (Op) sequence and repressing gene expression. P is the promoter region required to activate gene expression by the transcriptional activator that recruits RNA polymerase p l trpE trpD trpC trpB trpA (i) You identify a mutant unable to produce the amino acid tryptophan. Which of the following parts of the operon could have been mutated in this first mutant? trpR (circle as many answers as could be correct) Op trpE trpD trpC trpB trpA (ii) You identify a second mutant in which the trp operon is always expressed. Which of the following parts of the operon could have been mutated in this second mutant? trpR (circle as many answers as could be correct) Op trpE trpD trpC trpB trpA (ii) You identify a third mutant in which the trp operon is always expressed. Inducibility of the trp operon can be rescued by the trpR gene on a separate plasmid (i.e. in trans) Which of the following parts of the operon could have been mutated in this third mutant? trpR (circle as many answers as could be correct) Op trpE trpD trpC trpB trpA (iv) You identify a fourth mutant in which the trp operon is always expressed but which cannot be rescued by any single part of the operon on a separate plasmid. Which of the following parts of the operon could have been mutated in this fourth mutant? trpR (circle as many answers as could be correct) Op trpE trpD trpC trpB trpA (v) The ability of the Trp repressor to bind its Operator is controlled by the amino acid Tryptophan itself binding to TrpR protein. Would you expect binding Tryptophan to increase or decrease the ability of TrpR to bind DNA? Explain your answer (vi) Given your answer in (v) above, how do you think expression of the trpR gene is regulated so that TrpR protein can control the trp operon?Explanation / Answer
Please find the answers below:
Answer i: P, Op, trpA, trpB, trpC, trpD, trpD (Since all these genes are positive regulators, mutation in any of these can lead to prevention of gene expression)
Answer ii: trpR, Op (prevention of binding of trpR on operator would promote movement of RNA polymerase over the trp-genes and hence up-regulate gene expression)
Answer iii: Op (under this condition, the operator might not be essentially bound by the repressor for preventing gene expression and hence it might have undergone mutation for upregulating gene expression)
Answer iv: trpR (Since this component is essentially required to repress gene expression, its mutation and inability to bind to the operator leads to permanent activation of gene expression)
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