In Escherichia coli (E. coli), the tryptophan operon encodes structural genes, including trpC, trpB, and trpA, that encode the tryptophan synthetase protein complex. A repressor protein encoded by the trpR gene regulates the trp operon. In the presence of high levels of tryptophan, tryptophan molecules bind the repressor protein and allow the repressor to recognize and bind to the operator, called trpO, and repress the expression of the structural tryptophan synthetase genes. In the absence of tryptophan, the repressor is unable to bind the operator and gene expression can occur. Assume any minus symbol represents a null mutant. What is the type of expression of each functional structural gene for the three bacterial strains?

Explanation:

in the absence of tryptophan, Trp operon expresses itself, and in the presence of tryptophan, repressor binds to the operator region and stop the expression of trp operon.

1- In trpR is mutated then the trp operon will be in the constitution expressed condition even in the presence of tryptophan amino acid.

for trpA - No expression as TrpA is mutated. So no expression

for trpB - constitutive

for trpC -constitutive

2- In this condition, Oc is mutated in such a way that repressor protein can no longer bind to it.

So

for trpA- constitutive.

for trpB - No expression as TrpB is mutated. So no expression.

for trpC - No expression TrpC is mutated. So no expression.

3- in this condition repressor from the merodiploid operon will express and in the presence of trp it will block the expression of trp operon.

for trpA- repressible.

for trpB - repressible.

for trpC - repressible.

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