Question

1. If tryptophan interacts with a repressor protein (which it does in real life) and the trp operon is anabolic (which it is)

a. What would happen to tryptophan production levels and energy expenditure in a mutant bacterial cell that had a repressor protein that was incapable of binding to tryptophan (but was normal in every other way)

b. What would happen to tryptophan production levels and energy expenditure in a mutant bacterial cell that had a repressor protein that was incapable of binding to the operator (but was normal in every other way)

c. What would happen to tryptophan production levels and energy expenditure in a mutant bacterial cell that had a repressor protein that was incapable of letting go of tryptophan?

Answer 1

Tryptophan (trp) operon is an anabolic operon. In the presence of Trp, it is repressed by Trp repressor which binds to the operator and prevents the binding of RNA polymerase to the promotor.
In the absence of Trp, the repressor is unable to bind to the operator. Hence RNA polymerase can bind to the promoter and mediate transcription of structural genes.

i. If the repressor protein is unable to bind to Trp, it will lead to the constitutive activation of the operon. This is due to the fact that the repressor can bind to the operator only in complex with Trp.

ii. If the repressor protein is unable to bind to the operator, it will lead to the constitutive activation of the operon. This is due to the fact that the repressor binding to the operator essential to prevent RNA polymerase binding to the promoter.

iii. If the repressor protein is unable to let go of Trp, it will lead to the constitutive repression of the operon. This is due to the fact that the repressor can be relieved from the operator only when Trp is removed from it.