Question

select the correct answer. what is the determining factor for the transcription of structural genes of the trp operon?
a. the quantity of trp repressor protein made by the regulatory gene trpr
b. the ability of rna polymerase to bind to the promoter region
c. the availability of tryptophan in the environment
d. the location of a regulatory gene with reference to the other regions of the operon
e. the activity of the other operons present in bacteria

Explanation:

The trp operon is a repressible operon that controls the synthesis of tryptophan. The key factor determining the transcription of its structural genes is the availability of tryptophan in the environment. When tryptophan is abundant, it acts as a corepressor, binding to the trp repressor protein. This complex then binds to the operator, preventing RNA polymerase from transcribing the structural genes. When tryptophan is scarce, the repressor is inactive, and transcription occurs. Option A is incorrect because the quantity of the repressor alone isn't the determining factor; it's the interaction with tryptophan. Option B is incorrect as RNA polymerase can bind to the promoter, but the repressor - operator interaction (dependent on tryptophan) controls whether transcription proceeds. Option D is incorrect as the location of the regulatory gene doesn't determine transcription; it's the tryptophan - repressor interaction. Option E is incorrect as other operons don't affect the trp operon's transcription.

Answer:

C. the availability of tryptophan in the environment