Question

In the absence of trp, what happens to the genes within the trp operon?

1. regulator without tryp-binding prevents genes from transcribing.

2. regulator falls off the operator and structural genes get transcribed.

3. Lack of trp increases level of cAMP high, which leads to activation of CAP protein and gene expression.

4. active repressor binds to operator and genes do not get transcribed.

5. active activator binds to operator and transcription of the structural gene takes place.

Answer 1

Option-b the regulator falls off from the operator and structural genes gets transcribed

Trp operon is an example of negative regulation of gene expression which means that a repressor regulator protein binds to the operator in order to stop the transcription. In trp operon, The repressor senses the presence of tryptophan. If tryptophan is present in the cell , the repressor binds to tryptophan and blocks regulation so that the cell doesnt waste its energy for synthesising tryptophan. If tryptophan is absent, the repressor is in inactive form. It doesn't bind to operator thus switching on the transcription of tryptophan.

So in the absence of tryptophan, Structural genes are transcribed from trp operon as the repressive regulator trpR falls off from the operator

Other options are incorrect because

Genes are transcribed in the absence of tryptophan

Lack of glucose increases cAMP concentration which leads to CAP activation.

Trp operon works based on repressor protein and not activator protein.