Question

Compare and contrast the effect of a deletion in the operator of the lactose operon with one in the operator of the tryptophan operon?

Answer 1

Lac operon: (Lactose catabolism)

Lac operon involves the production of a repressor protein which binds to the operator in absence of the inducer, lactose. This inhibits the transcription of downstream structural genes by RNA polymerase.

When lactose is present, the repressor binds to lactose, as a result the operator binding site of the repressor gets engaged. Thus, the repressor cannot bind to the operator and transcription of the structural genes takes place which are involved in the catabolism of lactose.

If there is a mutation in the operator, the repressor protein cannot bind to the operator either in the presence or absence of lactose. Thus, the operon becomes constitutively active and produces the enzymes required for lactose catabolism even in the absence of lactose.

Tryptophan operon: (Tryptophan biosynthesis)

It involves a aporepressor which cannot alone bind to the operator.

Tryptophan is a corepressor and binds to the aporepressor, making it active. In presence of tryptophan, the aporepressor-corepressor complex can bind to the operator, inhibiting transcription of downstream genes, whose enzymes are responsible for biosynthesis of tryptophan.

So, when tryptophan is absent, the operon is active, while when tryptophan is present, the operon is inactive.

A mutation in the operator doesnot allow the binding of the aporepressor to the operator either in the presence or absence of tryptophan. So, the operon becomes constitutively active and produces the enzymes required for tryptophan biosynthesis even in the presence of tryptophan.