Question

Tryptophan operon is an example of _______ Operon
A) Inducible
B) Repressible
C) Both A and B
D) None of the above

Answer 1

Hint: Operans are clusters of structural genes under the influence of a single operator site and a regulatory gene that ensures that the expression of structural genes is regulated in a coordinated manner.

Complete Answer:
>The operon will be constitutively switched on (the genes will be expressed) when the repressor is inactivated. In the case of positive regulation, genes are expressed only when the active regulatory protein, e.g. the activator, is present.
>As a result, the operon will be shut off when the positive regulatory protein is absent or inactivated.In the case of negative regulation, genes in the operon are expressed unless the repressor protein is turned off.Inducible operons are activated in response to the metabolite (a small molecule undergoing metabolism) that controls the operon. Lac operon, for example, is caused in the presence of lactose (through the action of the metabolic by-product allolactose).
>Repressible operons are turned off in response to a small regulatory molecule. The trp operon, for example, is inhibited in the presence of tryptophan. Note that in this use, the terms are defined by the response to a small molecule.
>If tryptophan is present in the system, then E . Coli does not need to synthesise it; the trp operon is turned off. However, when the supply of tryptophan is limited, the operon control switch is switched on. >In the presence of tryptophan in the cell , two tryptophan molecules act as a co-repressor and bind to the trp repressor.
>This binding allows the inactive repressor to bind to the trp operator.The binding of the tryptophan-repressor complex to the operator prevents RNA polymerase from binding downstream genes and thus prevents their transcription. An operon that is turned on by binding of the inducer to the repressors is called an inducible operon.
>The trp operon is repressed by binding tryptophan to the repressor and is thus not an inducible operon, so options A, C, D do not hold true for it.

Hence the correct answer is option ‘B’.

Note:Operon is a cluster of co-ordinated genes. It includes structural genes (usually encoding enzymes), regulatory genes (encoding, e.g. activators or repressors) and regulatory sites (such as promoters and operators). The type of control is defined by the response of the operon when there is no regulatory protein present.