Question

Choose the correct answer from the alternatives given :
Repressible operon system is usually found in _(i)_ pathways. The pathway's end product serves as a _(ii)_ to activate the repressor, turn off enzyme synthesis and prevent overproduction of the end product of the pathway. Genes for this operon are usually switched _(iii)_ and the repressor is synthesised in an _(iv)_ form.

a. (i) anabolic, (ii) corepressor, (iii) on, (iv) inactive
b. (i) anabolic, (ii) inducer, (iii) off, (iv) active
c. (i) catabolic, (ii) inducer, (iii) off, (iv) active
d. (i) catabolic, (ii) corepressor, (iii) on, (iv) inactive

Answer 1

Hint: Repressible operon systems are regulated by a co-repressor. Its presence usually shuts down the operon in a feedback repression manner. Tryptophan operon is responsible for the synthesis of amino acid tryptophan. It is an example of a repressible operon system. Tryptophan synthesis requires the action of three enzymes which are synthesized by the coordinated activity of five different genes.

Complete answer:
• The addition of tryptophan stops the production of the three enzymes required for its biosynthesis and is thus referred to as a repressible system.
• In tryptophan operon, five structural genes, trp A, trp B, trp C, trp D, and trp E are responsible for the synthesis of three enzymes required to synthesize tryptophan.
• The operon contains a regulatory gene (R) that produces the repressor protein, called apo-repressor. Apo-repressor doesn’t bind to the operator directly and thus the operon is in ‘switched on’ position.
• When tryptophan is added, it functions as a co-repressor and binds to the apo-repressor. The apo-repressor bound to the co-repressor forms an activated repressor which then binds to the operator gene and blocks the function of RNA polymerase. In this way, it is ‘switched off’.
• Feedback repression is functional when there is no further need for the resulting product in the cell and thus no requirement of this anabolic pathway.
• It can now be said that the repressible operon system is usually found in anabolic pathways. The pathway's end product serves as a corepressor to activate the repressor, turn off enzyme synthesis, and prevent overproduction to the end product of the pathway. Genes for this operon are usually switched on and the repressor is synthesized in an inactive form.

Hence, the correct answer is option (a).

Note:
- Tryptophan repressor is a homodimer.
- The second mechanism of negative feedback in tryptophan operon is Attenuation.
- Jacob and Monod proposed the operon model for transcription regulation for the first time in 1961.