Question

Various mechanisms for regulation of the trp operon are described. Which is correct?

Select one:

a. Its transcription is down-regulated, when the Trp level is high.

b. Ribosome stalled due to a low Trp level induces an mRNA structure suitable for its continued transcription.

c. Both a and b

d. Neither a nor b

Answer 1

The right option is option (c) Both A and B

Explanation:-

The TRP operon is an example of the repressible operon type. This is different than inducible operon in which presence of a substrate initiate the transcription of the operon. In this operon system, if a molecule is present then the tanscription is down regulated.

This is because the end product of the pathway for which the gene products of the operon perform is the same molecule which turns it off.

In case of operon of trp, the enzymes are involved in biosynthesis of tryptophan but when there is tryptophan already, there is no need to make more tryptophan as it is useless. So it is downregulated when tryptophan levels are high in cell.

In the ribosome stallation process, there is sequence which codes for multiple tryptophan and if the polymerase make the mRNA of that sequence and there is tryptophan present that tryptophan will be charged onto tRNA and brought for translation as in prokaryotes both transcription and translation is happening at the same time. Now right after it, there is stem loop structure which forms and terminate the reaction before the whole opeon is transcribed. So in presence of tryptophan that happens.

Now when there is less tryptopha the ribosome will stall at that stretch of mRNA where there are multiple tryptophan codons as charged tRNA will not come and this stallation prevent the termination of the transcription.

So both the option A and B are correct.