Question

What would happen to the expression of the trp operon if you replace the key tryptophan codons in the leader peptide with tyrosine codons?

Answer 1

If tryptophan codon is replaced wit tyrosine codon than , this operon will Express always there will be no premature chain termination occur .

Explanation-

In normal condition tryptophan operon express and form tryptophan. When tryptophan concentration increases in cell , it lead to shut down of this operon, so it's a kind of repressable operon. Further this repression is enhanced by presence of leader sequence in 5' UTR of operon.

This leader sequence consists of 4 sequence which are complementary to each other

If 1-2 pair and 3- 4 pair , pre mature termination, terminate expression of operon

If 2- 3 pair anti terminator, allow expression of operon

This is mediated by presence of two tryptophan amino acid cofin at 11 and 12 of the leader sequence ( first sequence).

If tryptophan concentration is low- ribosome stall at first sequence, now 2 and 3 sequence pair -- form anti terminator- allow expression of tryptophan operon and synthesis of tryptophan.

If tryptophan concentration is high - rapidly tryptophan is added at codon 11 and 12 .--- ribosome rapidally each to 2nd sequence and now 3 and 4 pair firm terminator..expression of operon stop --- denotes that already has high tryptophan concentration in cell.

These 2 tryptophan plays a crucial role in regulation of the operon. If these two are replaced with tyrosine codons than the operon will always Express. As tryptophan mediated repression will not occur because now tryptophan is nor added in the mRNA.