Question

What would happen to expression of the trp operon if the 2 trp alleles (UGG) in the leader sequences were mutated to GGG (Gly) codons in the presence of tryptophan and absene of tryptophan?

Answer 1

Trp operon is an example of a negative repressible operon.
It is a biosynthetic operon that shows attenuation in the presence of tryptophan.
It contains a leader sequence that can sense the availability of trp levels in the cell.
If trp is abundant, the leader peptide forms a stem-loop structure which prevents that transcription of the structural genes in the operon.
If trp is less abundant, the leader peptide forms a different stem-loop structure which promotes that transcription of the structural genes in the operon.

The leader sequence contains codons for Trp which provide the operon with its sensitivity.
If Trp codons are replaced with glycine codons, the operon can not sense the levels of Trp in the cell as the ribosome does not stall at these codons if the cell contains ample Trp levels.

The operon will be active in the absence/presence of trp