Using recombinant DNA techniques, you have replaced the trp operon's leader peptide open reading frame in Escherichia coli with the one from the phe (phenylalanine) operon.

How will this affect the expression of the tryptophan structural genes in the presence or absence of tryptophan? Explain the rationale for your response.

Decreased transcription of structural genes of try operon irrespective of presence or absence of tryptophan. The leader sequence has a tryptophan codon in its sequence 1 that does not allow the formation of attenuator structure when tryptophan levels are low.

Explanation:

The leader peptide has a tryptophan codon in sequence 1. When tryptophan levels are low, translation of sequence 1 pauses at the tryptophan codon and the sequences 2 and 3 pair with each other. This does not allow pairing of sequences 3 and 4 and the formation of the attenuator is inhibited. Therefore, tryptophan operon is expressed when tryptophan levels are low.

When the leader peptide sequence is replaced with the one from phenylalanine operon, expression of tryptophan operon will be reduced. The replaced sequence lacks a tryptophan codon and therefore, the translation would not pause when the tryptophan levels are low. This would allow the ribosome to quickly translate the sequence 2. Sequences 3 and 4 will pair to form attenuator and the gene expression will be reduced irrespective of low levels of tryptophan.