Tryptophan Operon Attentuation presentaton first year college level.

1. Attenuation in Tryptophan Operon
Attenuation or more specifically transcription attenuation is a mechanism of gene regulation it was first Charles Yanofsky in the trp
operon of E. coli. in 1977. Attenuation acts post transcriptionally. Basically it is a mechanism for reducing expression of the tryptophan
operon when levels of tryptophan are high. However, rather than blocking initiation of transcription, attenuation prevents completion of
transcription. It involves the presence of a stop signal that indicates premature termination. The stop signal, referred to as the
attenuator, prevents the proper function of the ribosomal complex, stopping the process.
• The 5′ end of the polycistronic mRNA transcribed
from the trp operon has a leader sequence upstream
of the coding region of the trpE structural gene.
• This leader sequence encodes a 14 amino acid
leader peptide containing two tryptophan residues.
• The function of the leader sequence is to fine
tune expression of the trp operon based on the
availability of tryptophan inside the cell.

2. The leader sequence contains four regions
(numbered 1–4) that can form a variety of
base paired stem-loop (‘hairpin’) secondary
structures.
• The regions are: Region 1, region 2, region 3
and Region 4. Region 3 is complementary to
both region 2 and region 4.
• If region 3 and region 4 base pair with each other,
they form a loop like structure called attenuator
and it function as transcriptional termination.
If pairing occur between region 3 and region 2, then
no such attenuator form so that transcription
continues.
Attenuation depends on the fact that, in bacteria, ribosomes attach to mRNA as it is being synthesized and so translation starts
even before transcription of the whole mRNA is complete.