he trp operon of E. coli controls the biosynthesis
of tryptophan in the cell from the initial
precursor chorismic acid.
This operon contains genes for the production
of five proteins which are used to produce
three enzymes.
The products of the E and D genes form a multimeric
protein comprised of two copies of each protein
to produce the enzyme anthranilate synthetase.
This enzyme catalyzes the first two reactions
in the tryptophan pathway.
The next enzyme, which is responsible for
catalyzing the next two steps in the pathway
is indole glycerolphosphate synthetase and
it is the product of the C locus.
The final step in the reaction is the pathway
produces tryptophan from indole-glycerol phosphate
and serine.
This single step is catalyzed by tryptophan
synthetase, an enzyme that is a multimer of
two proteins that are the product of the B
and A genes.
As with all operons, the trp operon consists
of the repressor, promoter, operator and the
structural genes.
In this system, though, unlike the lac operon,
the gene for the repressor is not adjacent
to the promoter, but rather is located in
another part of the E. coli genome.
Another difference is that the operator resides
entirely within the promoter
The trp operon is a repressible system.
The primary difference between repressible
and inducible systems is the result that occurs
when the effector molecule binds to the repressor.
With inducible systems, the binding of the
effector molecule to the repressor greatly
reduces the affinity of the repressor for
the operator, the repressor is released and
transcription proceeds.
The lac operon is an example of an inducible
system.
With repressible systems, the binding of the
effector molecule to the repressor greatly
increases the affinity of repressor for the
operator and the repressor binds and stops
transcription.
Thus, for the trp operon , the addition of
tryptophan (the effector molecule) to the
E. coli environment shuts off the system because
the repressors binds at the operator.
Inducible system - the effector molecule interacts
with the repressor protein such that it can
not bind to the operator
Repressible system - the effector molecule
interacts with the repressor protein such
that it can bind to the operator
Attenuation of the trp Operon
One element of the trp operon is the leader
sequence (L) that in immediately 5' of 
the trpE gene.
This sequence about 160 bp is size also controls
the expression of the operon through a process
called attentuation.
This sequence has four domains (1-4).
Domain 3 (nucleotides 108-121) of the mRNA
can base pair with either domain 2 (nucleotides
74-94) or domain 4 (nucleotides 126-134).
If domain 3 pairs with domain 4, a stem and
loop structure forms on the mRNA and transcription
stops.
This structure forms when the level of tryptophan
is high in the cell.
If domain 3 pairs with domain 2, then the
stem and loop structure does not form and
transcription continues through the operon,
and all of the enzymes required for 
tryptophan 
biosynthesis 
are produce.
These events occur when tryptophan is low
in 
the cell.
If domain 4 is deleted, the stem and loop
structure can not form and transcription of
the remainder of the operon will occur even
in the presence of tryptophan.
Domain 4 is called the attenuator because
its presence is required to reduce (attenuate)
mRNA transcription in the presence of high
levels of tryptophan.
Domain 1 is also an important component of
the attenuation process.
The section of the leader sequence encodes
a 14 amino acid peptide that has two tryptophan
residues.
How does this entire attentuation process
work?
We will discuss the molecular events that
occur under conditions of high and low tryptophan.
trp Operon Transcription Under High Levels
of Tryptophan
When the cellular levels of tryptophan are
high, the levels of the tryptophan tRNA are
also high.
Immediately after transcription, the mRNA
moves quickly through the ribosome complex
and the small peptide is translated.
Translation is quick because of the high levels
of tryptophan tRNA.
Because of the quick translation, domain 2
becomes associated with the ribosome complex.
Then domain 3 binds with domain 4, and transcription
is attenuated because of the stem and loop
formation.
trp Operon Transcription Under Low Levels
of 
Tryptophan
Under low cellular levels of tryptophan, the
translation of the short peptide on domain
1 is slow.
Because 
of the slow translation, domain 2 does not
become associated with the ribosome.
Rather domain 2 associates with domain 3.
This 
structure permits the continued 
transcription 
of the operon.
Then the trpE-A genes are translated, and
the biosynthesis of 
tryptophan occurs.
