Trp Operon Overview
Trp structural genes
Attenuator Sequence
PTrp
O
1
AUG
E
AUG
UGGUGG
2
UGA
*
AUG
D
*
AUG
3
C
*
AUG
B
*
AUG
A
*
4
UUUUUUUUU
The Trp operon is controlled by negative regulation. A repressor protein (Trp repressor) binds to the operator sequence (O)
and prevents transcription. The repressor can only bind DNA if it is simultaneously binding a molecule of tryptophan. Thus
when trp is present the trp operon is repressed by the trp repressor and turned off.
When the amount of trp present in the bacteria decreases, trp dissociates from the trp repressor and the trp repressor can no
longer bind the operator and txn occurs. However, there is still some trp present (moderate amounts) so the operon does not
need to be actively transcribed
Within the attenuator sequence, there are two sets of imperfect, inverted repeats. Each pair of repeats is indicated above as a
colored arrow. Within repeat 1, are two trp codons (UGGUGG). Region 2 and region 3 (including dashed portion) form an
inverted repeat with a single mismatch. These repeats can therefore form three possible hairpin secondary structures in the
newly transcribed mRNA.
In the presence moderate amounts of trp, repeats 1 & 2 will form a hairpin, leaving repeat 3 (solid portion) and repeat 4 free to
form a hairpin. Formation of the 3-4 hairpin causes termination of transcription due to the presence of the 9 U’s immediately
following repeat 4. This is a classical rho-independent terminator sequence. (View Slides 2-4)
In the near absence of trp, ribosomes which bind and begin translation at the AUG just upstream of repeat 1 will pause when
they reach the two adjacent trp codons within repeat 1. The presence of the ribosome at this position prevents a hairpin from
forming between repeats1 and 2. Therefore, as RNA polymerase continues to transcribe the leader sequence, repeat 2 forms
a hairpin structure with repeat 3 (dashed and solid portions) as soon as repeat 3 is transcribed. With repeats 2 & 3 forming a
hairpin structure, repeat 4 remains single stranded and a termination hairpin is not formed. Termination does not occur and
there is no attenuation of transcription. RNA polymerase continues through the remainder of the leader region and on through
the structural genes E, D, C, B & A. (View Slides 5-7)
Next Slide
Attenuation occurs in the presence of moderate amounts of trp
1
AUG
UGGUGG
2
3
UGA
4
UUUUUUUUU
UUUUUUUUU
Ultimately, this is the secondary structure which will form if Trp is present
Next Slide
Attenuation is overcome when Trp levels are very low
1
AUG
2
UGGUGG
1
UGGUGG
4
UUUUUUUUU
2
AUG
3
UGA
UGA
3
4
UUUUUUUUU
Stalled Ribosome
Ultimately, this is the secondary structure formed at low concentrations of Trp
Next Slide
Attenuation occurs at moderately low levels of Trp
RNA polymerase
AUG
Transcription of the Trp operon has begun. RNA
polymerase has transcribed from the +1 into the trp
leader sequence. The line and boxes represent newly
synthesized mRNA from the trp operon.
UGG
The following 8 slides will play automatically to animate the process of attenuation.
To begin, click the button below
Attenuation occurs at moderately low levels of Trp
A ribosome has bound to the 1st AUG of the leader
sequence and begun translation. RNA polymerase has
continued transcription into region 2
Ribosome
RNA polymerase
1
AUG
UGGUGG
2
UGA
Attenuation occurs at moderately low levels of Trp
Trp
Trp-tRNA
Ribosome
Short peptide
RNA polymerase
1
AUG
UGGUGG
2
UGA
RNA polymerase continues transcription of the leader
sequence. Translation proceeds to the two trp codons. Since
there is some (moderate) amount of trp ( ) in the cell, TrptRNATrp is produced and can be used by the ribosome.
Attenuation occurs at moderately low levels of Trp
Termination
factor
Short peptide
Ribosome
RNA polymerase
1
AUG
UGGUGG
2
3
UGA
Since trp is present, translation continues through the two trp codons
on to the stop codon. Transcription has continued into region 3.
Attenuation occurs at moderately low levels of Trp
Translation terminates once the ribosome
reaches the stop codon. RNA polymerase
has continued transcription into region 4.
RNA polymerase
1
AUG
UGGUGG
2
UGA
Ribosome dissociates
after reaching stop
codon
Short peptide will
be degraded
3
4
Attenuation occurs at moderately low levels of Trp
Once the ribosome dissociates, regions 1 & 2 can and will form a hairpin structure.
RNA polymerase
AUG
UGGUGG
1
3
2
1-2 hairpin
Dissociated ribosome
Attenuation occurs at moderately low levels of Trp
With regions 1& 2 in a hairpin, regions 3 & 4 can form a similar structure
AUG
UGGUGG
1
RNA polymerase
2
UUUUUUUUU
3
4
Attenuation occurs at moderately low levels of Trp
Formation of the 3-4 hairpin, results in transcription termination
AUG
UGGUGG
UUUUUUUUU
RNA polymerase
dissociates after transcribing
poly U stretch
Next
Attenuation Animation
Click the button to see a faster animated sequence of steps for attenuation just showing the
events without all the text.
Attenuation Animation
AUG
UGG
RP
Attenuation Animation
1
AUG
UGGUGG
2
UGA
RP
Attenuation Animation
1
AUG
UGGUGG
2
UGA
RP
Attenuation Animation
1
AUG
UGGUGG
2
3
UGA
RP
Attenuation Animation
1
AUG
UGGUGG
2
3
4
UGA
RP
Attenuation Animation
3
1
2
RP
Attenuation Animation
AUG
UGGUGG
1
3
2
RP
Attenuation Animation
AUG
UGGUGG
1
2
UUU
RP
3
4
Attenuation Animation
AUG
UGGUGG
1
2
UUUUUUUUU
3
RP
4
Attenuation Animation
AUG
UGGUGG
UUUUUUUUU
Very low levels of Trp prevent attenuation allowing
Trp EDCBA genes to be transcribed
RNA polymerase
AUG
UGG
The following 5 slides show lack of attenuation in the near absence of Trp
Click the button below to view
Very low levels of Trp prevent attenuation allowing
Trp EDCBA genes to be transcribed
Ribosome begins translation, but stalls when it reaches 2 trp codons in leader sequence
Ribosome
RNA polymerase
1
AUG
UGGUGG
2
UGA
Very low levels of Trp prevent attenuation allowing
Trp EDCBA genes to be transcribed
RNA polymerase continues to transcribe the leader sequence
Ribosome
Paused on
Trp codons
RNA polymerase
1
AUG
UGGUGG
2
UGA
Very low levels of Trp prevent attenuation allowing
Trp EDCBA genes to be transcribed
Ribosome
Paused on
Trp codons
RNA polymerase
1
AUG
UGGUGG
2
UGA
3
4
Very low levels of Trp prevent attenuation allowing
Trp EDCBA genes to be transcribed
3
Ribosome
Paused on
Trp codons
2
4
1
AUG
UGGUGG
RNA polymerase
UGA
UUUUUUUUU
Since the ribosome is stalled on region 1, region 2 & 3 are avialable to form a hairpin
structure while RNA polymerase finishes transcribing region 4
Very low levels of Trp prevent attenuation allowing
Trp EDCBA genes to be transcribed
3
Ribosome
Paused on
Trp codons
2
4
1
AUG
UGGUGG
RNA polymerase
UGA
UUUUUUUUU
With the formation of the 2-3 hairpin, there is hairpin formed immediately upstream of the poly U stretch.
This allows RNA polymerase to continue transcription of the remainder of the operon
Replay