The tryptophan (trp) operon regulates gene expression in response to intracellular tryptophan levels through both repression and attenuation. When tryptophan is low, the trp repressor is inactive and transcription proceeds unimpeded. Additionally, ribosome stalling at trp codons in the 5' UTR prevents transcriptional termination through attenuation. Together, this maximizes production of tryptophan synthesis enzymes. When tryptophan is high, it activates the trp repressor which binds DNA and inhibits transcription. It also enables fast ribosome transit through the 5' UTR, allowing an alternative RNA structure that signals premature termination before the enzyme genes.

1. Tryptophan Operon

2. Precursor
Feedback
inhibition
Enzyme 1
Enzyme 2
Enzyme 3
Tryptophan
(a) (b)Regulation of enzyme
activity
Regulation of enzyme
production
Regulation
of gene
expression


trpE gene
trpD gene
trpC gene
trpB gene
trpA gene

3. DNA Function RNA/Protein Function
Trp R Gene for
repressor
Binds to operator to inhibit
transcription
P Promoter
O Operator
Trp E, D,
C, B, A
Structural
genes
Enzymes acting in pathway to
produce tryptophan.
Gene order correlates with
order of reactions in pathway.
5’ UTR
(Leader)
Premature termination of
transcription when trp levels are
high
Tryptophan Operon

4. • The operon can be switched off by a protein repressor
• The repressor prevents gene transcription by binding
to the operator and blocking RNA polymerase
• The repressor is the product of a separate regulatory
gene

5. • The repressor can be in an active or inactive form,
depending on the presence of other molecules
• A corepressor is a molecule that cooperates with a
repressor protein to switch an operon off
• For example, E. coli can synthesize the amino acid
tryptophan

6. • By default the trp operon is on and the genes for
tryptophan synthesis are transcribed
• When tryptophan is present, it binds to the trp
repressor protein, which turns the operon off
• The repressor is active only in the presence of its
corepressor tryptophan; thus the trp operon is turned
off (repressed) if tryptophan levels are high
© 2011 Pearson Education, Inc.

7. Control of Trp Operon Transcription
Trp Repressor is Inactive  Initial State: ON
Trp binding activates Repressor  Final State: OFF
tryptophan

8. Low tryptophan High tryptophan

9. Features of the 5’ UTR
•Contains complementary sequences that can form
hairpin structures when transcribed into RNA
•Codes for a stretch of U nucleotides that can act as
a termination signal after a hairpin structure
•Codes for several Trp codons as part of an
unstable protein product

10. 1. Operator site lies within the promoter
2. Allosteric transition
Allosteric protein-protein whose shape is changed
upon binding of a particular molecule  In the new
conformation the protein’s ability to react to a second
molecule is altered
3. Trp operon has another level of control  attenuation
4. Repressor lowers transcription 70-fold (as compared
to derepressed state)  attentuation permits another
10-fold control  total dynamic range of control =
700-fold

11. …
…
Attenuator Region of Trp Operon

12. Low tryptophan: transcription of trp operon genes RNA polymerase reads through attenuator.

13. High tryptophan: attenuation, premature termination  attenuator causes premature
termination of transcription
1. Attenuator region contains transcription stop signal (terminator)  not STOP codon!
2. The terminator consists of an inverted repeat followed by string of eight A-T pairs.

14. 3. The inverted repeat forms a hairpin loop.
4. When RNA polymerase reaches string of U’s…

15. …the polymerase pauses, the hairpin forms
 Transcript is released
 Termination occurs before transcription reaches the
trp (structural) genes

16. Attenuation gives some insight into how the operon is
shut down, but how does the cell activate trp operon
expression (i.e. defeat attenuation)?
preventing hairpin formation would destroy
termination signal  transcription would proceed

17. …
…
Mechanism of Attenuation

18. Key insight: mRNA produced from attenuator region can fold
into two different secondary structures
Stem loops: 1-2, 3-4 Stem loop: 2-3

19. 1.Formation of stem loop structures; 1-2 and 3-4
is more stable and results in the formation of a
termination (hairpin loop) structure/signal.
2.Formation of stem loop structure 2-3 would
result in the disruption of stem loops 1-2/3-4.
3.The stem loop structure formed between 2-3
does not result in termination signal 
transcription would proceed.
Q. How does the less stable structure (stem-loop
2-3) form?

20. The Importance of the Leader Region
-the 14 amino acid peptide formed from the leader sequence has 2
tryptophans.
-trp is a “rare” amino acid

21. 1. Recall that in bacteria, translation typically occurs
almost simultaneously with transcription.

22. 2. Thus, as soon as trp leader region is transcribed,
translation begins.
Consider LOW Trp Conditions
3. During low tryptophan concentration, ribosome will
stall at trp sites.
4. The trp site is right in the middle of region 1 of the
attenuator
 Meanwhile RNA polymerase continues to transcribe

23. The stalled ribosome prevents the formation of stem loops 1-2/3-4
and promote the formation of stem loop structure 2-3

24. 1. Stem loop structure 2-3 does not result in
transcriptional termination  whole operon mRNA
made.
2. What happens to the stalled ribosome?
(i) Since the genes in the operon have their own start
sites other ribosomes can come and translate those
proteins
(ii)Stalled ribosome can eventually either incorporate
trp-tRNA (+ 3 more a.a. before reaching stop codon)
or dissociate from mRNA

25. At HIGH Trp Conditions
1. When high levels of Trp-tRNA are present the two
tryptophan codons do not represent a barrier
translation  ribosome breezes through.
2. Ribosome continues through element 1 (no stalling)
and reaches stop signal (UGA)
3. With no ribosome  stem loops 1-2/2-3 form on
the mRNA  halting transcription before
polymerase has chance to reach trp structural
genes.

26. Effect on ribosome
and transcription at
HIGH Trp levels
Note: the 14 amino
acid leader peptide is
synthesized

27. -This mechanism involves: transcriptional-translational
coupling.
-Relies on rate of transcription & translation to be comparable
 if RNA polymerase >> ribosome, it might pass through
attenuator region before ribosome had a chance to stall at the
tryptophan codons.

28. Alternative RNA Structures from 5’ UTR
Termination signal due to
hairpin formed by 3+4 pairing
followed by string of uracils
No termination
signal formed
Formation of termination signal depends on
level of tryptophan carried by tRNA in the cell.

29. Attenuation
Premature Termination of Transcription
Ribosome translates
trp codons, preventing 2+3 pairing
3+4 pairing forms terminator

30. Antitermination
Ribosome stalls at trp codons,
allowing 2+3 pairing
Transcription continues
toward trp E, D, C. B, A

31. Summary of Trp Operon Regulation
Level of
Tryptophan
Trp Operon
Low
High
On
Trp repressor inactive
Lack of attenuation leads to high rate of
mRNA production
Off
Tryptophan + repressor = Active repressor
Reduction of mRNA production by attenuation