Trp operon

1. the tryptophan operon:

2. Tryptophan Operon
 Structural genes
 trp E, trpD, trpC trpB
& trpA
 Common promoter
 Regulatory Gene
 Apo-Repressor
 Inactive
 Operator
 Leader
 Co-repressor
 Tryptophan
R
Operon
Regulatory
Gene
P O E D C
5 Proteins
B A
L
Inactive repressor
(apo-repressor)

3. Tryptophan Operon
 Co-repressor --
tryptophan
 Absence of tryptophan
 Gene expression
R P O E D C
5 Proteins
B A
L
Inactive repressor
(apo-repressor)
Absence of Tryptophan
R P O E D C
No trp mRNA
B A
L
Presence of Tryptophan
Inactive repressor
(apo-repressor)
Trp
(co-repressor)
 Presence of tryptophan
 Activates repressor
 No gene expression
 Negative control

5. The trp operon encodes five structural genes
required for tryptophan synthesis.
These genes are regulated to efficiently express
only when tryptophan is limiting.
There are two layers of regulation involved:
(1) transcription repression by the Trp
repressor (initiation); (2) attenuation

6.  It also contains a repressive regulator gene
called trp R.
 Trp R has a promoter where RNA
polymerase binds and synthesizes mRNA
for a regulatory protein.
 The protein that is synthesized by trp R then
binds to the operator which then causes the
transcription to be blocked.

8. The Trp repressor
---the first layer of regulation
When tryptophan is present, it binds the Trp repressor
and induces a conformational change in that protein,
enabling it to bind the trp operator and prevent
transcription.
When the tryptophan concentration is low, the Trp
repressor is free of its corepressor and vacates its
operator, allowing the synthesis of trp mRNA to
commence from the adjacent promoter.

11. This section lies between the operator and the first gene
of the operon and is called the leader.
The leader encodes a short polypeptide and also contains
an attenuator sequence.

12. Attenuation
---the second layer of regulation
161 nucleotides of RNA are made from tryptophan
promoter before RNA polymerase encounters the first
codon of trpE.
Near the end of this leader sequence, and before
trpE , is a transcription terminator, composed of a
characteristic hairpin loop in the RNA.

13. The hairpin loop is followed by 8 uridine residues.
At this so-called attenuator , transcription usually
stops,yielding a leader RNA 139 nucleotides long.

14. Three features of the leader sequence:
1. There is a second hairpin (besides the terminator
hairpin) that can form between regions 1 and 2 of
the leader sequence.
2. region 2 also is complementary to region 3; thus ,
yet another hairpin consisting of regions 2 and 3
can form and when it does prevent the terminator
hairpin (3,4) from forming.

15. 3. The sequence encoding the leader peptide has a
striking feature : two tyrptophan codons in a row.
The function of these codons is to stop a ribosome
attempting to translate the leader peptide.

16.  The polypeptide encoded by the leader is short, just 14
amino acids long, and it includes two tryptophan (Trp)
residues. The tryptophans are important because:
1. If there is plenty of tryptophan, the ribosome won't have to
wait long for a tryptophan-carrying tRNA, and will rapidly
finish the leader polypeptide.
2. If there is little tryptophan, the ribosome will stall at the Trp
codons (waiting for a Trp-carrying tRNA) and will be slow
to finish translation of the leader.

17. Anti-terminator hair pin loop