Trp operon

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
PO E D C
5 Proteins
B AL
Inactive repressor
(apo-repressor)

3. Tryptophan Operon
Co-repressor --
tryptophan
Absence of tryptophan
Gene expression
R PO E D C
5 Proteins
B AL
Inactive repressor
(apo-repressor)
Absence of Tryptophan
R PO E D C
No trp mRNA
B AL
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. Italsocontainsarepressiveregulatorgene
calledtrpR.
TrpRhasapromoterwhereRNA
polymerasebindsandsynthesizesmRNA
foraregulatoryprotein.
TheproteinthatissynthesizedbytrpRthen
bindstotheoperatorwhichthencausesthe
transcriptiontobeblocked.

8. TheTrprepressor
---thefirstlayerofregulation
Whentryptophanispresent,itbindstheTrprepressor
andinducesaconformationalchangeinthatprotein,
enablingittobindthetrpoperatorandprevent
transcription.
Whenthetryptophanconcentrationislow,theTrp
repressorisfreeofitscorepressorandvacatesits
operator,allowingthesynthesisoftrpmRNAto
commencefromtheadjacentpromoter.

11. Thissectionliesbetweentheoperatorandthefirstgene
oftheoperonandiscalledtheleader.
Theleaderencodesashortpolypeptideandalsocontains
anattenuatorsequence.

12. Attenuation
---the second layer of regulation
161nucleotidesofRNAaremadefromtryptophan
promoterbeforeRNApolymeraseencountersthefirst
codonoftrpE.
Neartheendofthisleadersequence,andbefore
trpE,isatranscriptionterminator,composedofa
characteristichairpinloopintheRNA.

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 2of
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.Thesequenceencodingtheleaderpeptidehasa
strikingfeature:twotyrptophancodonsinarow.
Thefunctionofthesecodonsistostoparibosome
attemptingtotranslatetheleaderpeptide.

16. Thepolypeptideencodedbytheleaderisshort,just14
aminoacidslong,anditincludestwotryptophan(Trp)
residues.Thetryptophansareimportantbecause:
1.Ifthereisplentyoftryptophan,theribosomewon'thaveto
waitlongforatryptophan-carryingtRNA,andwillrapidly
finishtheleaderpolypeptide.
2.Ifthereislittletryptophan,theribosomewillstallattheTrp
codons(waitingforaTrp-carryingtRNA)andwillbeslow
tofinishtranslationoftheleader.

17. Anti-terminator hair pin loop