expalins about detailed mechanism of the trp operon and its attenuation

1. Tryptophan operon
By
Gandholi Neeraj Kalyan
Regd no. 21151
I Msc Biosciences
SSSIHL, PSN CAMPUS
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2. Outline
 What is operon ?
 History of the trp operon
 Structure of trp operon
 Tryptophan : Effect on negative control
 Transcriptional attenuation
 Acknowledgments
 References
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4. Types of operons
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Inducible operons
Genes are transcribed and
translated to form enzymes only
when the inducer molecule is
present in the environment
Ex: Lac operon
Repressible operons
• Found in anabolic pathway
• Functioning of operon is stopped
when concentration of end
product cross threshold value
• Ex: Trp operon in E.coli

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6. • It also contains the leader region trpL
coding for the leader peptide and an
RNA attenuator.
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9. Tryptophan: Effect on Negative Control
Low Tryptophan  no repression
Repression: tryptophan is a
co-repressor  binds
(inactive) apo-repressor
converting it to active
repressor
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Transcriptional attenuation
There are two different processes:
1. Translation mediated transcription attenuation.
2. RNA binding protein mediated attenuation

11. • The 14 amino acid peptide formed from the leader
sequence has 2 tryptophans.
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• Involves in transcriptional-translational coupling
attenuation.

12. LOW or NO Trp Conditions
• ribosome will stall at trp sites.
• The trp site is right in the
middle of region 1 of the
attenuator
• The stalled ribosome
prevents the formation of
stem loops 1-2 3-4 and
promote the formation of
stem loop structure 2-3
• whole operon mRNA made
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Translation mediated transcription attenuation

13. Translation mediated transcription attenuation
At HIGH Trp Conditions
• When high levels of Trp-tRNA
ribosome goes through.
• Ribosome continues through
element 1 (no stalling) and
reaches stop signal (UGA)
• With no ribosome stem loops 1-
2/3-4 form on the mRNA halting
transcription before polymerase
has chance to reach trp structural
genes.
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RNA binding protein-mediated attenuation
• In B. subtilis, transcriptional attenuation is
mediated by RNA binding protein called TRAP.
• TRAP forms multimer of 11 subunits
• TRAP is activated by tryptophan binding
• Then TRAP binds to leader segment form hairpin
and cause termination of transcription.
• Anti-TRAP molecule prevents TRAP from repressing
the trp operon.

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Summary
• The trp operon is a negatively induced operon
• Trp operon contains promoter region operator sequence regulatory
genes and also the structural genes
• The structural genes code for the various enzymes involved in the
tryptophan biosynthesis.
• Apo-repressor + co-repressor = holorepressor
(trp repressor) (tryptophan) active repressor
• In low trp conditions, stem loop 2-3 is formed, the transcription
continues
• In high trp conditions , stem loops 1-2, 3-4 are formed and transcription
is terminated.
• The RNA binding protein (TRAP) plays a role in mediation of the
attenuation.

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References
• https://www.slideshare.net/georgeoajr/trp-operon-107712378
• http://www.discoveryandinnovation.com/BIOL202/notes/lecture17.html
• https://en.wikipedia.org/wiki/Trp_operon
• https://www.youtube.com/watch?v=EvLy_1_Y3tk
• PBIO-102 Lecture notes

17. Acknowledgments
• I thank Dr. B.E Pradeep sir for giving me this wonderful
opportunity.
• I thank the pathfinder team for providing the substantial study
material which helped me a lot for the preparation of this
presentation.
• Finally, I thank my friends and my family members for their
constant support and encouragement.
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