Genetics - Ch 10

1
Genetics - Ch 10

• Molecular Mechanisms
• of Gene Regulation

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Control of Gene Expression
DNA -------------gt RNA ------------gt
protein
transcriptional control
translational control
Operon (prokaryotes) several adjacent genes
regulated together and coding for proteins
involved in a common process
Operator Repressor binding site Promoter
transcriptional activation site
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Repressible Vs. Inducible Operons

• Repressible System in biosynthetic
• (anabolic) pathways
• substrate present -gt operon turned off
• Trp Operon
  

Inducible System in degradative
(catabolic) pathways substrate present -gt
operon turned on Lac Operon
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Lac Operon
to metabolize (break down) lactose
constitutive
inducible

• Z protein ß-galactosidase
• cleaves lactose --gt glu gal

b) shifts bonds in lactose --gt allolactose
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Lac Operon Inducer
always some repressors in cell

• allolactose is an inducer
• makes repressor fall off operator
• transcription occurs

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Lac Operon Enhancer
CRP protein cAMP cAMP receptor
protein cyclic adenosine monophosphate

• CRP/cAMP enhancer
• binds to activator site upstream
• from Promoter
• helps RNA pol to attach to
• Promoter

low glucose --gt high cAMP high glucose --gt low
cAMP
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Lac Operon under Different Cell Conditions
what if?
no glu, high lac
high cAMP, high allolac
high transcription
high glu, no lac
low cAMP, low allolac
no transcription
no glu, no lac
high cAMP, no allolac
no transcription
low cAMP, some allolac not enhanced, induced
low transcription
some glu lac
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Mutated Lac Operon Areas
what if ?
I- no repressor Lac operon turned on
Oc repressor cant bind Lac operon turned on
to operator
I-d repressor cant bind Lac operon turned on
to operator
Is allolactose cant bind Lac operon turned
off to repressor
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Trp Operon
for production of tryptophan a.a.
10
Trp Operon Repressible System
Trp --gt Trp operon turned off
aporepessor (always present) combines with
co-repressor (tryptophan) to form functional
repressor
negative feedback loop
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Trp Operon Controlled by Attenuation
trpR P O 1 2 3 4 trpE trpD
trp C trp B trpA
attenuator
Leader
Attenuation

• attenuation can form
• under certain conditions
• base-pairing can occur
• between
• 1 - 2 2 - 3 3 - 4

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High levels of Tryptophan in Cell

• transcription translation occur simultaneously
  in Prokaryotes
• leader transcript (1) has 2 trp codons (UGGUGG)
• ribosomes moves fast along transcript
• stem-loop 3 - 4 fprms, poly Us after
• early termination of transcription,
• translation stops
• (only leader peptide forms
• - has no function)

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Low Levels of Tryptophan in Cell

• ribosome stalls at UGGUGG in leader transcript
  (1)
• stem-loop 2 - 3 forms, no poly U after
• transcription continues

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Low Levels of other Amino Acids

• ribosome stalls way early
• stem-loops 1-2 3-4 form, poly U after
• early termination of transcription