Download Lac Operon

Genetics - Ch 10
Molecular Mechanisms
of Gene Regulation
Control of Gene Expression
DNA ------------->
transcriptional
control
RNA ------------> protein
translational
control
Operon: (prokaryotes) several adjacent genes regulated
together and coding for proteins involved in a common
process
repressor gene
promoter operator
gene A gene B gene C
Operator: Repressor binding site
Promoter: transcriptional activation site
Repressible Vs. Inducible Operons
Repressible System: in biosynthetic
(anabolic) pathways
repressible
system
substrate present -> operon turned off
Trp Operon
Inducible System: in degradative
(catabolic) pathways
substrate present -> operon turned on
Lac Operon
Inducible
system
Lac Operon
to metabolize (break down) lactose
constitutive
inducible
Z protein= ß-galactosidase
a) cleaves lactose --> glu + gal
b) shifts bonds in lactose --> allolactose
Lac Operon: Inducer
always some repressors in cell
allolactose is an inducer:
•makes repressor fall off operator
•transcription occurs
Lac Operon: Enhancer
CRP protein:
cAMP receptor protein
CRP/cAMP: enhancer
•binds to activator site upstream
from Promoter
•helps RNA pol to attach to
Promoter
low glucose --> high cAMP
high glucose --> low cAMP
cAMP:
cyclic adenosine monophosphate
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
some glu & lac
low cAMP, some allolac
not enhanced, induced
low transcription
Mutated Lac Operon & Areas
what if …?
I-
no repressor
Lac operon turned on
Oc
repressor can’t bind
to operator
Lac operon turned on
I-d
repressor can’t bind
to operator
Lac operon turned on
Is
allolactose can’t bind
to repressor
Lac operon turned off
Trp Operon
for production of tryptophan a.a.
Trp Operon: Repressible System
[Trp] --> Trp operon turned off
trpR
aporepessor (always present)
combines with
co-repressor (tryptophan)
to form
functional repressor
negative feedback loop
trpR
Trp Operon Controlled by Attenuation
trpR
P O
1 2 3 4
Leader
trpE trpD trp C trp B trpA
attenuator
•attenuation can form
under certain conditions
•base-pairing can occur
between
1-2
2-3
3-4
Attenuation
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)
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
Low Levels of other Amino Acids
•ribosome stalls way early
•stem-loops 1-2 & 3-4 form, poly U after
•early termination of transcription