Chapter 17 Regulation of gene expression in bacteria: lac Operon of E. coli trp operon of E. coli.
Fine regulation in bacteria eg lac operon
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Transcript of Fine regulation in bacteria eg lac operon
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Fine control of transcription in bacteria
Fine control of transcription in bacteria
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B y S a m a neh P a l i z b a n
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The two well studied main mechanisms of transcriptional control of gene expression are:
1.The operons : genes involved in a metabolic pathway are regrouped into a gene cluster controlled by common regulatory sequences and proteins. The expression of these genes are then rapid and synchronized.
2. The cascades of gene expression : Under some environmental conditions, expression of a first set of genes can be “switch on”, and one or more of the products of this first gene set will “switch on” a second gene set. This event could be repeated many times to mobilize wider gene sets to achieve a special metabolic pathway.
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• The operon consists of several structural genes required for metabolism
• under the control of a single regulatory domain = coordinate regulation
• Repressor binding site = operator• Site for transcriptional activation = promoter
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Transcription Regulation in Prokaryotes
1. Negative Regulation = repressor binds to regulatory site to block transcription of active gene
2. Positive Regulation = Activator binds to regulatory site to stimulate transcription; gene is off in absence of activator
Negative regulation is more common in prokaryotes
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repressor protein blocks transcription
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Negative regulation:
1- Inducible operons:
- inducer antagonizes repressor to allow transcription initiation- activated by small molecule inducers- mode of regulation in degradative (catabolic) pathways
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Negative regulation:
1- Inducible operons
2. Repressible operons: - shut off by small molecule co-repressors; - mode of regulation in biosynthetic (anabolic) pathways
LAC OPERON- A catabolic operon
-Lactose degradation is regulated by the lactose (lac) operon
- Both positive and negative regulation
- Inducible - Repressilbe
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Lactose degradation is regulated by the lactose (lac) operon
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13 •The repressor is expressed constitutively (continuously) from the I gene and binds to the operator to block transcription
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15Conditions:Lactose –
Operon off (leaky)
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Conditions:Lactose +
18Conditions:Lactose + Glucose +
Operon off
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Adenylate cyclase
Active form
Conditions: glucose scarce
Subsequence: cAMP levels high
20Conditions:
Lactose + Glucose -
Abundant lac mRNA synthesized
operon on
21Positive Regulation of LAC operon
CRP-cAMP
cAMP
Structural genesP O CRP binding site
allo-lactose
22Negative Regulation of LAC operon
Repressor-allolactose
Allo-lactose
Structural genesP O CRP binding site
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