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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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