regulation of Transcription in prokaryotes can be done under operons and cascade of gene expression ; also we can divide it into negative and positive responses which the lac operon is a good example of it.

1. 1

2. Fine control of
transcription in bacteria
Fine control of
transcription in bacteria
2
By Samaneh
Palizban

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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

10. 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. 13 •The repressor is expressed constitutively (continuously)
from the I gene and binds to the operator to block
transcription

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Conditions:
Lactose –
Operon off (leaky)

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Conditions:
Lactose +

18. 18Conditions:
Lactose + Glucose +
Operon off

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Adenylate
cyclase
Active form
Conditions: glucose scarce
Subsequence: cAMP levels high

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Conditions:
Lactose + Glucose -
Abundant lac mRNA synthesized
operon on

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Positive Regulation of LAC operon
CRP-cAMP
cAMP
Structural genesP O
CRP binding site
allo-lactose

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Negative Regulation of LAC operon
Repressor-
allolactose
Allo-lactose
Structural genesP O
CRP binding site

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