Regulation of gene expression prokaryotes
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REGULATION OF GENE EXPRESSION PROKARYOTES. 3 LEVELS OF GENE EXPRESSION REGULATION. OVERVIEW OF REGULATORY MECHANISMS. CONTROL OF GENE EXPRESSION. CONTROL OF GENE EXPRESSION IN PROKARYOTES Enables bacteria to adjust their metabolism to environmental change

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REGULATION OF GENE EXPRESSION PROKARYOTES

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Regulation of gene expression prokaryotes

REGULATION OF GENE EXPRESSIONPROKARYOTES


Regulation of gene expression prokaryotes

3 LEVELS OF

GENE EXPRESSION

REGULATION


Regulation of gene expression prokaryotes

OVERVIEW OF

REGULATORY

MECHANISMS


Control of gene expression

CONTROL OF GENE EXPRESSION

  • CONTROL OF GENE EXPRESSION IN PROKARYOTES

    • Enables bacteria to adjust their metabolism to environmental change

    • Responses to environmental stimuli


Regulation of gene expression

REGULATION OF GENE EXPRESSION

  • REGULATION OF ENZYMATIC ACTIVITY


Enzyme regulation during metabolism control of enzymatic activity

ENZYME REGULATION DURING METABOLISM CONTROL OF ENZYMATIC ACTIVITY


Regulation by feedback inhibition

REGULATION BY FEEDBACK INHIBITION


Feedback inhibition

FEEDBACK INHIBITION

  • ISOLEUCINE SYNTHETIC PATHWAY


Catabolic operons

CATABOLIC OPERONS

INDUCIBLE ENZYMES


Control of gene expression1

CONTROL OF GENE EXPRESSION

  • CONTROL OF GENE EXPRESSION IN PROKARYOTES

    • Enzyme synthesis (Regulation of gene expression)

      • At the level of transcription of the genes coding for particular enzymes - control the # of enzyme molecules produced

      • Slower to take effect than feedback inhibition, but is more economical for the cell. It prevents unneeded protein synthesis for enzymes, as well as, unneeded pathway product

    • Examples illustrating regulation of a metabolic pathway is the tryptophan pathway in E. coli. Mechanisms for gene regulation were first discovered for E. coli

    • Current understanding of such regulatory mechanisms at the molecular level is primarily limited to bacterial systems

    • Reports on some eukaryotes & viruses*

      *Displacements of Prohead Protease Genes in the Late Operons of Double-Stranded-DNA Bacteriophages". Journal of Bacteriology. 1 March 2004. Retrieved 30 December 2012.


Control of gene expression2

CONTROL OF GENE EXPRESSION

  • OPERON MODEL-François Jacob and JaquesMonod (1961)-Regulated genes can be switched on/off depending on cell's metabolic needs

    • Basic Definitions

      • Operon = A regulated cluster of adjacent structural genes with related functions

      • Structural gene = Gene that codes for a polypeptide

        • Common in bacteria and phages

        • Has a single promoter region, so RNA polymerase will transcribe all structural genes on an all/none basis

        • Transcription produces a single polycistronic mRNA with coding sequences for all enzymes in a metabolic pathway . Prokaryotic transcription unit - 5 or more genes

        • Transcription —> Long mRNA molecule

        • Translation —> separate polypeptides


Lac operon

lac OPERON


Control of gene expression3

CONTROL OF GENE EXPRESSION

OPERONoperons.swf

  • OPERON MODEL-François Jacob and Jaques Monod (1961)

    • Basic Definitions

      • Polycistronic mRNA = A large mRNA molecule that is a transcript of several genes

        • Is translated into separate polypeptides

        • Contains stop and start codons for the translation of each polypeptide

      • Grouping structural genes into operons provides an advantage b/c:

        • Expression of all genes can be coordinated. When a cell needs the product of a metabolic pathway, all the necessary enzymes are synthesized at one time.

        • The entire operon can be controlled by a single operator


Control of gene expression in prokaryotes the lactose utilization operon

CONTROL OF GENE EXPRESSION IN PROKARYOTESTHE LACTOSE UTILIZATION OPERON

  • Basic Definitions

    • Inducible operon

    • Operator-(between promoter and structural genes/within promoter)-controls access to RNA polymerase to the structural genes

    • Repressor protein-binds to the operator and blocks the attachment of RNA polymerase to the promoter

      • Repressor protein encoded by regulatory gene

      • Corepressor-usually observed in biosynthetic operons (trp operon)

    • Structural genes

    • lac operon-a catabolic operon

      • lacI-regulatory gene-encodes repressor protein


Control of gene expression in prokaryotesthe lactose utilization operon

CONTROL OF GENE EXPRESSION IN PROKARYOTESTHE LACTOSE UTILIZATION OPERON

  • Structural Genes

    • lacZ-b-galactosidase (lactose Glucose + Galactose)

    • lacY-permease

    • lacA-transacetylase

  • Inducer-allolactose (an isomer of lactose)

    • Inducer present-operon active-synthesis of enzymes for metabolism of lactose

    • Inducer absent-operon inactive-active repressor binding to the operator prevents access to RNA polymerase

      • Basal levels of lactose metabolic enzymes due to unstable interaction between repressor protein and operator

  • AN EXAMPLE OF NEGATIVE REGULATION


Animation ch 8 operons induction

lac OPERON

Animation Ch. 8 Operons Induction

Regulatory

gene

Promoter

Operator

lacZ

lacI

DNA

No

RNA

made

3

mRNA

RNA

polymerase

5

Active

repressor

Protein

(a) Lactose absent, repressor active, operon off

lac operon

lacZ

DNA

lacI

lacY

lacA

RNA

polymerase

3

mRNA

mRNA 5

5

Permease

-Galactosidase

Transacetylase

Protein

Inactive

repressor

Allolactose

(inducer)

(b) Lactose present, repressor inactive, operon on


Lac repressor interacting with dna

LAC REPRESSOR INTERACTING WITH DNA


Regulation of gene expression prokaryotes

(Constitutive synthesis)


Overview

OVERVIEW

  • POSITIVE & NEGATIVE CONTROL OF GENE EXPRESSION


Regulation of gene expression prokaryotes

This one

Lac operon: CAP (Catabolic activator protein)


Lac operon1

lac OPERON

  • POSITIVE REGULATION OF THE LAC OPERON

    • Positive control of a regulatory system occurs only if an activator molecule interacts directly with the genome to turn on transcription (lac operon)

    • lac operon is under dual regulation that includes negative control by repressor protein and positive control by cAMP receptor protein (CAP)

    • CAP (gen: crp) = An allosteric protein that binds cAMP and activates transcription binding to an operon's promoter region (enhances the promoter's affinity for RNA polymerase)

    • cAMP-CAP-positive activator of lactose metabolic enzyme synthesis (facilitates RNA pol. binding to the promoter-if glucose is absent

    • Glu absent: cAMP high Glu present: cAMP low


Lac operon2

lac OPERON

  • POSITIVE CONTROL

    • E. coli preferentially uses glucose over lactose as a substrate for glycolysis (Higher efficiency)

    • Therefore, normal expression of the lac operon requires:

      • Presence of lactose

      • Absence of glucose (crp:cAMP receptor protein gene )

    • When Glu concentration decreases, cAMP increases


Lac operon3

lac OPERON

  • POSITIVE CONTROL

  • How is CAP affected by the absence or presence of glucose?

    • When glucose missing, cell accumulates cAMP, a nucleotide derived from ATP.

    • cAMP activates CAP so that it can bind to the lac promoter

    • When glucose concentration rises, glucose catabolism decreases the cAMP concentration


Positive regulation of the lac operon

POSITIVE REGULATION OF THE lac OPERON

[GLUCOSE]

Present

Absent

[cAMP] becomes scarce

[cAMP] rises

cAMP loses CAP

cAMP binds CAP

cAMP-CAP complex

binds lac promoter

CRP disengages from

lac promoter

Efficient transcription

of lac operon

Slow transcription

of lac operon


Lac operon4

lac OPERON

Self quiz link: http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter12/animation_quiz_4.html

* lac operonhttp://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter12/animation_quiz_4.html

lac operonhttp://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter18/animations.html

lac operonoperons_induction.swf


Dual regulation of lac operon

DUAL REGULATION OF lac OPERON

  • IN THIS TYPE OF REGULATION

    • Negative control by repressor determines whether or not the operon will transcribe the structural genes

    • Positive control by CRP determines the rate of transcription (slow vs. efficient)

    • E. colieconomizes on RNA/protein synthesis with the help of these negative and positive controls

    • CRP is an activator of several different operons that program catabolic pathways

    • Glucose's presence deactivates CRP. This, slows the synthesis of those enzymes a cell needs to use catabolites other than glucose

    • E. coli preferentially uses glucose as its primary carbon and energy source, and the enzymes for glucose catabolism are coded for by unregulated genes that are continuously transcribed (constitutive)


Dual regulation of the lac operon

DUAL REGULATION OF THE lac OPERON

  • IN THIS TYPE OF REGULATION

    • Therefore, when glucose is present, CRP does not work and the cell's systems for using secondary energy sources are inactive

    • When glucose is absent, the cell metabolizes alternate energy sources

      • The cAMP level rises, CRP is activated and transcription begins of operons that program the use of alternate energy sources (e.g., lactose)

      • Which operon is actually transcribed depends upon nutrient availability

        • Example: If lactose is present, the lac operon will be switched on as allolactose inactivates the repressor


Anabolic operons

ANABOLIC OPERONS

REPRESSIBLE ENZYMES


Repressible operons

REPRESSIBLE OPERONS

  • REPRESSIBLE ENZYMES

    • Their synthesis is inhibited by the specific metabolite

      • trp operon

        • trp present-operon inactive-trp is the corepressor

        • trp absent-operon active

  • BACTERIA ARE REMARKABLE IN THEIR ABILITY TO ADAPT TO A VARIETY OF ENVIRONMENTSS BY THEIR ELABORATE CONTINGENCY OF MECHANISMS TO CONTROL ENZYME SYNTHESIS AND HENCE METABOLIC PATHWAYS


Regulation of gene expression prokaryotes

trp operon

Promoter

Promoter

Genes of operon

DNA

trpD

trpB

trpA

trpE

trpC

trpR

Operator

Regulatory

gene

Stop codon

Start codon

3

mRNA 5

RNA

polymerase

mRNA

5

D

E

C

B

A

Protein

Inactive

repressor

Polypeptide subunits that make up

enzymes for tryptophan synthesis

(a) Tryptophan absent, repressor inactive, operon on

DNA

No RNA made

mRNA

Protein

Active

repressor

Tryptophan

(corepressor)

(b) Tryptophan present, repressor active, operon off


Trp operon

trp OPERON

Additional regulation mechanism: Attenuation of trp operon:

http://www.youtube.com/watch?v=8aAYtMa3GFU

Animation Ch. 8 Operons repression

trp operon

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter18/animations.html


Repressible vs inducible operons

REPRESSIBLE VS. INDUCIBLE OPERONS

REPRESSIBLE (trp)

Their genes are switched on until metabolite activates the repressor

They function in anabolic pathways

Pathway’s end product switches off its own production by repressing enzyme synthesis

ANABOLIC

INDUCIBLE (Lac)

Their genes are switched off until a specific metabolite inactivates the repressor

They function in catabolic pathways

Enzyme synthesis is switched on by the nutrient the pathway uses

CATABOLIC


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