GENE GENE REGULATIONREGULATION

GENE REGULATIONGENE REGULATION Virtually every cell in your Virtually every cell in your

body contains a complete set body contains a complete set of genesof genes

But they are not all turned on But they are not all turned on in every tissuein every tissue

Each cell in your body Each cell in your body expresses only a small subset expresses only a small subset of genes at any timeof genes at any time

During development different During development different cells express different sets of cells express different sets of genes in a precisely regulated genes in a precisely regulated fashionfashion

GENE REGULATIONGENE REGULATION Gene regulation occurs at the Gene regulation occurs at the

level of transcription or level of transcription or production of mRNAproduction of mRNA

A given cell transcribes only A given cell transcribes only a specific set of genes and a specific set of genes and not othersnot others

Insulin is made by pancreatic Insulin is made by pancreatic cellscells

CENTRAL DOGMACENTRAL DOGMA

Genetic information always goes Genetic information always goes from DNA to RNA to proteinfrom DNA to RNA to protein

Gene regulation has been well Gene regulation has been well studied in E. colistudied in E. coli

When a bacterial cell encounters a When a bacterial cell encounters a potential food source it will potential food source it will manufacture the enzymes necessary manufacture the enzymes necessary to metabolize that foodto metabolize that food

Gene RegulationGene Regulation

In addition to sugars like glucose and In addition to sugars like glucose and lactose Elactose E. coli . coli cells also require amino cells also require amino acidsacids

One essential aa is tryptophan. One essential aa is tryptophan.

When E. When E. colicoli is swimming in is swimming in tryptophan (milk & poultry) it will tryptophan (milk & poultry) it will absorb the amino acids from the absorb the amino acids from the mediamedia

When tryptophan is not present in the When tryptophan is not present in the media then the cell must manufacture media then the cell must manufacture its’ own amino acidsits’ own amino acids

TrpTrp Operon Operon E. coliE. coli uses several proteins encoded uses several proteins encoded

by a cluster of 5 genes to manufacture by a cluster of 5 genes to manufacture the amino acid tryptophanthe amino acid tryptophan

All 5 genes are transcribed together as All 5 genes are transcribed together as a unit called an operon, which a unit called an operon, which produces a single long piece of mRNA produces a single long piece of mRNA for all the genesfor all the genes

R

Operon

RegulatoryGene

P O E D C

5 Proteins

B AL

Inactive repressor (apo-repressor)

RNA polymerase binds to a promoter RNA polymerase binds to a promoter located at the beginning of the first located at the beginning of the first gene and proceeds down the DNA gene and proceeds down the DNA transcribing the genes in sequencetranscribing the genes in sequence

The tryptophane gene is turned on The tryptophane gene is turned on when there is no tryptophan in the when there is no tryptophan in the media media

the the trptrp gene is a repressible gene gene is a repressible gene ((Genes whose expression is turned off by Genes whose expression is turned off by the presence of some substance (co-the presence of some substance (co-repressor)repressor)))

That is when the cell wants to make That is when the cell wants to make its’ own tryptophanits’ own tryptophan

Fig. 16.6

Tryptophan OperonTryptophan Operon

Co-repressor -- Co-repressor -- tryptophantryptophan Absence of Absence of

tryptophantryptophan Gene expressionGene expression

R P O E D C

5 Proteins

B AL

Inactive repressor (apo-repressor)

Absence of Tryptophan

R P O E D C

No trp mRNA

B AL

Presence of Tryptophan

Inactive repressor (apo-repressor)

Trp(co-repressor)

Presence of tryptophanPresence of tryptophan Activates repressorActivates repressor No gene expressionNo gene expression

Negative controlNegative control

GENE REGULATIONGENE REGULATION In addition to amino In addition to amino

acids, acids, E. coliE. coli cells also cells also metabolize sugars in their metabolize sugars in their environmentenvironment

In 1959 Jacques Monod In 1959 Jacques Monod and Fracois Jacob looked and Fracois Jacob looked at the ability of at the ability of E. coli E. coli cells to digest the sugar cells to digest the sugar lactoselactose

GENE REGULATIONGENE REGULATION In the presence of the sugar In the presence of the sugar

lactose, lactose, E. coliE. coli makes an makes an enzyme called enzyme called beta beta galactosidasegalactosidase

Beta galactosidaseBeta galactosidase breaks breaks down the sugar lactose so the down the sugar lactose so the E. coliE. coli can digest it for food can digest it for food

It is the LAC Z gene in It is the LAC Z gene in E coliE coli that codes for the enzyme that codes for the enzyme beta galactosidasebeta galactosidase

Lac Z GeneLac Z Gene

Lac Z is an inducible gene ( Lac Z is an inducible gene ( Genes Genes whose expression is turned on by the whose expression is turned on by the presence of some substance)presence of some substance)

E. coliE. coli cells can not make the cells can not make the sugar lactosesugar lactose

They can only have lactose when They can only have lactose when it is present in their environmentit is present in their environment

Then they turn on genes to break Then they turn on genes to break down lactosedown lactose

GENE REGULATIONGENE REGULATION

The The E. coliE. coli bacteria only needs bacteria only needs beta beta galactosidasegalactosidase if there is lactose in if there is lactose in the environment to digestthe environment to digest

There is no point in making the There is no point in making the enzyme if there is no lactose sugar enzyme if there is no lactose sugar to break downto break down

It is the combination of the It is the combination of the promoter and the DNA that regulate promoter and the DNA that regulate when a gene will be transcribedwhen a gene will be transcribed

GENE REGULATIONGENE REGULATION This combination of a promoter This combination of a promoter

and a gene is called anand a gene is called an OPERONOPERON Operon is a cluster of genes Operon is a cluster of genes

encoding related enzymes that encoding related enzymes that are regulated togetherare regulated together

i

Operon

RegulatoryGene

p o z y a DNA

m-RNA

-Galactosidase

Permease

Transacetylase

Protein

LAC Z GENELAC Z GENE

E. coliE. coli regulate the production of regulate the production of Beta GalactocidaseBeta Galactocidase by using a by using a regulatory protein called a regulatory protein called a repressorrepressor

The repressor binds to the lac Z gene The repressor binds to the lac Z gene at a site between the promotor and at a site between the promotor and the start of the coding sequencethe start of the coding sequence

The site the repressor binds to is The site the repressor binds to is called the called the operatoroperator

LAC Z GENELAC Z GENE

Normally the Normally the repressor sits on the repressor sits on the operator repressing operator repressing transcription of the transcription of the lac Z genelac Z gene

In the presence of In the presence of lactose the lactose the repressor binds to repressor binds to the sugar and this the sugar and this allows the allows the polymerase to move polymerase to move down the lac Z genedown the lac Z gene

i p o z y a

No lac mRNA

Absence of lactose

Active

i p o z y a

-Galactosidase Permease Transacetylase

Presence of lactose

Inactive

LAC Z GENELAC Z GENE

When there is When there is no sugar left the no sugar left the repressor will repressor will return to its return to its spot on the spot on the chromosome chromosome and stop the and stop the transcription of transcription of the lac Z genethe lac Z gene

i p o z y a

No lac mRNA

Absence of lactose

Active

Negative control(bound repressor inhibits

transcription)

Catabolite RepressionCatabolite Repression (Glucose Effect) (Glucose Effect)

Definition: Definition: Control of an Control of an operon by operon by glucoseglucose

+ glucose

- glucose

Time (hr)

Un

its

of g

alac

tosi

das

e

+ lactose

Glucose added

Catabolic Catabolic operonsoperons

Mechanism of Catabolite Mechanism of Catabolite RepressionRepression

c-AMPc-AMP CAP (CRP) CAP (CRP)

proteinprotein CAP-cAMP CAP-cAMP

complexcomplex Promoter Promoter

activationactivation

Absence of glucose

i p o z y a

Active

-Galactosidase Permease Transacetylase

CAP

Inactive

ATPAdenyl cyclase

c-AMP

Maximum expression

Positive controlPositive control (bound activator facilitates trancription) (bound activator facilitates trancription)

Mechanism of Catabolite Mechanism of Catabolite RepressionRepression

GlucoseGlucose:cAM:cAMPP

CAP (CRP) CAP (CRP) proteinprotein

No CAP-cAMP No CAP-cAMP complexcomplex No Promoter No Promoter

activationactivation

Presence of glucose

i p o z y a

Inactive

ATPAdenyl cyclase

CAP X

-Galactosidase Permease Transacetylase

Low level expression

GENE REGULATIONGENE REGULATION In eukaryotic organisms like In eukaryotic organisms like

ourselves there are several ourselves there are several methods of regulating methods of regulating protein productionprotein production

Most regulatory sequences Most regulatory sequences are found upstream from the are found upstream from the promoterpromoter

Genes are controlled by Genes are controlled by regulatory elements in the regulatory elements in the promoter region that act like promoter region that act like on/off switches or dimmer on/off switches or dimmer switchesswitches

GENE REGULATIONGENE REGULATION Specific transcription factors Specific transcription factors

bind to these regulatory bind to these regulatory elements and regulate elements and regulate transcriptiontranscription

Regulatory elements may be Regulatory elements may be tissue specific and will tissue specific and will activate their gene only in activate their gene only in one kind of tissueone kind of tissue

Sometimes the expression of Sometimes the expression of a gene requires the function a gene requires the function of two or more different of two or more different regulatory elementsregulatory elements