Transcript 5. kuliah gene regulation

GENE
REGULATION
GENE REGULATION
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Virtually every cell in your body
contains a complete set of genes
But they are not all turned on in every
tissue
Each cell in your body expresses only a
small subset of genes at any time
During development different cells
express different sets of genes in a
precisely regulated fashion
GENE REGULATION
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Gene regulation occurs at the level of
transcription or production of mRNA
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A given cell transcribes only a specific
set of genes and not others
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Insulin is made by pancreatic cells
CENTRAL DOGMA
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Genetic information always goes from DNA to
RNA to protein
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Gene regulation has been well studied in E. coli
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When a bacterial cell encounters a potential
food source it will manufacture the enzymes
necessary to metabolize that food
Gene Regulation
In addition to sugars like glucose and lactose
E. coli cells also require amino acids
 One essential aa is tryptophan.
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When E. coli is swimming in tryptophan (milk
& poultry) it will absorb the amino acids from
the media
 When tryptophan is not present in the media
then the cell must manufacture its’ own amino
acids
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Trp Operon
E. coli uses several proteins encoded by a
cluster of 5 genes to manufacture the amino
acid tryptophan
All 5 genes are transcribed together as a unit
called an operon, which produces a single long
piece of mRNA for all the genes
Regulatory
Gene
R
Inactive repressor
(apo-repressor)
Operon
P
O
L
E
D
C
5 Proteins
B
A
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RNA polymerase binds to a promoter
located at the beginning of the first gene
and proceeds down the DNA transcribing
the genes in sequence
The tryptophane gene is turned on when
there is no tryptophan in the media
the trp gene is a repressible gene (Genes
whose expression is turned off by the presence
of some substance (co-repressor))
That is when the cell wants to make its’ own
tryptophan
Fig. 16.6
Tryptophan Operon
Absence of Tryptophan
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Co-repressor -tryptophan
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Gene expression
Presence of tryptophan
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P
O
L
E
D
C
B
A
B
A
Absence of tryptophan
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R
Inactive repressor
(apo-repressor)
5 Proteins
Activates repressor
No gene expression
Presence of Tryptophan
Negative control
R
P
O
L
E
D
C
No trp mRNA
Inactive repressor
(apo-repressor)
Trp
(co-repressor)
GENE REGULATION
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In addition to amino acids, E.
coli cells also metabolize sugars
in their environment
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In 1959 Jacques Monod and
Fracois Jacob looked at the ability
of E. coli cells to digest the sugar
lactose
GENE REGULATION
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In the presence of the sugar lactose, E.
coli makes an enzyme called beta
galactosidase
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Beta galactosidase breaks down the
sugar lactose so the E. coli can digest it
for food
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It is the LAC Z gene in E coli that
codes for the enzyme beta
galactosidase
Lac Z Gene
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Lac Z is an inducible gene ( Genes whose
expression is turned on by the presence of some
substance)
E. coli cells can not make the sugar lactose
They can only have lactose when it is present
in their environment
Then they turn on genes to break down
lactose
GENE REGULATION
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The E. coli bacteria only needs beta
galactosidase if there is lactose in the
environment to digest
There is no point in making the enzyme if
there is no lactose sugar to break down
It is the combination of the promoter and the
DNA that regulate when a gene will be
transcribed
GENE REGULATION
 This combination of a promoter and a
gene is called an OPERON
 Operon is a cluster of genes encoding
related enzymes that are regulated together
Regulatory
Gene
i
Operon
p
o
z
y
a
DNA
m-RNA
Protein
Transacetylase
b-Galactosidase
Permease
LAC Z GENE
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E. coli regulate the production of Beta
Galactocidase by using a regulatory protein
called a repressor
The repressor binds to the lac Z gene at a site
between the promotor and the start of the
coding sequence
The site the repressor binds to is called the
operator
LAC Z GENE
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Normally the
repressor sits on the
operator repressing
transcription of the
lac Z gene
In the presence of
lactose the
repressor binds to
the sugar and this
allows the
polymerase to move
Absence of lactose
i
z
o
p
a
y
Active
No lac mRNA
Presence of lactose
i
p
o
z
y
a
Inactive
b-Galactosidase Permease Transacetylase
LAC Z GENE
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When there is no
sugar left the
repressor will return
to its spot on the
chromosome and stop
the transcription of
the lac Z gene
Absence of lactose
i
p
o
z
y
Active
No lac mRNA
a
Negative control
(bound repressor inhibits transcription)
Catabolite Repression
(Glucose Effect)
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Definition:
Control of an
operon by glucose
Catabolic operons
Units of galactosidase
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- glucose
Glucose
added
+ glucose
Time (hr)
+ lactose
Mechanism of Catabolite Repression
Absence of glucose
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c-AMP
CAP (CRP)
protein
CAP-cAMP
complex
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Promoter
activation
Adenyl cyclase
c-AMP
ATP
CAP
i
p
z
o
y
a
Active
Inactive
b-Galactosidase Permease Transacetylase
Maximum expression
Positive control (bound activator facilitates trancription)
Mechanism of Catabolite Repression
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Glucose:cAMP
CAP (CRP) protein
No CAP-cAMP
complex
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No Promoter
activation
Presence of glucose
Adenyl cyclase
ATP
X
CAP
i
p
o
z
y
a
Inactive
b-Galactosidase Permease Transacetylase
Low level expression
GENE REGULATION
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In eukaryotic organisms like ourselves
there are several methods of regulating
protein production
Most regulatory sequences are found
upstream from the promoter
Genes are controlled by regulatory
elements in the promoter region that
act like on/off switches or dimmer
switches
GENE REGULATION
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Specific transcription factors bind to
these regulatory elements and regulate
transcription
Regulatory elements may be tissue
specific and will activate their gene only
in one kind of tissue
Sometimes the expression of a gene
requires the function of two or more
different regulatory elements