Transcript operons operons operons - local.brookings.k12.sd.us

OPERONS
* Indicated slides borrowed from: Kim Foglia
Image from: http://www.eldontaylor.com/blog/wp-content/uploads/bigstock-On-Off-Switch-Shows-Energy-Sup-519917201-300x300.jpg
Slide by Kim Foglia modified
PROKARYOTES
Genes with related functions grouped together
= Operon
• example: all enzymes in a metabolic pathway
– promoter = RNA polymerase binding site
• single promoter controls transcription of all genes in operon
• transcribed as one unit & a single mRNA is made
– operator = DNA binding site of repressor protein
– regulatory genes = code for regulatory proteins (EX: repressors)
When gene is turned ON:
Polymerase binds promoter
mRNA is made (transcription)
mRNA is turned into proteins by ribosomes
DNA
RNA
TATA
polymerase
1
2
3
4
mRNA
promoter
operator
enzyme1
gene1
enzyme2
gene2
enzyme3
gene3
enzyme4
gene4
gene1
gene2
gene3
gene4
gene1
gene2
gene3
gene4
RNA
polymerase
TATA
promoter
repressor
operator
Slide by Kim Foglia modified
DNA
When gene is turned OFF
Repressor binds operator
RNA polymerase can’t transcribe gene
• REPRESSABLE OPERONS
Usually ON/repressor usually ACTIVE
Can be turned off (repressed)
Genes for enzymes that make product always needed
EX: trp operon
makes enzymes used in essential amino acid synthesis
• INDUCIBLE OPERONS
Usually OFF/repressor INACTIVE
Can be turned on (induced)
Genes for enzymes that are only needed sometimes
EX: lac operon
makes enzymes used in lactose digestion
DIFFERENCE BETWEEN THE TWO IS
WHAT MAKES REPRESSOR ACTIVE OR INACTIVE!
REPRESSOR is usually INACTIVE
Cells need to make tryptophan
REPRESSOR is usually ACTIVE
Cells don’t need to make lactose
digesting enzymes if no lactose present
If tryptophan is available, don’t
need to make it
If lactose is available, need to digest it
Tryptophan ACTIVATES repressor
TURNS GENE OFF
Lactose INACTIVATES repressor
TURNS GENE ON
Repressible trp operon:
Codes for enzymes that synthesize tryptophan
Synthesis pathway model
When excess tryptophan is present, it binds to trp
repressor protein & triggers repressor to bind to
DNA
– blocks (represses) transcription
RNA
polymerase
trp repressor
TATA
gene1
gene2
gene3
gene4
DNA
trp
repressor
promoter
operator
trp
repressor protein
= INACTIVE
trp
trp
trp
trp
trp
trp
conformational change in
repressor protein makes it
ACTIVE!
SLIDE FROM: Kim Foglia
trp
trp
repressor
tryptophan
trp
tryptophan – repressor protein
Complex = ACTIVE
SLIDE FROM: Kim Foglia
Inducible lac operon
codes for enzymes for lactose digestion
lac
lac
lac
Digestive pathway model
lac
lac
lac
When lactose is present, binds to
lac repressor protein & triggers repressor
to release DNA
lac
RNA
lac repressor
TATA
polymerase
gene1
gene2
gene3
gene4
1
2
3
4
enzyme1
enzyme2
enzyme3
enzyme4
mRNA
promoter
– induces transcription
operator
repressor
lac
conformational change in
repressor protein makes it
INACTIVE!
lac
repressor
DNA
repressor protein =ACTIVE
lactose
lactose – repressor protein
Complex = INACTIVE
Using REPRESSORS to TURN OFF genes
=
NEGATIVE CONTROL
REPRESSIBLE and INDUCIBLE operons
ARE BOTH TYPES OF
NEGATIVE CONTROL ! ! ! !
http://3.bp.blogspot.com/-tArOpdcSKlQ/VDyOFD4hJ8I/AAAAAAAABw8/rEaxBPVNf-g/s1600/Electical%2BLight%2Bswitch...cartoon.jpeg
Using ENHANCER REGIONS and
ACTIVATORS to TURN ON genes =
POSITIVE CONTROL
http://cdn.xl.thumbs.canstockphoto.com/canstock8046278.jpg
POSTIVE CONTROL of Lactose operon
What happens when concentration of glucose is LOW?
LOW GLUCOSE → HIGH cAMP
GLUCOSE IS FOOD OF CHOICE
cAMP makes CAP active; starts transcription
Image from:http://image.slidesharecdn.com/18regulationofgeneexpression-130613012903-phpapp02/95/18-regulation-of-gene-expression-15-638.jpg?cb=1371087103
POSTIVE CONTROL of Lactose operon
What happens if BOTH GLUCOSE AND LACTOSE are
present?
GLUCOSE IS FOOD OF CHOICE
Repressor is inactive but CAP activator is not activated
Gene is unable to turn on at significant rate
Image from:http://image.slidesharecdn.com/18regulationofgeneexpression-130613012903-phpapp02/95/18-regulation-of-gene-expression-15-638.jpg?cb=1371087103
WHAT ABOUT EUKARYOTES
• Genes for proteins that work together in a
pathway are spread out on different
chromosomes (NO OPERONS)
• Separate control sequences for each gene
• BOTH POSTIVE (enhancers) and NEGATIVE
(repressors) control
ACTIVATORS BIND TO ENHANCER region
Image from: https://cellularphysiology.wikispaces.com/file/view/18_09ActivatorAction_3-L.jpg/465203668/18_09ActivatorAction_3-L.jpg
MAKE A CONNECTION
WHERE DO ACTIVATOR proteins come from?
Images from
http://images.slideplayer.com/1/273672/slides/slide_49.jpg
http://image.slidesharecdn.com/45lecturepresentation-101204060035-phpapp01/95/chapter-45-textbook-presentation-33-728.jpg?cb=1291442560
ACTIVATED ENHANCERS attach to
TRANSCRIPTION FACTORS and FOLD DNA
back onto itself
Image from: https://cellularphysiology.wikispaces.com/file/view/18_09ActivatorAction_3-L.jpg/465203668/18_09ActivatorAction_3-L.jpg
RNA POLYMERASE attaches to
TRANSCRIPTION INTIATION COMPLEX to
start transcription (GENE is turned ON!)
Image from: https://cellularphysiology.wikispaces.com/file/view/18_09ActivatorAction_3-L.jpg/465203668/18_09ActivatorAction_3-L.jpg
Different genes
have different
enhancer regions
and are
controlled by
different
activators
Image from: http://image.slidesharecdn.com/18regulationofgeneexpression-130613012903-phpapp02/95/18-regulation-of-gene-expression-35-638.jpg?cb=1371087103
DIFFERENT ENHANCER sequences can
TURN ON a gene in DIFFERENT KINDS of cells at
DIFFERENT TIMES.
Image from: https://cellularphysiology.wikispaces.com/file/view/18_09ActivatorAction_3-L.jpg/465203668/18_09ActivatorAction_3-L.jpg