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How does protein recognize and bind to a specific DNA sequence?
DNA binding motifs (on protein) (recognize DNA sequence)
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Motif:
- Repeated structure
- eg: Helix - turn - helix motif
- eg: Leucine zipper
- eg: homeodomain
- eg: zinc finger
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Operon:
- (prokayotes only)
- -grouping of genes
- -stretch of DNA used to make enzymes for a particular metabolic pathway
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Trp operon:
- trp - operon make enzyme so cell can make aa tryprotein
5prime....regulatory region...(operator/promoter)....coding region (5 genes long)...3 prime
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Operator:
where transcription factor attaches
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When is operon turned on/off?
- 1) if tryprophon is absent, cell makes it:
- RNA polymer binds to promoter and
- Transcription occurs
2) If tryptophan is present, operon should turn off
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Transcription factor:
Transcription factor---> Tryptophones attach
Repressable - strong promoter usually on
- eg: LAC operon "weak promoter" often off (has to been turned on
- E. Coli and lactose
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Operons:
- LAC operon - makes enzyme that breaksdown lactose
- -promoter= weak (generally off) eg E coli.
- -Needs to be activated to turn on
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E. Coli - Cap Protein
- If (glucose) is low, then bacteria need to use another sugar
- eg: Lactose
- If (glucose) is low, then cAMP rises,
- cAMP binds to CAP
- and together they bind to DNA
- If (glucose) is low and (lactose) is high then,
- Operon is turned on, transcription occurs.
- If (glucose) is low and (lactose) is low,
- No transcription
- (Repressor binds to operator and RNA polymerase cant bind)
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CAP:
Transcription Factor and activator
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When glucose is low and lactose is high:
- some lactose molecules bind the repressor
- Repressor falls off
- RNA polymerase binds to promoter
- Transcription occurs
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Glucose and Lactose chart:
- Glucose +, Lactose + = operon off, CAP is not bound
- Glucose +, Lactose - = operon off, repressor is bound/cap is not
- Glucose -, Lactose - = operon off, repressor is bound
- glucose-, lactose + = operon on, CAP is bound, repressor is not.
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Eukaryotes Gene Regulation:
- 1) more complex
- 2) Transcription factors - MANY
- 3) No operons - distant sites on DNA
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transcription factors
- (made of protein)
- 1) general - on/off switch
- 2) specific (activators/co-activators)
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Activators:
Modulate rate/speed of transcription
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Where does control occur:
@ transcription initiation site
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Control can be either:
- positive or negative
- Positive- increases frequency of initiation (activators)
- Negative - decreases freq. of initiation (repressors)
both occur on the operator
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