BIOB11H3 Lecture Notes - Lecture 20: Ubiquitin, Ferritin, Cytosol

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Lecture #20
Date: Thursday, July 15th/2010
Control of gene expression: prokaryotes
- Bacteria often need to turn genes on and off rapidly in response to changing environmental
o e.g. presence of certain nutrients
- addition of inducer; e.g. lactose
o it is a waste to make E-galactose when there is no lactose ? transcription is stopped for
that gene
- polycistronic ร† multiple proteins
- control of promoter controls entire pathway in response to environment change
The regulation of operons in bacteria
- operator ร† encodes within promoter
- repressor ร†represses transcription
o e.g. lactose
The Lac operon: an inducible operon
- bacteria = glucose lovers
- make sure lactose present and cAMP levels low for high levels of lac operon transcription
- the concentration of cAMP is inversely proportional to the concentration of glucose:
o glucose high ร† cAMP low
o glucose low ร† cAMP high
- cAMP-CRP binding complex binds to the lac promoter and enhances attachment of RNA pol
o enhances lac operon transcription
- low glucose & lactose available: high (constitutive) level of expression
- high glucose & lactose available: low (basal) level of expression
o chosen b/w the two: glucose & lactose; ?๎€š}๎€ž๎‚v[๎‚š๎€ƒร๎€‚v๎‚š๎€ƒ๎‚š}๎€ƒ๎‚š๎‚Œ๎€‚v๎‚๎€๎‚Œ]๎€๎€ž๎€ƒo๎€‚๎€๎€ƒ}๎‚‰๎€ž๎‚Œ}v
o b/c cAMP-CRP enhances leร€๎€žo๎€ƒ}(๎€ƒ๎€žร†๎‚‰๎‚Œ๎€ž๎‚๎‚]}vV๎€ƒ๎€ยต๎‚š๎€ƒ]๎‚š[๎‚๎€ƒv}๎‚š๎€ƒ๎‚šZ๎€ž๎‚Œ๎€ž
The Trp operon: a repressible operon
- Tryptophan = corepressor
Control of gene expression: eukaryotes
- Control of eukaryotic gene expression is most complex and occurs primarily at 3 levels:
o Transcriptional-level control: whether or not a gene is transcribed
o Processing-level control: the mechanism controlling conversion of pre-mRNA to mRNA
o Translational-level control: whether or not an mRNA is translated; how often and at
what rate
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inverse proportionality
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Transcriptional-level control
- Transcription factors; modifications: methylation/deacetylation
- Different growth conditions: in different environments, different genes are expressed ?mRNAs
are also different
- Different stimuli (signals) ร†make different TFs (transcription factors) ร† bind different DNA site
(sequence) ร† regulate genes
An aside: DNA microarrays
- Yeast are grown in 2 different conditions:
o Glucose-rich media vs.
o Ethanol rich media
- They turn on or turn off genes that are needed or not needed in the different environment in
order to survive
- Sometimes the fluorescence emitted by this type of interaction is yellow:
o Meaning both red and green cDNAs present; both yeasts express this gene
o e.g. actin or tubulin
- If there is no colour: that gene is transcribed under neither growth condition
- Certain genes are expressed only under certain conditions
Structure of TFs
- TFs are proteins that contain multiple domains:
o DNA binding domain
o Activation domain
o Dimerization-domain:
ร†binds another protein of identical structure
ร†forms a dimer; function in both DNA binding and activation
- Each domain folds independently; have different functions
DNA binding domains of TFs
- Related structures that interact with DNA are called ^motifs_
- An example includes HMG box = high mobility group
- Other examples:
o Zinc-finger motif
๎‚ƒ Zinc fingers occur in tandem (e.g. 3,5,9 repeats in a row)
๎‚ƒ TFIIIA = transcription of 5S rRNA
o Helix-loop-helix motif
๎‚ƒ Have a basic amino acid region (i.e. lysine, arginine, histodine etc.); involved in
interacting with DNA
๎‚ƒ Transcription factors associated with this motif are dimers (hetero- or homo-
dimers); interacting together of two similar structures = dimerization
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