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Lecture 20

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

Biological Sciences
Course Code
Aarti Ashok

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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 Æ multiple proteins
- control of promoter controls entire pathway in response to environment change
The regulation of operons in bacteria
- operator Æ encodes within promoter
- repressor Ærepresses 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 Æ cAMP low
o glucose low Æ 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; ?}v[Áv}v]o}}v
o b/c cAMP-CRP enhances leÀo}(Æ]}vVµ][v}Z
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
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) Æmake different TFs (transcription factors) Æ bind different DNA site
(sequence) Æ 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:
Æbinds another protein of identical structure
Æforms 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
Zinc fingers occur in tandem (e.g. 3,5,9 repeats in a row)
TFIIIA = transcription of 5S rRNA
o Helix-loop-helix motif
Have a basic amino acid region (i.e. lysine, arginine, histodine etc.); involved in
interacting with DNA
Transcription factors associated with this motif are dimers (hetero- or homo-
dimers); interacting together of two similar structures = dimerization
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