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Transcriptional
Control
AHMP 5406
Lac Operon
Bacterial example of a genetic switch
Operon = group of genes transcribed from
a single promoter
Lac Operon
Lac operon controlled
by
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
Lac repressor protein
CAP
Lac operon controls
expression of lacZ
gene
Transcription allows
bacterium to use
lactose
Regulation of Transcription in Euks.
Promoters can be affected by regulatory
proteins bound 1000’s of bp’s away
Require several transcription factors
Chromatin packaging can help regulation
of transcription
Gene control regions
Control expression of genes



Spread over long stretches of DNA
Promoter
Regulatory sequences
Where regulatory proteins attach
Gene activator proteins
Have 2 domains


One recognizes regulatory
sequences
Activation domain –
increases rate of
transcription initiation
Influence position of RNA
polymerase II holoenzyme


Partially assembled
Finished at the promoter
Chromatin remodeling
Can be directed by
activator proteins
Recruit remodeling
proteins

HATs (histone acetyl
transferases)
Histone acetylation increases
accessibility

Chromatin remodeling
complex
Transcriptional Synergy
Many activators have a greater effect than
a single activator
Repressor proteins
Down regulate
transcription
Usually do not compete for
RNA pol binding sites
Protein-protein interaction
Co-activators/repressors
Do not bind directly onto DNA
Usually too weak to act on their own
Bind to other regulatory proteins
Post Transcriptional Controls
Occur after RNA pol is bound to promoter

Later in pathway
Not as common as previous transcriptional
controls
Transcription attenuation
Premature termination of transcription

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In bacteria transcript inherently forms 3D
structure
Interferes with transcription process
What if the protein is needed?
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
Regulatory proteins bind to nascent transcript
Stabilize RNA to allow mRNA maturation
Alternative Splicing
RNA can be spliced in
several ways
Can make many
different proteins from
one transcript
Can be tissue specific
Drosophila example

38K genes from one
transcript
RNA editing
Change in mRNA sequence after
transcription
Can change reading frame

Indels
Can change codon
Deamination of adenine


Adenosine deaminase acting on RNA
enzyme (ADAR)
Produces inosine = A,U, C
C to U


Can make stop codons that truncate
protein
Ex. apolipoprotein-B mRNA
mRNA localization
Advantage to have
mRNA where protein
will be translated
Localization signals


found in 3’UTR
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