Transcript Regulation of Gene Expression

Control of Transcription
DNA has “on” and “off” switches
Activator –protein that binds near gene’s
promoter region
- allows RNA polymerase to
transcribe (allows it to fit)
Repressor – protein that binds to DNA and
prevents RNA polymerase from binding
-coded for by “regulator” gene
The Operon Model
Operon – region of DNA with group of genes for
proteins with related functions
(see diagram)
-All of the genes in the operon are controlled by
activity at the promoter & the operator
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Gene for
repressor protein
Promoter
for I gene
PI
CAP-binding
site
Promoter for
lac operon
I
CAP Plac O
Regulatory region
Operator
Gene for
permease
Gene for
b-galactosidase
Z
Gene for
transacetylase
Y
Coding region
lac control system
A
The “Lac” Operon
-in E. coli bacteria
-genes for enzymes to break down lactose sugar
1. Regulator gene codes for Lac repressor protein
2. Repressor binds with operator region
If no lactose present:
3. RNA polymerase can not bind to promoter, no enzyme made and
operon is “off”
If lactose is present:
3. Lactose binds with repressor, repressor no longer binds to
operator
4. RNA polymerase transcribes the structural genes
5. Translation occurs, enzymes are made
6. Lactose is metabolized
*Conservation of resources – enzymes
only made when needed*
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RNA
polymerase
CAP
I
cAMP
CAP
Plac
O
Promoter
lac operon is "repressed"
Repressor
Y
Z
Operator
A
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CAP
I
RNA
polymerase
Plac
CAP
Allolactose
(inducer)
O
Promoter
cAMP
lac operon is "induced"
Z
Y
Operator
A
Glucose
Lactose
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+
+
RNA-polymrase
CAPbinding binding site
(promoter) Operator
site
lacZ gene
Operon ? because CAP is not bound
Repressor
+
–
Operon ? both because lac repressor is
bound and CAP is not
RNA polymerase
–
–
CAP
Operon ? because lac repressor is bound
RNA polymerase
–
+
CAP
mRNA synthesis
Operon ? because CAP is bound and lac
repressor is not
RNA in Gene regulation
“Small RNA’s”
Process of RNA interference (RNAi) or “RNA silencing”
-double stranded RNA is cut into small pieces by enzyme
called dicer
-pieces then unravel into single strands
1. miRNA – micro RNA
-binds to a specific RNA thus blocking it from being
translated
-reversible
2. siRNA – small interfering RNA
-binds to specific mRNA, destroys it
Purposes: (evolutionary origins)
1. Inactivate viral DNA transcription
2. Inactivate transposons – renegade “jumping genes”
3. Regulation
Nucleosomes
X- Inactivation