Transcript Control of Gene Expression

Chapter 16
Control of Gene
Expression
Topics to discuss
• DNA binding proteins
• Prokaryotic gene regulation
– negative inducible: lac operon
– negative repressible: trp operon
• Eukaryotic gene regulation
6 groups of DNA-binding
regulatory proteins have been
identified
Prokaryotic Gene Regulation
• Operons
• Operons provide coordinate expression
A model operon
Now let’s look at 2 (of four)
groups of operons
 Negative inducible operons
 Negative repressible operons
Negative inducible operon
INDUCIBLE
Turned on
repressor
Now let’s look at 2 specific operons
• The Lactose operon: negative inducible
• The Tryptophan operon: negative repressible
Francois Jacob and Jacques
Monod 1961
• Studied lactose metabolism/operon in prokayotes
J. Monod
Won the Nobel Prize in 1965
Sources of prokaryotic energy
• 1. Disaccharide
• Lactose
• 2. Monosaccharide
• Glucose
Sources of prokaryotic energy
• Monosaccharide
• Glucose
• Disaccharide
• Lactose
Building the lac operon
Lac Operon and Proposed
Arrangement with control I gene
I
P
O
Z
Not technically part of the “operon”
Y
A
Lactose Metabolism Requires
Coordination of all these genes!
Negative inducible operon
(assume only lactose is present)
Next: let’s review just the pattern of
expression when:
• When only lactose is present.
• When only glucose is present.
• Glucose and lactose are present in the cell
“Why does the presence of glucose influence lactose
Lac
metabolism?
To answer this question you need
to understand this reaction
ATP
Adenylyl cyclase
Cyclic AMP
+
catabolite activator protein
Glucose may inhibit this enzyme
Now put the pieces all together.
How CAP and cAMP affects the
promoter
So….Conditions: Only lactose is present
But…Conditions: Only glucose
And Finally….Glucose and Lactose
Summary
Trypthophan Operon
• 5 genes involved in the synthesis of the
amino acid trypthophan
Negative repressible operon
2 shapes to the mRNA
When trp is high
When trp is low
Ribosome does not stall
Ribosome stalls
Changes in Chromatin structure and eukaryotic gene regulation
• Histone acetylation
• DNA methylation
• RNA silencing
Do eukaryotes show coordinate
gene regulation?
Yes, the same response element may be found in
related genes.
Also remember gene expression through RNA interference!