Transcript Molecular Biology of Gene Function

Chapter 16
Control of Gene
Expression
Topics to discuss
• DNA binding proteins
• Prokaryotic gene regulation
– Lac operon
– Trp operon
• Eukaryotic gene regulation
Today’s lecture
6 groups of DNA-binding
regulatory proteins have been
identified
Prokaryotic Gene Regulation
Nutritional enviroment and a bug’s growth!
• Operons
• Operons provide coordinate expression
A model operon
Now let’s look at 2 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
• The Tryptophan operon
Francois Jacob and Jacques
Monod 1961
• Studied lactose metabolism in prokayotes
J. Monod
Won the Nobel Prize in 1965
Sources of prokaryotic energy
• 1. Disaccharide
• Lactose
• 2. Monosaccharide
• Glucose
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
Negative inducible operon
Conditions: No glucose but lactose is present
Lactose Metabolism Requires
Coordination of all these genes!
Laboratory Exercise: Mutational
Analysis of the lac operon
Try out your ability to reason
through the questions.
• Predict if you expect to obtain the proteins
from the lacZ and lacY genes IF certain
mutations are present in the operon.
The next three slides are the “key”
to the mutations.
Mutational Key:
•
•
•
Any + means wildtype.
Any – means mutant.
Two other mutants: “Oc and Is”
•
I- cells synthesize full levels in the presence or absence of inducerTranscription Turned ON. Inhibitor cannot bind to DNA
•
P- the DNA promoter mutation : cannot bind RNA polymerase
Transcription OFF
•
Is super-suppressors can bind DNA but not inducer. DNA and
Transcription Turned OFF.
•
Oc is an DNA mutation: repressor cannot bind. Transcription ON.
Examples
I s=the Super-repressor
Allolactose CANNOT BIND
VERSUS
I - = DNA binding site mutated,
prevents binding, allows transcription
DNA binding site
Next: 3 important terms of DNA
control.
• Constitutive activity
• Trans acting
• Cis acting
Β-galactosidase
No Lactose
1
I+P+O+Z+Y+
2
I-P+O+Z+Y+
3
I+P-O+Z+Y+
4
I+P+OcZ+Y+
5
I+P+O+Z-Y-/ I-P+O+Z+Y+
6
I+P+OcZ+Y-/ I+P+O+ZY+
7
I+P+OcZ-Y+/ I-P+O+Z+Y-
8
ISP+O+Z+Y-/ I-P+O+Z-Y+
9
I+P-OcZ+Y+/ I+P+O+Z-Y-
-
Lactose
+
Permease
No lactose
-
Lactose
+
Negative inducible operon
Question: what happens when both
lactose and glucose are present
together in the same cell?
To answer this question you first need to
understand this reaction
ATP
Adenylyl cyclase
Cyclic AMP
+
catabolite activator protein
(helix-turn-helix)
Glucose may inhibit this enzyme
• cAMP and glucose levels are inversely proportional.
The promoter needs to be in an
ideal conformation for RNA
polymerase.
How CAP and cAMP affects the
promoter
END of PART I
Trypthophan Operon
Trypthophan Operon
• 5 genes involved in the synthesis of the
amino acid trypthophan
Negative repressible operon
2 shapes to the mRNA
attenuation
Eukaryotic Gene Regulation
Do eukaryotes show coordinate
gene regulation?
Yes, the same response element may be found in
related genes.
Changes in Chromatin structure and eukaryotic gene
regulation
• Chromatin Structure
– DNase hypersensitivity (
• Histone acetylation
• DNA methylation