Download Ch 16 Lac Operon

Chapter 16
Control of Gene
Expression
Topics to discuss
• DNA binding proteins
• Prokaryotic gene regulation
• 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 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
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
Lactose Metabolism Requires
Coordination of all these genes!
Negative inducible operon
Conditions: No glucose but lactose is present
Lactose Metabolism Requires
Coordination of all these genes!
Lac Operon and Proposed
Arrangement with control I gene
I
P
O
Z
Not technically part of the “operon”
Y
A
Overview of: Mutational Analysis of the lac operon
3-24p. Lab manua
Monod and Jacob
Discovery of lac operon control
Try out your ability to reason
through the questions.
I gene
P gene
Z,A,Y
genes
O
I+
P+
Z+,A+, Y+
O+
I-
P-
Z- or A-, or
Y-
Oc
Is
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
+
+
+
+
+
-
Now, back to glucose and the lac
operon!
Lac
Question: what happens when both
lactose and glucose are present
together in the same cell?
Conditions: glucose present and lactose
Need to understand this reaction
ATP
Adenylyl cyclase
Cyclic AMP
+
catabolite activator protein
Glucose may inhibit this enzyme
The promoter needs to be in an
ideal conformation for RNA
polymerase.
How CAP and cAMP affects the
promoter
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
• DNase hypersensitivity
• Histone acetylation
• DNA methylation
For Your Information
Next 4 slides
Experimental tests on the
repressor:
• I+ cells synthesize full levels of the lac enzymes only in the
presence of an inducer
• I- mutations NO DNA BINDING ACTIVITY, TRANSCRIPTION
OCCURS EVEN IN ABSENCE OF INDUCER!
• Is mutation NO LACTOSE BINDING ACTIVITY but has DNA
BINDING ACTIVITY. NO TRANSCRIPTION EVEN WITH
INDUCER!
• Oc mutation: constitutive activity
Trans (across) acting product
Oc= constitutive operator, notice
how it affects transcription.
Mutations to inducer binding
domain of repressor
Extra slides
Table I.
Table 2.
Table 3.
Allosteric affect of DNA binding
protein
The Lac promoter and its role in
lac transcription
Repressor has two binding
sites.
Lactose
binding site
DNA Operator
binding site
ITPG: Binds repressor but is not
cleaved.