Transcript Principles and Definitions

Genetic Regulatory Mechanisms
Dr. Jeffrey Patton
Associate Professor
Pathology, Microbiology, and Immunology
USC-School of Medicine
Control of Gene Expression
• Transcriptional control
• Clustering of genes with related function
• Coordinate control of genes with related
function
• Polycistronic mRNA
Inducible Genes - Operon Model
• Definition: Genes whose expression is
turned on by the presence of some
substance
– Lactose induces expression of the lac
genes
– An antibiotic induces the expression of a
resistance gene
• Catabolic pathways
Lactose Operon
• Structural genes
– lac z, lac y, & lac a
– Promoter
– Polycistronic
mRNA
• Regulatory gene
– Repressor
• Operator
• Operon
• Inducer - lactose
Regulator
yGene
i
Operon
p
o
z
y
a
DNA
m-RNA
Protein
Transacetylase
b-Galactosidase
Permease
Lactose Operon
• Inducer -- lactose
– Absence
• Active repressor
• No expression
– Presence
• Inactivation of
repressor
• Expression
• Negative control
Absence of lactose
i
p
o
z
y
a
Activ
e
No lac mRNA
Presence of lactose
i
p
o
z
y
a
Inactiv
e
b-Galactosidase Permease Transacetylase
Catabolite Repression
(Glucose Effect)
• Definition: Control
of an operon by
glucose
• Catabolic operons
Units of ß-galactosidase
- glucose
Glucose
added
+ glucose
Time (hr)
+ lactose
Mechanism of Catabolite Repression
• c-AMP
• CAP (CRP)
protein
• CAP-cAMP
complex
– Promoter
activation
• Positive control
Absence of glucose
c-AMP
CAP
i
p
z
o
Adenyl
cyclase
ATP
y
a
Active
Inactive
b-Galactosidase Permease Transacetylase
Maximum expression
Mechanism of Catabolite Repression
• Glucose:cAMP

• CAP (CRP)
protein
• No CAP-cAMP
complex
– No Promoter
activation
Presence of glucose
Adenyl
cyclase
X
CAP
i
p
o
z
y
ATP
a
Inactive
b-Galactosidase Permease Transacetylase
Low level expression
Repressible Genes - Operon Model
• Definition: Genes whose expression is
turned off by the presence of some
substance (co-repressor)
– Tryptophan represses the trp genes
• Biosynthetic pathways
– Co-repressor is typically the end product of
the pathway
Tryptophan Operon
• Structural genes
– trp E, trpD, trpC
trpB & trpA
– Common promoter
• Regulatory Gene
– Apo-Repressor
Regulatory
Gene
R
Operon
P
O
L
E
D
C
• Inactive
•
•
•
•
Operator
Leader
Operon
Co-repressor
– Tryptophan
Inactive
repressor (aporepressor)
5
Proteins
B
A
Tryptophan Operon
Absence of Tryptophan
• Co-repressor -tryptophan
R
P
O
L
E
D
C
B
A
– Absence of
tryptophan
• Gene expression
– Presence of
tryptophan
Inactive
repressor (aporepressor)
• Activates repressor
• No gene
expression
• Negative control
• Role of tryptophan
5
Proteins
Presence of Tryptophan
R
Inactive
repressor (aporepressor)
P
O
L
E
D
C
No trp mRNA
Trp
(co-repressor)
B
A
Attenuation
• Definition:
Premature
termination of
transcription
• Leader region
–
–
–
–
Leader transcript
Translation start
Translation stop
Tryptophan
codons
L
P
O
1
2
3
4
2
3
4
E
DNA
UGA
AUG
1
2 trp codons
RNA
Attenuation
3
1
2
UUUUUUU
3
• Coupled transcription
and translation
2
– region 1 : region 2
– region 2 : region 3
– region 3 : region 4
4
• Mutually exclusive
mRNA secondary
structure
4
1
UUUUUUU
Attenuation
High tryptophanyl-t-RNA
Low tryptophanyl-t-RNA
UGA
AUG
UGA
AUG
1
1
3
2
3
2
4
4
2 trp codons
3
3
4
2
2 trp codons
UUUUUUU
1
2
Attenuation
4
UUUUUUU
1
No Attenuation
Regulation of Enzyme Activity
• Feed back inhibition
• Epigenetic modification
– Post translational modifications
• Phosphorylation/dephosphorylation
• Adenylation/deadenylation