Download Operons: The Basic Concept

Operons: The Basic Concept
• In bacteria, genes are often clustered into
operons, composed of
– An operator, an “on-off” switch
– A promoter
– Genes for metabolic enzymes
• Bacterial cells divide by binary fission
– Which is preceded by replication of the bacterial
chromosome
Replication
fork
Origin of
replication
Termination
of replication
Figure 18.14
Mutation and Genetic Recombination as
Sources of Genetic Variation
• Since bacteria can reproduce rapidly
– New mutations can quickly increase a population’s
genetic diversity
• An operon
– Is usually turned “on”
– Can be switched off by a protein called a repressor
• The trp operon: regulated synthesis of
repressible enzymes
trp operon
Promoter
DNA
Promoter
Genes of operon
trpD
trpC
trpE
trpR
trpB
trpA
Operator
Regulatory
gene
mRNA
5
3
RNA
polymerase
Start codon
Stop codon
mRNA 5
E
Protein
Inactive
repressor
D
C
B
Polypeptides that make up
enzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the
promoter and transcribes the operon’s genes.
Figure 18.21a
A
DNA
No RNA made
mRNA
Protein
Active
repressor
Tryptophan
(corepressor)
(b) Tryptophan present, repressor active, operon off. As tryptophan
accumulates, it inhibits its own production by activating the repressor protein.
Figure 18.21b
Repressible and Inducible Operons: Two Types of
Negative Gene Regulation
• In a repressible operon
– Binding of a specific repressor protein to the
operator shuts off transcription
• In an inducible operon
– Binding of an inducer to an innately inactive
repressor inactivates the repressor and turns on
transcription
• The lac operon: regulated synthesis of inducible
enzymes
Promoter
Regulatory
gene
DNA
Operator
lacl
lacZ
3
mRNA
Protein
No
RNA
made
RNA
polymerase
5
Active
repressor
(a) Lactose absent, repressor active, operon off. The lac repressor is innately active, and in
the absence of lactose it switches off the operon by binding to the operator.
Figure 18.22a
lac operon
DNA
lacl
lacz
3
mRNA
5
lacA
RNA
polymerase
mRNA
mRNA 55'
-Galactosidase
Protein
Allolactose
(inducer)
lacY
Permease
Transacetylase
Inactive
repressor
(b) Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses
the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced.
Figure 18.22b
• Inducible enzymes
– Usually function in catabolic pathways
• Repressible enzymes
– Usually function in anabolic pathways
• Regulation of both the trp and lac operons
– Involves the negative control of genes, because the
operons are switched off by the active form of the
repressor protein
Positive Gene Regulation
• Some operons are also subject to positive
control
– Via a stimulatory activator protein, such as
catabolite activator protein (CAP)
• In E. coli, when glucose, a preferred food source,
is scarce
– The lac operon is activated by the binding of a
regulatory protein, catabolite activator protein (CAP)
Promoter
DNA
lacl
lacZ
CAP-binding site
Active
CAP
cAMP
Inactive
CAP
Figure 18.23a
RNA
polymerase
can bind
and transcribe
Operator
Inactive lac
repressor
(a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized.
If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces
large amounts of mRNA for the lactose pathway.
• When glucose levels in an E. coli cell increase
– CAP detaches from the lac operon, turning it off
Promoter
DNA
lacl
lacZ
CAP-binding site
Operator
RNA
polymerase
can’t bind
Inactive
CAP
Inactive lac
repressor
(b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized.
When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription.
Figure 18.23b