Chapter 28
Type Lecture
Reviewed
Status In progress
Chapter 28 - gene regulation in bacteria
Many bacteria genes are clustered and regulated in
operons
Many bacterial genes are transcribed and regulated together in an operon
Operon = cluster of genes sharing a promoter and regulatory sequences
The lac operon is governed by more than repressor
binding
Lac Operon undergoes Positive regulation
Glucose preferred energy source in E.coli
Availability of glucose governs expression of lactose-digesting genes via
catabolite repression
When glucose is present, lactose genes are turned off
When glucose is present catabolite repression restricts expression of genes
required for catabolism of other sugars
Mediated by cAMP - coactivator
cAMP receptor protein (CRP)/ or catabolite gene activator protein (CAP) –
activator protein
Glucose absent
When glucose is absent:
CRP-cAMP binds near promoter - stimulates transcription 50x
Chapter 28 1
, CRP-cAMP positive regulatory element responsive to glucose levels
BUT:
CRP-cAMP only works when the Lac repressor has dissociated
Dissociation of the repressor from the lac operon little effect unless CRP-cAMP
is present
When CRP not bound the wild-type promoter is relatively weak
CRP interacts directly with α subunit of RNA polymerase
Open complex of RNA polymerase and the promoter form readily in the
presence of CRP- cAMP
Glucose present
In the presence of glucose cAMP is inhibited
As [cAMP] declines CRP binding to DNA declines
Decrease in expression of lac operon
No lactose present
If lactose is absent
Whether [glucose] is high or low
repressor stays bound
no transcription even when CRP-cAMP bind
Lactose present (glucose levels still regulate)
In the presence of both glucose and lactose, bacterial cells prefer to use
glucose.
Repressor dissociates in presence of lactose
Glucose prevents induction of the lac operon.
binding of CRP – cAMP complex to the CRP binding site is required for
induction of the lac operon
In the presence of glucose synthesis of cAMP inhibited
Summary
Chapter 28 2
, 1. Lactose must be present to form allolactose to bind to repressor and cause it to
dissociate from operator
reducing repression
2. [Glucose] must be low so that cAMP can increase, bind to CRP, and the complex
can bind near the promoter
causing activation
When lactose is low, repressor is bound:
inhibition
When lactose is high, repressor dissociates
permitting transcription
When glucose is high, CRP is not bound and
transcription is dampened
When glucose is low, cAMP is high and CRP is bound
activation
Transcription Attenuation regulates Amino acid
Biosynthetic Enzymes
Genes for enzymes needed to synthesize a given amino acid are clustered in an
operon
When amino acid is abundant operon repressed
Transcription begins but is then halted by a stop signal (attenuator)
The attenuator sequence is in the 5’-region of a leader sequence and it can
make the ribosome stall
The trp operon is regulated by transcription
attenuation
When Trp is abundant, it binds to repressor, causes it to bind to operator and
slow expression of genes for Trp synthesis
Trp repressor is a homodimer
Has helix-turn-helix motifs that interact with DNA via the major groove
Chapter 28 3
, The trp operator site overlaps the promoter so binding of the repressor blocks
binding of RNA polymerase
Transcription is fine-tuned by a second regulatory process called transcription
attenuation
The role of the Attenuator
The trp operon attenuation mechanism uses signals encoded in four sequences
within a 162 nucleotide leader region
The leader contains a region known as the attenuator made up of sequences 3
&4
Transcription attenuation relies on the fact that in bacteria, transcription and
translation can proceed simultaneously
The attenuator determines:
If transcription will be attenuated at the end of the leader /
If transcription will continue into the genes for Trp synthesis
The leader region can form different stem-loop
structures
Leader is 162 nucleotides long
Includes segments 1-4
If segments 3 and 4 base-pair, they form attenuation signal
G≡C rich stem loop structure followed by series of U residues
Acts as transcription terminator
If segments 2 and 3 base-pair, attenuator structure cannot form
transcription proceeds and the trp synthetic enzymes are made
Loop formed does not obstruct transcription
Abundance of tRNATrp leads to formation of the
attenuator
Sequence 1 of the leader sequence contains two Trp codons
Chapter 28 4
Type Lecture
Reviewed
Status In progress
Chapter 28 - gene regulation in bacteria
Many bacteria genes are clustered and regulated in
operons
Many bacterial genes are transcribed and regulated together in an operon
Operon = cluster of genes sharing a promoter and regulatory sequences
The lac operon is governed by more than repressor
binding
Lac Operon undergoes Positive regulation
Glucose preferred energy source in E.coli
Availability of glucose governs expression of lactose-digesting genes via
catabolite repression
When glucose is present, lactose genes are turned off
When glucose is present catabolite repression restricts expression of genes
required for catabolism of other sugars
Mediated by cAMP - coactivator
cAMP receptor protein (CRP)/ or catabolite gene activator protein (CAP) –
activator protein
Glucose absent
When glucose is absent:
CRP-cAMP binds near promoter - stimulates transcription 50x
Chapter 28 1
, CRP-cAMP positive regulatory element responsive to glucose levels
BUT:
CRP-cAMP only works when the Lac repressor has dissociated
Dissociation of the repressor from the lac operon little effect unless CRP-cAMP
is present
When CRP not bound the wild-type promoter is relatively weak
CRP interacts directly with α subunit of RNA polymerase
Open complex of RNA polymerase and the promoter form readily in the
presence of CRP- cAMP
Glucose present
In the presence of glucose cAMP is inhibited
As [cAMP] declines CRP binding to DNA declines
Decrease in expression of lac operon
No lactose present
If lactose is absent
Whether [glucose] is high or low
repressor stays bound
no transcription even when CRP-cAMP bind
Lactose present (glucose levels still regulate)
In the presence of both glucose and lactose, bacterial cells prefer to use
glucose.
Repressor dissociates in presence of lactose
Glucose prevents induction of the lac operon.
binding of CRP – cAMP complex to the CRP binding site is required for
induction of the lac operon
In the presence of glucose synthesis of cAMP inhibited
Summary
Chapter 28 2
, 1. Lactose must be present to form allolactose to bind to repressor and cause it to
dissociate from operator
reducing repression
2. [Glucose] must be low so that cAMP can increase, bind to CRP, and the complex
can bind near the promoter
causing activation
When lactose is low, repressor is bound:
inhibition
When lactose is high, repressor dissociates
permitting transcription
When glucose is high, CRP is not bound and
transcription is dampened
When glucose is low, cAMP is high and CRP is bound
activation
Transcription Attenuation regulates Amino acid
Biosynthetic Enzymes
Genes for enzymes needed to synthesize a given amino acid are clustered in an
operon
When amino acid is abundant operon repressed
Transcription begins but is then halted by a stop signal (attenuator)
The attenuator sequence is in the 5’-region of a leader sequence and it can
make the ribosome stall
The trp operon is regulated by transcription
attenuation
When Trp is abundant, it binds to repressor, causes it to bind to operator and
slow expression of genes for Trp synthesis
Trp repressor is a homodimer
Has helix-turn-helix motifs that interact with DNA via the major groove
Chapter 28 3
, The trp operator site overlaps the promoter so binding of the repressor blocks
binding of RNA polymerase
Transcription is fine-tuned by a second regulatory process called transcription
attenuation
The role of the Attenuator
The trp operon attenuation mechanism uses signals encoded in four sequences
within a 162 nucleotide leader region
The leader contains a region known as the attenuator made up of sequences 3
&4
Transcription attenuation relies on the fact that in bacteria, transcription and
translation can proceed simultaneously
The attenuator determines:
If transcription will be attenuated at the end of the leader /
If transcription will continue into the genes for Trp synthesis
The leader region can form different stem-loop
structures
Leader is 162 nucleotides long
Includes segments 1-4
If segments 3 and 4 base-pair, they form attenuation signal
G≡C rich stem loop structure followed by series of U residues
Acts as transcription terminator
If segments 2 and 3 base-pair, attenuator structure cannot form
transcription proceeds and the trp synthetic enzymes are made
Loop formed does not obstruct transcription
Abundance of tRNATrp leads to formation of the
attenuator
Sequence 1 of the leader sequence contains two Trp codons
Chapter 28 4
