Download DNA-binding proteins

7.1 Major Modes of Gene Regulation
• Gene expression: transcription of gene into mRNA followed
by translation of mRNA into protein (Figure 7.1)
• Most proteins are enzymes that carry out biochemical
reactions
• Constitutive proteins are needed at the same level all the
time
• Microbial genomes encode many proteins that are not
needed all the time
• Regulation helps conserve energy and resources by fine
tuning protein levels
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Levels of
Regulation
A
Snapshot
–35 –10
+1
Promoter
RBS Structural gene Terminator
DNA 5′
3′
5′
3′
Activation Repression
Transcription (making RNA)
RBS
RNA
Start codon
Stop codon
5′
3′
5′-UTR
3′-UTR
Translation (making protein)
Feedback inhibition
Mechanisms of
controlling
enzyme activity
Protein
Degradation
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Protein–protein interactions
Covalent modifications
Figure 7.1
II. DNA-Binding Proteins and Transcriptional
Regulation
• 7.2 DNA-Binding Proteins
• 7.3 Negative Control:
Repression and Induction
• 7.4 Positive Control: Activation
• 7.5 Global Control and the lac Operon
• 7.6 Transcriptional Controls in Archaea
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7.2 DNA-Binding Proteins
• mRNA transcripts generally have a short half-life
• Prevents the production of unneeded proteins
• Regulation of transcription typically requires
proteins that can bind to DNA
• Small molecules influence the binding of
regulatory proteins to DNA
• Proteins actually regulate transcription
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7.2 DNA-Binding Proteins
• Most DNA-binding proteins interact with DNA in a
sequence-specific manner
• Specificity provided by interactions between amino acid
side chains and chemical groups on the bases and
sugar–phosphate backbone of DNA
• Major groove of DNA is the main site of protein binding
• Inverted repeats frequently are binding site for regulatory
proteins
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7.2 DNA-Binding Proteins
• Homodimeric proteins: proteins composed of two
identical polypeptides
• Protein dimers interact with inverted repeats on
DNA
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Types of
DNA
Repeats
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Mirror repeat
7.2 DNA-Binding Proteins
• Several classes of protein domains are critical for
proper binding of proteins to DNA
• Helix-turn-helix (Figure 7.4)-one class of DNA binding
domain
• First helix is the recognition helix
• Second helix is the stabilizing helix
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A perfect fit for the major groove
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© 2015 Pearson Education, Inc.
7.2 DNA-Binding Proteins
• Classes of protein domains
• Zinc finger
• Protein structure that binds a zinc ion
• Eukaryotic regulatory proteins use zinc fingers for DNA
binding
• Leucine zipper
• Contains regularly spaced leucine residues
• Function is to hold two recognition helices in the correct
orientation
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Three important types of DNA binding
domains
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7.3 Negative Control: Repression and Induction
• Defined as control that prevents transcription
• Several mechanisms in bacteria
• These systems are greatly influenced by environment in
which the organism is growing
• Through presence or absence of specific small
molecules
• Interactions between small molecules and DNA-binding
proteins result in control of transcription or translation
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7.3 Negative Control: Repression and Induction
• Early on, microbiologists realized that bacteria
could respond to environmental signals by starting
or stopping to make enzymes: adaptation.
• Induction-occurs when environmental signal
triggers the synthesis of an enzyme
• Repression-occurs when environmental signal
prevents the synthesis of an enzyme
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7.3 Negative Control: Repression and Induction
Induction: production of an enzyme in response to a signal
(Figure 7.6)
• Typically affects catabolic enzymes (e.g., lac operon)
• Enzymes are synthesized only when they are needed
• No wasted energy
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Induction
Total protein
Cell number
β-Galactosidase
Lactose added
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Figure 7.6
7.3 Negative Control: Repression and Induction
• Repression: preventing the synthesis of an enzyme in
response to a signal (Figure 7.5)
• Enzymes affected by repression make up a small fraction
of total proteins
• Typically affects anabolic enzymes
(e.g., arginine biosynthesis)
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Repression
Cell number
Total protein
Arginine added
Arginine
biosynthesis
enzymes
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Figure 7.5
7.3 Negative Control: Repression and Induction
• Inducer: a substance that induces enzyme synthesis
• Corepressor: a substance that represses enzyme
synthesis (not same as repressor)
• Effectors: collective term for inducers and repressors
• Effectors affect transcription indirectly by binding to specific
DNA-binding proteins
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7.3 Negative Control: Repression and Induction
• Paradigm system is the lactose (lac) operon of E. coli.
• Operon: group of genes related in that they work together
• Structural genes code for enzymes or soldier proteins
• Regulatory genes control the structural genes (Figure 7.7)
• Enzyme induction can also be controlled by a repressor
• Addition of inducer inactivates a repressor protein (not same as
corepressor substance) , and transcription can proceed (Figure 7.8)
• Repressor's role is to prevent enzyme synthesis, so it is
called negative control
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7.3 Negative Control: Repression and Induction
• Structural genes of lac operon: (contiguous)
• Lac Z
• Lac Y
• Lac A
• Regulatory genes of the lac operon: (contiguous,
overlapping or separate)
• Lac operator or Lac O
• Lac promoter or Lac P
• Lac repressor or Lac I-coded by an independent gene with its own
promoter-always active at a low level: constitutive
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lac Promoter lac Operator
RNA
polymerase
lacZ
lacY
lacA
Transcription blocked
Repressor
lac Promoter lac Operator
RNA
polymerase
lacZ
lacY
lacA
Transcription proceeds
Repressor
Inducer
(allolactose)
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Figure 7.8
7.3 Negative Control: Repression and Induction
• Structural genes of arg operon: (contiguous)
• Arg C
• Arg B
• Arg H
• Regulatory genes of the lac operon: (contiguous,
overlapping or separate)
• Arg operator or Arg O
• Arg promoter or Arg P
• Arg repressor or Arg R-coded by an independent gene with its own
promoter-always active at a low level
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arg Promoter arg Operator
argC
argB
RNA
polymerase
argH
Transcription proceeds
Repressor
arg Promoter arg Operator
RNA
polymerase
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argC
argB
argH
Corepressor
Transcription blocked
(arginine)
Repressor
Figure 7.7
Summary: Induction and repression work by way of
a protein that changes shape in response to a signal
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7.4 Positive Control: Activation
• Positive control: regulator protein activates the
binding of RNA polymerase to DNA (Figure 7.9)
• Maltose catabolism in E. coli
• Maltose activator protein cannot bind to DNA unless it
first binds maltose
• Activator proteins bind specifically to certain DNA
sequence
• Called activator-binding site, not operator
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Activatorbinding site
mal Promoter
malE
malF
malG
No transcription
RNA
polymerase
Maltose activator protein
Activatorbinding site
mal Promoter
malE
RNA
polymerase
malF
malG
Transcription proceeds
Maltose activator protein
Inducer
(maltose)
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Figure 7.9
7.4 Positive Control: Activation
• Promoters of positively controlled operons only weakly bind
RNA polymerase
• Activator protein helps RNA polymerase recognize
promoter
• May cause a change in DNA structure
• May interact directly with RNA polymerase
• Activator-binding site may be close to the promoter or be
several hundred base pairs away (Figure 7.11)
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Activatorbinding site
Promoter
RNA
polymerase
Transcription
proceeds
Activator protein
Promoter
RNA
polymerase
Activator protein
Transcription
proceeds
Activatorbinding site
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Figure 7.11
7.4 Positive Control: Activation
• Genes for maltose are spread out over the
chromosome in several operons (Figure 7.12)
• Each operon has an activator-binding site
• Multiple operons controlled by the same regulatory
protein are called a regulon
• Regulons also exist for negatively controlled
systems
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GF
B
EK
M
A
Y
Z
oriC
Mal regulatory
protein
Lac regulatory
protein
T
P
Q
Maltose operons make
up maltose regulon
Lactose operon
Direction of transcription
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Figure 7.12
7.5 Global Control and the lac Operon
• Global control systems: regulate expression of many
different genes simultaneously
• Catabolite repression is an example of global control
• Synthesis of unrelated catabolic enzymes is repressed if glucose is
present in growth medium (Figure 7.13)
• lac operon is under control of catabolite repression
• Ensures that the "best" carbon and energy source is used first
• Diauxic growth: two exponential growth phases
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Growth on
lactose
Glucose
exhausted
Growth on
glucose
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Figure 7.13
7.5 Global Control and the lac Operon
• Cyclic AMP and CRP
• In catabolite repression, transcription is controlled by
an activator protein and is a form of positive control
(Figure 7.15)
• Cyclic AMP receptor protein (CRP) is the activator
protein
• Cyclic AMP is a key molecule in many metabolic
control systems
• Derived from a nucleic acid precursor
• Is a regulatory nucleotide
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CRP protein
cAMP
RNA
polymerase
Binding of CRP recruits
RNA polymerase
DNA
lacI
Transcription
mRNA
lacI
lac Structural genes
C
P
O
lacZ
Active
repressor
binds to
operator
and blocks
transcription.
lacY
lacA
Transcription
mRNA
lacZ
lacY
lacA
Translation
Translation
LacI
Inducer
LacZ
Active
repressor
LacY
LacA
Lactose catabolism
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Inactive
repressor
Figure 7.15
7.5 Global Control and the lac Operon
• Dozens of catabolic operons are affected by
catabolite repression
• Enzymes for degrading lactose, maltose, and other
common carbon sources
• Flagellar genes are also controlled by catabolite
repression
• No need to swim in search of nutrients
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7.6 Transcription Controls in Archaea
• Archaea use DNA-binding proteins to control
transcription
• More closely resembles control by Bacteria than
Eukarya
• Repressor proteins in Archaea
• NrpR is an example of an archaeal repressor protein
from Methanococcus maripaludis (Figure 7.16)
• Represses genes involved in nitrogen metabolism
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NrpR
DNA
NrpR blocks TFB
and TBP binding;
no transcription.
BRE TATA
INIT
NrpR binds
α-ketoglutarate.
α-Ketoglutarate
(
)
NH3
When NrpR is
released, TBP and
TFB can bind.
Glutamate
NrpR
TFB
TBP
Transcription
proceeds.
RNA polymerase
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Figure 7.16
III. Sensing and Signal Transduction
• 7.7 Two-Component Regulatory Systems
• 7.8 Regulation of Chemotaxis
• 7.9 Quorum Sensing
• 7.10 Other Global Control Networks
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7.7 Two-Component Regulatory Systems
• Prokaryotes regulate cellular metabolism in
response to environmental fluctuations
• External signal is transmitted directly to the target
• External signal is detected by sensor and transmitted to
regulatory machinery (signal transduction)
• Most signal transduction systems are two-component
regulatory systems
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7.7 Two-Component Regulatory Systems
• Two-component regulatory systems (Figure 7.17)
• Made up of two different proteins:
• Sensor kinase (in cytoplasmic membrane): detects
environmental signal and autophosphorylates
• Response regulator (in cytoplasm): DNA-binding protein
that regulates transcription
• Also has feedback loop
• Terminates signal
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Environmental signal
Sensor kinase
ATP
Cytoplasmic
membrane
ADP
His
His
P
P
Response regulator
Phosphatase
activity
P
P
RNA
polymerase
Promoter
Transcription blocked
Operator
DNA
Structural genes
Shows regulator blocking transcription but activation can also occur
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7.7 Two-Component Regulatory Systems
• Almost 50 different two-component systems in E. coli
• Examples include phosphate assimilation, nitrogen metabolism,
and osmotic pressure response (Figure 7.18)
• Some Archaea also have two-component regulatory
systems
• Some signal transduction systems have multiple regulatory
elements
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7.8 Regulation of Chemotaxis
• Modified two-component system used in
chemotaxis to
• Sense temporal changes in attractants or repellents
• Regulate flagellar rotation
• Three main steps (Figure 7.19)
1. Response to signal
2. Controlling flagellar rotation
3. Adaptation
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7.8 Regulation of Chemotaxis
• Step 1: Response to signal
• Sensory proteins in cytoplasmic membrane sense
attractants and repellents
• Methyl-accepting chemotaxis proteins (MCPs)
• Bind attractant or repellent and initiate flagellar rotation
• Step 2: Controlling flagellar rotation
• Controlled by CheY protein
• CheY results in counterclockwise rotation and runs
• CheY-P results in clockwise rotation and tumbling
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Repellents bind to MCP and
trigger phosphorylation of
CheA-CheW complex.
MCP
CheR
MCP is both
methylated
and
demethylated.
CheB
P
CheW
CheA
ATP
ADP
CheY
CheA-CheW
phosphorylate
CheY and CheB.
Flagellar
motor
CheY-P binds to
flagellar switch.
CheY
P
CheZ
CheB
Cytoplasm
CheZ dephosphorylates
CheY-P.
Flagellum
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Figure 7.19
Summary
• CheY = counterclockwise rotation = run
• But CheY-P = clockwise = tumble
• Repellents increase CheY-P therefore tumbling
and direction change
• Works through two component system using MCP
and other Che proteins
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7.8 Regulation of Chemotaxis
• Step 3: Adaptation
• Feedback loop
• Allows the system to reset itself to continue to sense the
presence of a signal
• Involves modification of MCPs with methyl group
• Degree of methylation controls sensitivity to attractant and
repellent
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7.9 Quorum Sensing
• Prokaryotes can respond to the presence of other
cells of the same species
• Quorum sensing: mechanism by which bacteria
assess their population density
• Ensures that a sufficient number of cells are present
before initiating a response that, to be effective, requires
a certain cell density (e.g., toxin production in
pathogenic bacterium)
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7.9 Quorum Sensing
• Each species of bacterium produces a specific
autoinducer molecule (Figure 7.20)
• Diffuses freely across the cell envelope
• Reaches high concentrations inside cell only if many
cells are near
• Binds to specific activator protein and triggers
transcription of specific genes
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Acyl homoserine lactone (AHL)
Activator protein
AHL
AHL
Quorumspecific
proteins
Other cells
of the same
species
Chromosome
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AHL synthase
Figure 7.20
7.9 Quorum Sensing
• Several different classes of autoinducers
• Acyl homoserine lactone (AHL) was the first autoinducer
to be identified
• Quorum sensing first discovered as mechanism
regulating light production in bacteria including
Aliivibrio fischeri (Figure 7.21)
• Lux operon encodes bioluminescence
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7.9 Quorum Sensing
• Examples of quorum sensing
• Virulence factors
• Switching from free-living to growing as a biofilm
• Quorum sensing is present in some microbial
eukaryotes
• Quorum sensing likely exists in Archaea
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7.9 Quorum Sensing-example
• Virulence factors
• Staphylococcus aureus
• Secretes small peptides that damage host cells or alter
host's immune system
• Under control of autoinducing peptide (AIP)
• Activates several proteins that lead to production of
virulence proteins (Figure 7.22b)
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Binding of AIP to
ArgC leads to
auto-phosphorylation.
AIP
Cytoplasmic
membrane
ArgB
ArgC
ATP
P
Pre-AIP
Cytoplasm
ADP
ArgC phosphorylates
ArgA.
Pre-AIP is
converted to
AIP by ArgB
and exported
out of the cell.
ArgA
P
ArgA-P activates
expression of genes
required for pre-AIP
and virulence proteins.
+ +
Basal transcription
D
C
B
argA
Virulence proteins
Genes encoding virulence
Virulence factor production in Staphylococcus
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Figure 7.22b
Summary
• Staphylococcus aureus is an opportunitist
pathogen
• ArgA-P activates arg genes above basal level
• Also activates virulence factors
Exoenzymes to digest tissue
Exotoxins as poisons (gastroenteritis)
Role in toxic shock syndrome (TSS)
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7.9 Quorum Sensing
• Biofilm formation
• Pseudomonas aeruginosa
• Produces polysaccharides that increase pathogenicity
and antibiotic resistance
• Two quorum-sensing systems
• Produces AHLs and cyclic di-guanosine monophosphate
(c-di-GMP)
• Leads to exopolysaccharide production and flagella
synthesis (Figure 7.23)
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Production
of AHLs and
c-di-GMP
Increasing
cell
population
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Exopolysaccharide
production and
flagella synthesis
Attachment
Mature
biofilm
Figure 7.23
7.10 Other Global Control Networks
• Nitrogen utilization: regulation by alternate sigma
• Under limiting conditions nitrogen uptake and use
becomes very important
• Requires expression of new genes
• The genes do not have consensus promoter sequence,
therefore not recognized by regular RNA pol and sigma
factor
• Nitrogen stress results in expression of alternate sigma:
sigma54 from RpoN gene
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7.13 Nitrogen Fixation, Nitrogenase, and
Heterocyst Formation
• Nitrogen fixation is process of reducing N2 to NH3
• Only certain prokaryotes can fix nitrogen
• Reaction is catalyzed by nitrogenase
• Composed of dinitrogenase and dinitrogenase
reductase
• Sensitive to the presence of oxygen
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7.13 Nitrogen Fixation, Nitrogenase, and
Heterocyst Formation
• Highly regulated process because it is such an
energy-demanding process
• Nif regulon coordinates regulation of genes
essential to nitrogen fixation (Figure 7.27)
• Oxygen and ammonia are the two main regulatory
effectors
• Complex regulation uses multiple strategies
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Nitrogenase
proteins
Dinitrogenase
reductase
FeMo-co
synthesis
Mo
processing
FeMo-co
synthesis
Dinitrogenase
Dinitrogenase
Homocitrate
reductase synthesis
processing
Regulators
Metal center
Positive Negative
biosynthesis
Flavodoxin
FeMo-co
synthesis
β
Electron
transport
Pyruvate
flavodoxin
oxidoreductase
α
FeMo-co
insertion into
dinitrogenase
nif DNA
Q
B
A
L
F
M Z W V S U
X
N
E
Y
T
K
D
H
J
RNA
© 2015 Pearson Education, Inc.
Figure 7.27
7.13 Nitrogen Fixation, Nitrogenase, and
Heterocyst Formation
• Heterocyst-specialized cells for nitrogen fixation in
some cyanobacteria (Figure 7.28)
• Requires metabolic and morphological changes
• Formation of thickened envelope (3 layers)
• Inactivation of photosystem II (it releases oxygen)
• Expression of nitrogenase
• Patterning of heterocyst differentiation
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Fixed N flow
[α-Ketoglutarate]
NtcA activates
hetR expression
Heterocyst
Vegetative cells
Vegetative cells
Fixed C flow
A filament of Anabaena
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Heterocyst—vegetative cell interactions
HetR activates genes necessary
for heterocyst formation
Triggering heterocyst formation
Figure 7.28
V. RNA-Based Regulation
• Uses non-coding RNAs or non-coding regions of
transcripts for regulation
• 7.14 Regulatory RNAs: Small RNAs (i.e.
antisense or cis-RNA and trans-RNA)
• 7.15 Riboswitches
• 7.16 Attenuation
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7.14 Regulatory RNA: Small RNAs
• Small RNAs work by:
• Block or open a ribosome-binding site (RBS)
• Increase or decrease degradation of mRNA (i.e. mRNA
half-life)
• May also act at level of transcription
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Translation inhibition/stimulation
1.
mRNA
5′
3′
3′
sRNA
RNA degradation/protection
1.
5′
5′
3′
RBS
mRNA
5′
RBS
3′
3′
sRNA
5′
RBS
5′
3′
RBS
Ribonuclease
Translation
No translation
Translation
No translation
3′
2.
5′
RBS
2.
5′
3′
5′
3′
RBS
5′
3′
RBS
3′
5′
5′
3′
RBS
Ribonuclease
No translation
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Translation
No translation
Translation
Figure 7.29
7.14 Regulatory RNA: Small RNAs
• Antisense RNAs first discovered in plasmids,
phages, transposons
• RNA-OUT of Tn10 (IS10) decreases transposase
expression
• Most antisense regulators have not been studied
• How do we know they exist?
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7.14 Regulatory RNA: Small RNAs and
Antisense RNA
• Second classic example
• AntiQ RNA from Enterococcus faecalis plasmid pCF10
• Small RNA binds to growing mRNA transcript for
conjugation genes
• Alters secondary structure to block further transcription
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7.14 Regulatory RNA: Small RNAs and
Antisense RNA
• Types of small RNAs (cont'd)
• Trans-RNAs are encoded in the intergenic region (not
within gene they regulate)
• Limited complementarity with target
• Usually require help from a protein for binding
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7.14 Regulatory RNA: Small RNAs and
Antisense RNA
• Hfq protein
• Facilitates RNA binding
• Also has regulatory functions independent
of RNA
• “Global” regulator
• http://www.rcsb.org/pdb/pv/pv.do?pdbid=1K
Q2&bionumber=1
© 2015 Pearson Education, Inc.
Small regulatory
RNA
mRNA
5′
Hfq
protein
© 2015 Pearson Education, Inc.
3′
Small regulatory
RNA recognition sequence
Figure 7.30
Useful Reference
•Annu Rev Genet. Author manuscript; available in PMC 2011 Jan 28.
•Annu Rev Genet. 2010; 44: 167–188.
•doi: 10.1146/annurev-genet-102209-163523
•PMCID: PMC3030471
•NIHMSID: NIHMS236247
•Bacterial antisense RNAs: How many are there and what are they doing?
•Maureen Kiley Thomason1,2 and Gisela Storz1
•1 Cell Biology and Metabolism Program, Eunice Kennedy Shriver National Institute of Child
Health and Human Development, Bethesda, MD 20892-5430
•2 Department of Biochemistry and Molecular & Cell Biology, Georgetown University Medical
Center, Washington, DC 20007
•Maureen Kiley Thomason: vog.hin.liam@amyelik; Gisela Storz: vog.hin.xileh@zrots
•Abstract
•Antisense RNAs encoded on the DNA strand opposite another gene have the potential to
form extensive base pairing interactions with the corresponding sense RNA. Unlike other
smaller regulatory RNAs in bacteria, antisense RNAs range in size, from tens to thousands
of nucleotides. The numbers of antisense RNAs reported for different bacteria vary
extensively but hundreds have been suggested in some species. If all of these reported
antisense RNAs are expressed at levels sufficient to regulate the genes encoded opposite
them, antisense RNAs could significantly impact gene expression in bacteria. Here we
review the evidence for these RNA regulators and describe what is known about the
functions and mechanisms of action for some of these RNAs. Important considerations for
future research as well as potential applications are also discussed.
© 2015 Pearson Education, Inc.
7.15 Riboswitches
• Riboswitches: RNA domains in an mRNA
molecule that can bind small molecules to control
translation of mRNA (Figure 7.31)
• Located at 5′ end of mRNA
• Binding results from folding of RNA into a 3-D structure
• Similar to a protein recognizing a substrate
• Found in some bacteria, fungi, and plants
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 7.31
7.15 Riboswitches
• Riboswitch example: SAM riboswitch or (SAM Box
riboswitch) in Bacillus subtilis
• Regulates expression of genes required for
methionine metabolism at level of translation of
mRNA
No SAM: transcription of
mRNA takes place
SAM causes shape change that
allows formation of a
terminator
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7.16 Attenuation
• Transcriptional control that functions by
premature termination of mRNA synthesis
• Trp operon of E. coli
• Additional control level above and beyond induction or
repression
• Presence of trp down regulates expression of genes for trp
biosynthesis
• Depends on control sequences in 5’UTR or trp leader
© 2015 Pearson Education, Inc.
trp structural genes
P O L
DNA
trpE
trpD
trpC
trpB
trpA
Trp Leader
Met-Lys-Ala-lle-Phe-Val-Leu-Lys-Gly-Trp-Trp-Arg-Thr-Ser
Trp mRNA gets translated as it is made
When there is trp in the cell the trp leader can be transcribed
No trp in the cell transcription stalls
© 2015 Pearson Education, Inc.
Figure 7.32
Excess tryptophan:
transcription
terminated
Leader sequence
DNA
Direction of
transcription
Base
pairing
Ribosome
2
5′
3
1
Trp-rich
leader
peptide
Transcription
terminated and
tryptophan
structural
genes not
transcribed
Direction of
translation
Leader sequence
DNA
Direction of
transcription
Translation
stalled
2
5′
1
Direction of
translation
© 2015 Pearson Education, Inc.
4
mRNA
Limiting tryptophan:
transcription
proceeds
Leader
peptide
RNA
polymerase
terminates
3
RNA
polymerase
continues
4
Transcription
continues and
tryptophan
structural genes
transcribed
Figure 7.33
VI. Regulation of Enzymes and Other Proteins
Post-translational
• 7.17 Feedback Inhibition
• 7.18 Post-Translational Regulation
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7.17 Feedback Inhibition
• Feedback inhibition: mechanism for turning off the
reactions in a biosynthetic pathway (Figure 7.34a)
• End product of the pathway binds to the first enzyme in
the pathway, thus inhibiting its activity
• Inhibited enzyme is an allosteric enzyme (Figure 7.34b)
• Two binding sites: active and allosteric
• Easily reversible reaction
• Fast-acting
© 2015 Pearson Education, Inc.
7.17 Feedback Inhibition
• Some pathways controlled by feedback inhibition
use isoenzymes, different enzymes that catalyze
the same reaction but are subject to different
regulatory controls
• Rhodopseudomonas palustris
• Highly versatile bacterium with multiple life styles
• Switches between different nitrogenase isoenzymes
depending on conditions
© 2015 Pearson Education, Inc.
7.18 Post-Translational Covalent Modification
• Biosynthetic enzymes can also be regulated by
covalent modifications
• Regulation involves a small molecule attached to or
removed from the protein (Figure 7.35)
• Results in conformational change that inhibits activity
• Common modifiers include adenosine monophosphate
(AMP), adenosine diphosphate (ADP), inorganic
phosphate (PO42-), and methyl groups (CH3)
© 2015 Pearson Education, Inc.