Chapter 10
Lecture Outline
Molecular Regulation
Microbial Gene Organization
DNA-dep. RNA
polymerase
binds
Promotor
Helps align mRNA in
ribosomes
Leader
Activator/Operator
Structural genes
On-Off switch
(transcribed)
Operon
•Multiple genes transcribed from one promoter
•Genes are transcribed together
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Regulating Gene Expression

Microbes monitor two different compartments
 Intracellular


Concentrations of vitamins, sugars, amino acids, nucleotides
Control of de novo synthesis and degrading enzymes
 Extracellular


What type of environment (e.g., pond water, gastrointestinal
tract, in a host cell)
Adjustment of gene expression for best protection/invasion
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Regulating Gene Expression

Microbes must sense their environment
 Receptors
on cell surface
Receptors must transmit information to
chromosome
 Signals from activated receptor alter gene
expression

 Change
transcription rate
 Change translation rate
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Response to Conditions Outside of the Cell


Sensing the Environment
Two-component signal transduction

Sensor kinase protein in plasma
membrane

Binds to signal




Nutrient
Chemical cue
Activates itself via phosphorylation
Cytoplasmic response regulator


Takes phosphate from sensor
Binds chromosome

Alters transcription rate of multiple genes
Or
Activator
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Microbial Control of Gene Expression
Occurs at Multiple Levels

Alteration of DNA sequences


Control of transcription





E.g., phase variation
Operators = sequences on DNA / repressors = proteins binding
to DNA
Activators = sequences on DNA / activators = proteins binding to
DNA
Control of mRNA stability
Translational control
Post-translational control

Modifying protein activity by chemical modification like
phosphorylation, methylation, acetylation etc.
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Control of Gene Transcription

Regulatory proteins bind to DNA regulatory sequences that control
gene transcription

Operators: binding of regulator (repressor) down regulates expression
of target genes
 Activators: binding of regulator (activator) increases expression of
target genes



Regulatory proteins (regulators) bind to small molecular weight
compounds (ligands)
Different regulators bind to different ligands
Once ligand is bound to regulator the ability of regulator to bind to
DNA is altered
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Gene Regulation via Repressors



Repressors (protein) bind to operators (DNA sequence)
Upon binding to the operators repressors inhibit gene
transcription
Two scenarios are possible:

Active repressor is removed from operator by ligand (inducer)
binding



Gene was off  now gene on
Gene induction
Inactive repressor binds to operator after binding of ligand (corepressor)


Gene was on  now gene off
Gene repression



If ligand diminishes and not available for repressor binding, repressor
releases from operator
Gene was off  now gene on
Gene derepression
Microbiology: An Evolving Science
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Gene Regulation via Repressors
Lactose operon
Tryptophan operon
Active
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Inactive
9
The E. coli lac Operon

Lactose (milk sugar) is used as nutrient
 Cannot


Lactose permease allows entry
Proton motive force used to move lactose inside cell
 Must

pass through plasma membrane
be converted to glucose to be digested
b-galactosidase converts lactose to glucose


Humans also make b-galactosidase
If not, person is lactose-intolerant
Inducer
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The E. coli lac Operon
The lacZ gene encodes b-galactosidase
 The lacY gene encodes lactose permease

 Need
both proteins to digest lactose
Role unclear
Microbiology: An Evolving Science
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The E. coli lac Operon

Repressor protein LacI blocks transcription


Active repressor binds to operator
Repressor responds to presence of lactose


Binds inducer (allolactose) or DNA, not both
Add lactose in the absence of b-galactosidase 
allolactose accumulates  repressor falls off operator
allolactose
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The E. coli lac Operon
Animation: The lac Operon
Click box to launch animation
Microbiology: An Evolving Science
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Concept Quiz
What is the phenotype of an E. coli strain with
a mutation in the lac operon operator such
that Lac repressor could never bind to the
operator?
The strain never transcribes lac genes.
b. The strain transcribes lacZ and lacY only
when lactose is present.
c. The strain always transcribes lacZ and
lacY, even without lactose present.
a.
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Trp Operon

Cell must make the amino acid tryptophan
 Requires
many proteins, made from one operon
 When tryptophan is plentiful, cell stops synthesis

Trp repressor must bind tryptophan to bind DNA
 Opposite
of lac repressor
Repressor + Tryptophan
Transcription repressed
Microbiology: An Evolving Science
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Gene Regulation via Activators


Activators proteins typically bind poorly to
activator DNA unless an inducer is present
If inducer concentration diminishes activator
protein can no longer bind to activator DNA and
gene transcription ceases
Microbiology: An Evolving Science
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Sigma Factor Regulation



Allow coordinated expression of large gene sets
Normally degraded
Under stress no degradation

Binding to core RNA Polymerase
 Sigma factor/RNA polymerase complex binds to various heat-shock genes


By controlling s factor expression large gene sets can be controlled
s factors regulate transcription of all genes


Alternative s factors used for special cases





s70 initiates transcription at most genes
Sporulation in B. subtilis—Bs s28
Stationary phase—s38
Heat-shock and stress response—s32
Flagellar synthesis—s28
Control of ratio of s factors determines global control of protein
synthesis
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Sigma Factor Regulation

Temperature-sensitive mRNA structure
translation of s32 only at high
temperature.
 Allows

Proteolysis rapidly removes s factors
 Rapid


turnover allows more exact control
s70 degraded rapidly at 42°C
Synthesis of proteins that inhibit s factors
 Anti-s

factors block s activity until needed
Anti-anti-s factors respond to environment
Microbiology: An Evolving Science
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Small Regulatory RNAs



Many intergenic regions encode small untranslated RNA
(sRNA)
Small RNAs regulate transcription or stability of specific
mRNA nmolecules
Does not require protein translation


Economical regulation
Antisense RNA base-pairs to mRNA

Usually prevents translation


Until removed via endonuclease
Universal method of gene control—found
in all creatures
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Quorum Sensing

Cells work together at high cell density
 V.
fischeri becomes bioluminescent
 Many bacteria form biofilms
 Exoenzyme production
 Toxin production

Send signal chemical to other cells
 Chemical

Autoinducer
 Binds

accumulation = high cell density
to sensor in cell
Sensor activates transcription
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Microbial Communication through
Quorum Sensing
Autoinducer diffuses into the medium
where it accumulates. At threshold
concentration AI diffuses back into the cell
and binds to activator protein LuxR.
LuxI synthesizes
the autoinducer
homoserine
lactone
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Peptides
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