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Chapter 17 Regulation of Gene Expression in Bacteria and Bacteriophages. Copyright © 2010 Pearson Education Inc. Chapter 19: Regulation of Gene Expression in Bacteria and Bacteriophages. Through evolutionary processes, organisms have developed ways to compensate for

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Chapter 17Regulation of Gene Expression in Bacteria and Bacteriophages

Copyright © 2010 Pearson Education Inc.


Chapter 19: Regulation of Gene Expression in

Bacteria and Bacteriophages

Through evolutionary processes,

organisms have developed ways

to compensate for

environmental changes.

Alter gene activity to optimize

growth and reproduction in a

given environment.


Two Types of Genes

1) Regulated Genes – activity is controlled in response to the

needs of a cell or organism.

2) Constitutive genes - (housekeeping genes) always active

(e.g. protein synthesis and Glucose metabolism)

Basic Mechanism of Gene Regulations in Bacteria

Bacteria have developed ways to turn off genes whose products are

not needed and for turning on genes whose products are

needed in each environment.

The turning of genes off or on requires interaction between

regulatory proteins and DNA sequences.


Inducible Gene Expression

When a gene is turned on by the addition of a substance, it is called inducible gene.

The regulator substance is called an inducer, which are members of a class

of small molecules = effectors

Controlling site is near protein coding sequence.

The addition of inducer leads to induction.


Induction of genes required for lactose utilization in E. coli

E. coli grow in simple media containing salts, nitrogen sources, and glucose.

If lactose (or other sugar) is added instead of glucose,

a number of enzymes are rapidly synthesized.

*Inducer of Lactose operon

When Lactose is only sugar, three enzymes are synthesized

1) B-galactosidase

2) Lactose permease

3) Trans acetylase (function poorly understood)


All 3 genes are clustered on the genome and are transcribed

onto a single mRNA called a polygenic mRNA or a polycistronic mRNA

Chain terminating mutation

Nonsense mutants (chain terminating mutant) were used to determine that all 3 genes

were on the same mRNA.

jacob monod s operon model for the regulation of the lac genes
Jacob & Monod’s Operon Model for the regulation of the lac genes
  • Operon is a cluster of genes, the expression of which are regulated together by operator regulator protein interactions, plus the operator region itself and the promoter.

In E. coli, the lac operon is under

negative, positive and inducible control

Lac I+ gene encodes lac repressor protein made constitutively,

which will bind operator region of the lac operon.

Few repressors are present in cell since promoter is relatively weak.


Absence of lactose

Lac operon is under negative control:

There is a low level of lac gene expression because the repressor binds

and unbinds allowing for low amounts of protein such as

B-galactosidose and permease to be generated


Presence of lactose

Inducible Control

If in the presence of lactose, the above B-galactosidose produces

inducer molecules, allolactose, which is the inducer.


Experiments by Jacob and Monod

Partial diploid that has F‘ plasmid

Without inducer

With inducer


Mutation in Lac I gene, which generates a mutant repressor that cannot bind to the operator



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Mutation in Lac I gene, which generates a mutant repressor

that cannot bind to the operator

With inducer


Lac Operon experiments

Dominant Effect

Mutation in Lac I that

cannot bind Inducer but can bind

the operator


Lac Repressor model for tetramer protein structure

Has four polypeptides and

each polypeptide is from

repressor gene.

**This data convinced many scientist (at the time) that all genes

were under negative control due to the binding of a repressor. **


Positive control of the lac operon.

Turns on expression of the lac operon.

Ensures that lac operon stays on when lactose is the sole carbon source,

but not in the presence of glucose.

Glucose is used preferentially over lactose.


Positive control of the lac operon.

Glucose inactivates adenylate cyclase

Glucose removes remaining cAMP by

Activating Phosphodiesterase

In presence of glucose, concentration of

positive regulator (CAP-cAMP complex)

that binds the lac operon for increased gene

activity is reduced.

Because the presence of glucose reduces

the amount of cAMP in cell.


Basepair sequence of the lac I gene promoter

Shine-Dalgarno sequence

GUG start site

Basepair sequence of the controlling sites, promoter and operator, for the lac operon.

tryptophan operon
Tryptophan Operon

All necessary amino acids may not be present in a growth medium. If a specific amino acid is missing, the bacteria has certain operons that enable the bacterial cell to manufacture that amino acid.

For example the Tryptophan Operon

Has five structural genes


Two mechanisms of regulations for tryptophan operon

# 1 Repressor/operator interaction with tryptophan as the effector

molecule: Tryptophan binds the aporepressor (trpR) and then

binds operator to turn off the gene

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Two mechanisms of regulations for tryptophan operon

# 2 Attenuation Control Regulatory Leader region

Determines if initiated transcripts include other

structural genes or not.

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Operon called repressible operon (# 1). No transcripts in presence

of tryptophan.

The biosynthetic pathway is catalyzed by a specific enzyme

(at each step) which is coded by a specific gene or genes.

Presence of tryptophan in medium keeps operon turned off

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# 2 Attenuation Control

Absence of tryptophan or in the presence of low amounts of tryptophan:

Under severe tryptophan starvation  all long transcripts

gene activity at maximum

Under less severe situation gene Long and short transcripts

Expression at less than maximum

Greater the amount of tryptophan the greater the number of short transcripts

Attenuation controls = > terminates transcription producing short transcripts


RNA polymerase response to

these mRNA secondary structures


Transcription and translation are tightly coupled in prokaryotes

Attenuation occurs at the mRNA level and can reduce transcription of trp-operon 8-or-10 fold.

*Last long enough for

Ribosome to load onto mRNA.

Position of ribosome on leader transcript determines if transcription is

terminated or not.

If starved for trytophan = lack trp-tRNA

If no trp-tRNA, ribosome stalls at trp codons. With Ribosome

on region one, the 1-2 loop can’t form.

So, the 2-3 loop for antitermination forms. Thus region 3 can’t

pair with region 4 and the RNA polymerase can now continues.


Starved for tryptophan

Transcription continues

++ trptophan

Termination signal

For RNA polymerase,

Which stops transcription


Attenuated-controlled bacterial operons

Regulation of other amino acid biosynthesis operons

Leader Peptides of other attenuated-controlled Bacterial operons

Isoleucine and leucine


The ara Operon of E. coli: Positive and Negative Control

At the same time that Jacob &

Monod were doing their work,

Englesberg, et. al. were studying

The regulation of the arabinose

(ara) operon of E. coli.

They found that instead of being

regulated with a negative control

mechanism as seen in the lac operon,

the ara operon was primarily under

Positive control. Although their

conclusion were not widely accepted,

biochemical and molecular test proved

that they were correct.


In the lac operon, allolactose would bind the repressor to remove it from the operator so that the

Polymerase could bind the operator and start transcription.

In the ara operon, two molecules of AraC protein bind and act as a bridge from the operator (araO2 )

And to the promoter region ara I1 which creates a loop that prevents the binding of CAP-cAMP.

With the addition of arabionose, the arabionose bound AraC protein is allosterically modified to bind to

ara I2, which allows CAP-cAMP to bind the CAP site and positive regulate gene expression occurs.


However, for the ara operon to function, glucose can not be

Present. If present, it will eliminate cAMP and focus on the

Utilization of glucose

**Positive regulation of activators is now known to occur in a

Variety of prokaryotic systems and in all eukaryotes.


Bacteriophage Gene regulation

Regulation of gene expression in the lytic cycle and lysogeny

in baceriophage lambda (λ)

Excellent model for developmental switches in eukaryotic systems

After λ infection of bacteria, a choice is made between lytic and lysogenic pathways

  • Linear chromosome is circularized
  • in host
  • Transcription begins at PL & PR
  • PL promoter for left early operon
  • PR promoter for right early operon
  • These promoters are on different DNA
  • Strands.

Depends on a genetic switch, which involves competition between

the products of the CI gene (the repressor) and the Cro gene (the Cro protein)

regulator of CI gene.



Important info


cI gene

Cro gene

N gene

N = protein is the antiterminator

that allows RNA transcription

past transcription terminator signals.



cII protein stimulates

synthesis of cI repressor

which competes with

Cro protein.

Decision on which pathway

is taken is determined by the

amount of λ repressor or

Cro protein that is bound

to PR or OR region.

cI protein

Integration of λ