DEFINITIONS

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Chapter 7 Operons Fine Control of Prokaryotic
Transcription
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Control of gene expression is essential to
life. Grouping functionally related genes
together so they can be regulated
together. Operon
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7.1 The lac Operon
Lactose metabolism in E.coli is carried out by
two enzymes, with possible involvement by a
third. The genes for all three enzymes are
clustered together and transcribed together from
one promoter, yielding a ploycistronic message.
These three genes, linked in function, are
therefore also linked in expression. They turned
off and on together.
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Negative control of the lac operon
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Conversion of lactose to allolactose(inducer)
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Discovery of the OperonMerodiploid with one
wild-type gene and one
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Conclusion Jacob and Monond, by skillful genetic
analysis, were able to develop the operon concept.
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Repressor-Operator Interactions

• Cohn and colleagues demonstrated with a
  filter-binding assay that lac repressor binds to
  lac operator. Furthermore ,this experiment showed
  that a genetically defined constitutive lac
  operator has lower than normal affinity for the
  lac repressor, demonstrating that the sites
  defined genetically and biochemically as the
  operator are one and the same.

-IPTG
O
OC
No operator
IPTG
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Positive control of the lac operon
Positive control of the lac operon, and certain
other inducible operons that code for
sugar-metabolizing enzymes, is mediated by a
factor called catabolite activator protein (CAP),
which, in conjunction with cyclic-AMP, stimulates
transcription. Depressed by glucose
Wild-type CAP
cAMP has many effects, and may indirectly inhibit
some step in expression of the lacZ gene in vitro.
Mutant CAP
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The Mechanism of CAP Action

• The CAP-cAMP complex stimulates transcription of
  the lac operon by binding to an activator site
  adjacent to the promoter and helping RNA
  polymerase bind to the promoter.

CAP-cAMP allow formation of an open promoter
complex
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Electrophoresis of CAP-cAMP promoter complexes
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Hypothesis for CAP-cAMP activation of lac
transcription
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7.2 the ara Operon

• The ara Operon of E.coli is another
  catabolite-repressible operon.
• Three interesting features to compare with the
  lac operon
• 1.two ara operators exist araO1 and araO2.
• 2.the CAP-binding site is about 200bp upstream of
  the ara promoter, yet CAP can still stimulate
  transcription.
• 3.the operon has another system of negative
  regulation, mediated by the AraC protein

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Proteins must bind to the same face of the DNA to
interact by looping out the DNA
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Control of the ara operon
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Autoregulation of araC

• AraC controls its own synthesis by binding to
  araO1 and preventing leftward transcription of
  the araC gene.

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7.3The trp operon

• The lac operon codes for catabolic enzymes.
• The trp operon codes for anabolic enzymes.

Negative control of the trp operon.
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Attenuation in the trp operon
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Mechanism of attenuation

• Attenuation impose an extra level of control on
  an operon, over and above the repressor-operator
  system. It operates by causing premature
  termination of transcription of the operon when
  the operons products are abundant.

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Two structures available to the leader-attenuator
transcript
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Sequence of the leader Overriding attenuation
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Chapter 8 Major Shifts in Prokaryotic
Transcription
Operon the ways in which bacterial control the
transcription of a very limited number of genes
at a time. Radical shifts in gene expression
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8.1 modification of the host RNA polymerase
during phage infection

• Temporal control of transcription in phage
  SPO1-infected B.subtilis.

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• Specificities of polymerases B and C.
• B contain gp28
• C contain gp33 and gp34.

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8.2 The RNA polymerase encoded in phage T7
Temporal control of transcription in phage
T7-infected E.coli
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8.3 control of transcription during sporulation
When the bacterium B.subtilis sporulates, a whole
new set of sporulation-specific genes is turned
on, and many, but not all, vegetative genes are
turned off. This switch takes place largely at
the transcription level. It is accomplished by
several new s-factors that displace the
vegetative s-factors from the core RNA polymerase
and direct transcription of sporulation genes
instead of vegetative genes. Each s-factor has
its own preferred promoter sequence.
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8.4 genes with mutiple promoters
Some prokaryotic genes must be transcribed under
conditions where two different s-factors are
active. These genes are equipped with two
different promoters, each recognized by one of
the two s-factors. This ensures their expression
no matter which factor is present and allows for
differential control under different conditions.
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8.5 The E.coli heat shock genes

• The heat shock response in E.coli is governed by
  an alternative s-factors, s32(sH) which displaces
  s70(sA) and directs the RNA polymerase to the
  heat shock gene promoter. The accumulation of s32
  and enhanced translation of the mRNA encoding
  s32.

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