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The lac Operon

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inducer (usually a small molecule) is needed to allow initiation of transcription. ... -galactosidase (cleaves lactose into glucose and galactose) ... – PowerPoint PPT presentation

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Title: The lac Operon


1
The lac Operon
2
Diauxic Growth Curve
Time in hours
3
What was happening?
The cells use glucose first. When
they run out of glucose, they use lactose.
4
Beta galactosidase is not expressed unless 1)
There is no glucose. 2) There is lactose.
5
Two categories of transcription regulation
  • Negative control repressor protein present which
    prevents transcription,inducer (usually a small
    molecule) is needed to allow initiation of
    transcription.
  • Positive control activator protein induces
    transcription. No repressor must be overridden.

6
Repressor Proteins
  • Repressor proteins bind to an operator site
    within the promoter.
  • Binding of a repressor prevents binding of RNA
    polymerase
  • The repressor is an allosteric protein that can
    be activated by binding to a small molecule

7
Repressor Proteins
  • A repressor is a protein product made by one gene
    in the bacterium in order to control a second
    gene by turning it off when its product is not
    wanted.

8
Activator Proteins
  • Activator proteins bind to DNA sequences and then
    interact with RNA polymerase to enhance its
    binding.
  • Activator proteins can be allosteric and bind to
    small molecules before they can bind to DNA.

9
Activator Proteins
10
Regulation of tryptophan synthesis
11
The operon
A group of genes whose expression is coordinated
by an operator (F. Jacob et al.) 1960.
12
The promoter
A genetic element necessary for the expression of
an operon (F. Jacob et al.) 1964.
13
Lactose metabolism in E. coli.
  • Enzymes necessary for lactose metabolism
  • ß-galactosidase (cleaves lactose into glucose and
    galactose)
  • lactose permease (transports lactose into the
    cell)
  • transacetylase (function unknown)

14
The lac operon
  • lacZ (ß-galactosidase)
  • lacY (lactose permease)
  • lacA (transacetylase)

15
Negative control of lac operon
16
  • Glucose levels are correlated to the
    concentration of cAMP via the enzyme that
    synthesizes cAMP, adenylyl cyclase.
  • High glucose levels adenylyl cyclase
    low cAMP levels
  • Low glucose levels adenylyl cyclase
    high cAMP levels
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