Thanksgiving Dinner

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Thanksgiving Dinner

• How will all the bacteria in your colon deal with
  all the food you eat?

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Bacteria - Prokaryotes
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Intestinal Bacteria are Dependent on Us for
Nutrients

• Bacteria can synthesize amino acids however,
  they prefer to obtain them from their
  environment.
• You will soon be eating tons of turkey, a rich
  source of tryptophan. However, currently you may
  not be providing your intestinal bacteria with
  all the tryptophan they need.
• Where will the bacteria get tryptophan from?
• They prefer to use glucose as a carbon source,
  but they can also use lactose.
• Hopefully, you currently drinking lots of milk,
  but if you dont you may soon be lots of eating
  pumpkin pie with whipped cream.
• How do the bacteria respond to a diet rich in
  lactose?

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Bacteria must respond quickly to our whims

• Bacteria have the capability to synthesize
  tryptophan if needed. However, if the
  concentrations of tryptophan are high the
  bacteria inhibits this biosynthetic pathway

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trp operon

• The five genes coding for the proteins (enzymes)
  that make up the tryptophan biosynthetic pathway
  are clustered together on the bacterial
  chromosome.
• A single promoter makes a single mRNA molecule.

TATA
trpE
trpD
trpC
trpB
trpA
STOP
mRNA
mRNA
trpE
trpD
trpC
trpB
trpA
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Figure 18.19 Regulation of a metabolic pathway
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Inhibition of Gene Expression

• Tryptophan inhibits the first enzyme in the
  biosynthetic pathway
• Tryptophan also inhibits the transcription of the
  genes that encode the enzymes in the biosynthetic
  pathway.
• When tryptophan levels drop, more enzymes are
  synthesized and the first enzyme in the
  biosynthetic pathway is no longer inhibited.

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How is transcription of the trp operon regulated?
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Figure 18.20a The trp operon regulated
synthesis of repressible enzymes
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Figure 18.20b The trp operon regulated
synthesis of repressible enzymes (Layer 1)
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Figure 18.20b The trp operon regulated
synthesis of repressible enzymes (Layer 2)
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Repressible and inducible operons

• Trp operon a repressible operon
• Transcription is inhibited when a specific small
  molecule (tryptophan) binds to a regulatory
  (repressor) protein
• Lac operon an inducible operon
• Transcription is stimulated when a specific small
  molecule (allolactose) binds to an regulatory
  protein.

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Lactose metabolism

• In bacteria, the enzyme beta-galactosidase breaks
  lactose down into glucose and galactose.
• The lac operon contains the gene for
  b-galactosidase and two other enzymes utilized in
  lactose metabolism.
• Allolactose, an isomer of lactose, which is
  present in small amounts, controls that lac operon

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Figure 18.21a The lac operon regulated
synthesis of inducible enzymes
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Figure 18.21b The lac operon regulated
synthesis of inducible enzymes
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Quick response times

• The trp and lac regulatory proteins are always
  being made (constitutive expression).
• In the case of lactose, it only takes 15 minutes
  for the number of b-galactosidase molecules to
  increase 10,000X.