The ara Operon in E' coli

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The ara Operon in E. coli

• Control of arabinose metabolism
• No arabinose, no expression of the genes encoding
  for the related enzymes
• With arabinose and glucose, still little
  expression of the genes
• With arabinose but no glucose, the genes are
  expressed

glucose
arabinose
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Cis and Trans Factors

• Promoter
• pBAD
• Operators
• araO1 controls gene araC which is a regulatory
  protein
• araO2 controls PBAD
• araI
• araPc site
• AraC protein
• Arabinose

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Regulatory Control of ara Genes
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Autoregulation of araC
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The trp Operon in E. coli

• Anabolic pathway vs. catabolic pathway
• Tryptophan is a signal to turn off the operon

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Cis and trans Factors

• trpO, P
• Trp leader and attenuator
• Aporepressor
• Tryptophan (co-repressor)

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70-fold repression
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Additional Control of the trp Operon---Attenuatio
n
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Pre-mature Termination of Transcription
Attenuation
Structure genes
O, P
Leader attenuator
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(No Transcript)
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Illustration of two alternative mRNA structures
in the attenuator
1
2
3
4
A
Transcription is attenuated
B
Attenuation is defeated
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Attenuation of the trp Operon