Metabolism is the sum of the chemical reactions that occur in an organism. It can be described as a series intersecting, enzyme catalyzed pathways that either build or break components down. Some of these pathways can be regulated by controlling the production of the enzyme catalysts. In this chapter, we take a closer look at this method of regulation by examining two pathways: tryptophan synthesis and lactose catabolism.

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Class Notes

[Introduction] [The Trp Operon] [Repressible vs Inducible Systems]

[The Lac Operon] [Negative vs Positive Control]


Trp Operon

  • The genes for the five enzymes in the Trp synthesis pathway are clustered on the same chromosome in what is called the Trp operon.
  • The Trp operon has three components:
    • Five Structural Genes:
      • These genes contain the genetic code for the five enzymes in the Trp synthesis pathway
    • One Promoter:
      • DNA segment where RNA polymerase binds and starts transcription
    • One Operator:
      • DNA segment found between the promoter and structural genes. It determines if transcription will take place. If the operator is turned "on", transcription will occur.
  • When nothing is bonded to the operator, the operon is "on".
    • RNA polymerase binds to the promoter and transcription is initiated.
    • The five structural genes are transcribed to one mRNA strand.
    • The mRNA will then be translated into the enzymes that control the Trp synthesis pathway.
  • The operon is turned "off" by a specific protein called the repressor.
    • The repressor is a product of the regulator gene which is found some distance from the operon.
      • Transcription of the regulator produces mRNA which is translated into the repressor.
      • The repressor is inactive in this form and can not bind properly to the operator with this conformation.
    • To become active and bind properly to he operator, a co-repressor must associate with the repressor.
      • The co--repressor for this system is Trp
        • This makes sense because E. coli does not want to synthesize Trp if it is available from the environment
        • The more Trp available, the more that can associate with repressor molecules.
    • An active repressor binds to the operator blocking the attachment of RNA polymerase to the promoter.
    • Without RNA polymerase, transcription and translation of the structural genes can't occur and the enzymes needed for Trp synthesis are not made.

Repressible vs Inducible Systems

The Lac Operon

Negative vs Positive Control