GENE REGULATION

Each cell expresses, or turns on, only a fraction of its genes. The rest of the genes are repressed, or turned off. The process of turning genes on and off is known as gene regulation. Gene regulation also allows cells to react quickly to changes in their environments. Gene regulation can occur at any point during gene expression, but most commonly occurs at the level of transcription (when the information in a gene's DNA is transferred to mRNA). Signals from the environment or from other cells activate proteins called transcription factors. These proteins bind to regulatory regions of a gene and increase or decrease the level of transcription. By controlling the level of transcription, this process can determine the amount of protein product that is made by a gene at any given time.

Introduction

• E. coli   lives in colon. It has a metabolic pathway that allows for the synthesis of the amino acid tryptophan (Trp).
  • This pathway starts with a precurser molecule and proceeds through five enzyme catalyzed steps before reaching the final Trp product.
• It is important that   E. coli   be able to control the rate of Trp synthesis because the amount of Trp available from the environment varies considerably.
  • If we eat a meal with little or no Trp, the   E. coli   in our gut must compensate by making more.
  • If we eat a meal rich in Trp,   E. coli   doesn't want to waste valuable resources or energy to produce the amino acid because it is readily available for use.
• Therefore,   E. coli   uses the amount of Trp present to regulate the pathway.
  • If levels are not adequate, the rate of Trp synthesis is increased.
  • If levels are adequate, the rate of Trp synthesis is inhibited.
• There are two ways of regulating the Trp pathway:
  • The first method works to decrease the synthesis of Trp by inhibiting the first enzyme in the pathway, preventing the rest of the pathway from proceeding.
  • What inhibits the first enzyme? Trp does!
    • The more Trp in the cell, the more that can bind to the first enzyme and prevent it from catalyzing the first step.
  • This method of regulation is feedback inhibition in which the end product of a pathway acts as an inhibitor of an enzyme in that pathway.
• The other method of control stops the production of the enzymes in the pathway at the transcription level.
  • Remember that enzymes are proteins that must be transcribed and translated from the genetic code.
  • If the genes for the enzymes are not transcribed to mRNA, then translation to the enzymes cannot occur.
  • Without enzymes, there is no Trp synthesis.
• This method of control is called regulation of gene expression because control is taking place at the genetic level. This method of control will now be examined in detail.

Trp Operon:

 

Fig. 21. Structure of 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 cannot bind properly to the operator with this conformation.
• To become active and bind properly to the 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.