16.3.3 Gene Control: Transcription Factors

• Prokaryotes use operons to control the expression of genes in cells
• Eukaryotes also use transcription factors to control gene expression
• A transcription factor is a protein that controls the transcription of genes by binding to a specific region of DNA
• They ensure that genes are being expressed in the correct cells, at the correct time and to the right level
• It is estimated that ~10% of human genes code for transcription factors
  • There are several types of transcription factors that have varying effects on gene expression
  • This is still a relatively young area of research and scientists are working hard to understand how all the different transcription factors function
  • Transcription factors allow organisms to respond to their environment
  • Some hormones achieve their effect via transcription factors

How transcription factors work

• Some transcription factors bind to the promoter region of a gene
  • This binding can either allow or prevent the transcription of the gene from taking place

• The presence of a transcription factor will either increase or decrease the rate of transcription of a gene
• For example, PIF is a transcription factor found in plants that activates the transcription of the amylase gene

A transcription factor binding to the promoter region of a gene which allows RNA polymerase to bind and for transcription to occur

• Plant cells use transcription factors in a similar way to animal cells
• Gibberellin is a hormone found in plants (e.g. wheat and barley) that controls seed germination by stimulating the synthesis of the enzyme amylase
• It does this by influencing transcription of the amylase gene
  • When gibberellin is applied to a germinating seed there is an increased amount of the mRNA for amylase present

Mechanism

• The breakdown of DELLA protein by gibberellin is necessary for the synthesis of amylase
• The following components are involved:
  • Repressor protein DELLA
  • Transcription factor PIF
  • Promoter of amylase gene
  • Amylase gene
  • Gibberellin
  • Gibberellin receptor and enzyme

• The process occurs as follows:
• DELLA protein is bound to PIF, preventing it from binding to the promoter of the amylase gene so no transcription can occur
• Gibberellin binds to a gibberellin receptor and enzyme which starts the breakdown of DELLA
• PIF is no longer bound to DELLA protein and so it binds to the promoter of the amylase gene
• Transcription of amylase gene begins
• Amylase is produced

The breakdown of DELLA protein by Gibberellin allows the transcription factor PIF to bind to the promoter for the amylase gene and for transcription to initiate