Gene Control: Transcription Factors (CIE A Level Biology)

• 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

Transcription Factor Diagram

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

Gibberellin Control Diagram

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