Gene Control: Transcription Factors (OCR A Level Biology)

• Eukaryotes can use transcription factors to control gene expression
  • Transcription factors are proteins that bind to specific regions of DNA to control the transcription of genes

• 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 (i.e. the region of DNA 'upstream' of the gene that controls the expression of the 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

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

Gene control: oestrogen

• In mammals, the hormone oestrogen is involved in controlling the oestrus cycle and also in sperm production
• Oestrogen is a lipid-soluble molecule and can therefore diffuse through the plasma membrane of cells
• It then moves to the nucleus and binds to an oestrogen receptor
• These receptors are actually transcription factors that are able to initiate transcription for many different genes by binding to their promoter regions
• Once bound, oestrogen causes a change in the shape of the receptor
• As a result, the receptor moves away from the protein complex it is normally attached to and binds to the promoter region of one of its target genes
• This allows RNA polymerase to bind and to begin transcribing that gene

A summary of how oestrogen can stimulate the transcription of a gene

Gene control: gibberellin

• 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 (the one involved is called PIF)
  • Promoter of amylase gene
  • Amylase gene
  • Gibberellin
  • Gibberellin receptor and enzyme

• The process occurs as follows:
• DELLA protein is bound to the transcription factor, 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
• The transcription factor 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