Gene Control: Transcription Factors (OCR A Level Biology): Revision Note

Gene Control: Transcription Factors

• 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