7.2.1 Regulation of Gene Expression by Proteins

The function of the promoter

• Only some DNA sequences code for the production of polypeptides, these are called coding sequences
• Non-coding sequences produce functional RNA molecules like transfer RNA (tRNA) or are involved in the regulation of gene expression such as enhancers, silencers and promoters
• The promoter is a non-coding sequence located near to a gene
  • The promoter is not itself transcribed
• The promoter acts as the binding site for RNA Polymerase during the initiation of transcription
• Binding of RNA Polymerase to the promoter is under the control of various regulatory proteins

Gene expression varies in different cells

• Genes are not expressed equally in every cell
  • Essential genes needed for the survival of an organism are expressed all the time
    • eg. Genes for the main enzymes in the respiratory pathways or ATP synthase
  • Other genes are only expressed when needed and at levels that make specific amounts of protein
    • eg. The gene for rhodopsin that is only expressed in light-sensitive receptor cells of the eye
• Regulatory mechanisms exist to ensure the correct genes are expressed at the correct time
  • These mechanisms are different between prokaryotes and eukaryotes but both employ transcription factors and other proteins that bind to specific sequences in DNA

Regulation of gene expression in eukaryotes

• Eukaryotes regulate gene expression in response to variations in their environment
• Specific proteins bind to DNA to regulate transcription and ensure that only the genes required are being expressed in the correct cells, at the correct time and to the right level
  • This is key to how processes of cellular differentiation and development in multicellular organisms are controlled
• Regulatory transcription factor proteins include activators and repressors, they are unique to a specific gene
  • Activator proteins bind to enhancer sequences and increase the rate of transcription
  • Repressor proteins bind to silencer sequences and decrease or block transcription
• General transcription factors are a type of transcription factors that bind directly to the promoter to help initiate transcription
  • This helps RNA polymerase to attach to the promoter and start transcribing the gene
  • In eukaryotes, several general transcription factors are needed for transcription

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

Regulation of gene expression in prokaryotes

• Unlike in eukaryotes, only one general transcription factor and RNA polymerase is needed to initiate transcription
• When prokaryotes need to respond to environmental changes, additional proteins interact directly with target regions of DNA to alter the level of gene expression
• For example, the genes involved in the breakdown and metabolism of lactose by Escherichia coli are repressed in the absence of lactose
  • When no lactose is available, a repressor protein binds to DNA near the promoter of the genes for the proteins that degrade lactose (lacZ, lacY and lacA) 
    • The repressor physically blocks RNA polymerase from accessing that section of the bacteria's genome
  • If lactose is present, the repressor protein is released from the DNA allowing RNA polymerase to begin transcription
  • The genes are expressed and the lactose-degrading enzymes are produced
    • Lactose can be broken down and used for energy generation in this way
  • Once all the available lactose is metabolised, the genes are repressed again
  • This mechanism of negative feedback ensures that a cell’s resources are not wasted making proteins that are not needed

In the absence of lactose the repressor protein binds which prevents RNA polymerase from initiating transcription of genes coding for enzymes used to metabolise lactose.

When lactose is present it binds to the repressor protein allowing RNA polymerase to bind to the promoter and begin transcription