Lac operon

• The lac operon is an example of controlling gene expression at the transcriptional level, so before the protein is even made
• Operons are a set of structural genes that are regulated by the same regulatory gene
• If the operator sequence is bound by a repressor protein (a type of transcription factor), it prevents RNA polymerase from binding to the promoter region and therefore no mRNA will be produced.
• The lac operon exists to prevent the unneccessary use of resources to produce lactase and only when lactose is present and there is little glucose present
• The conditions for when the lac operon is active:

  Lactose is present

  No glucose
• When lactose is present but there is also glucose, the lac operon is not activated as glucose is used first
• When lactose is present the repressor protein coded by the regulatory gene (LacI) bind to lactose and changes it structure making it no longer complimentary to the operator which allows RNA polymerase to bind and transcribe the structural genes to produce lactase.

  But in order for RNA polymerase to bind to the promoter it also needs the help of the cAMP-CAP complex which only forms when glucose is no present
• In the lac operon in regards to cap and camp.

  when glucose levels are low cAMP accumulates and binds to the allosteric site of CAP and activates it and forms a cAMP-CAP complex and binds to the promoter sequence and helps RNA polymerase transcribe the genes
• Catabolite activation occurs when glucose is absent so cAMP levels increase and activate the cAMP-CAP complex which then binds to the promoter and helps RNA polymerase transcribe the structural genes