GENE EXPRESSION VIA LAC OPERON

• Prokaryotic vs. Eukaryotic cells

  • Prokaryote: No nucleus, No membrane bound organelles, DNA free in cytoplasm in loops and plasmids
  • Eukaryote: Nucleus, Membrane bound organelles, DNA in nucleus
• Lactose

  A disaccharide (carbohydrate) made up of a glucose molecule and a galactose molecule
• Glucose

  A monosaccharide (carbohydrate)
• Operon

  A group of genes that are transcribed at the same time, usually control an important biochemical process, only found in prokaryotic cells
• Lac operon

  Consists of three genes which code for proteins, each involved in processing sugar lactose (lactose metabolism)
• Lac operon genes

  • Lac z gene encodes for the enzyme Beta-galactosidase, this enzyme hydrolyses (splits) lactose into glucose and galactose
  • Lac y gene encodes a permease (transport lactose into the cell)
  • Lac a gene encodes the transacetylase enzyme (helps metabolise lactose)
• E.coli can use either lactose OR glucose as an energy source
• Lactose has to be hydrolysed (digested) first into glucose and galactose
• The bacterium prefers to use glucose when it can
• Operon

  Consists of three elements: The genes that it controls, A promoter region where RNA polymerase can bind, An operator region between the promoter and the first gene which acts as an on off switch
• Bacteria respond to variations in the environment by altering their gene expression pattern
• The lac operon is efficient - it is wasteful to synthesis lactose metabolising enzymes in the absence of lactose
• Lac operon regulation

  1. In the absence of inducer lactose, the gene Lacl produces a repressor protein which binds to the operator site and prevents transcription of structural genes by blocking the RNA polymerase

  2. When the lactose is present (not glucose), it binds to the repressor from binding to the operator. The operator induces RNA polymerase to bind to the promotor and mRNA transcribes the structural genes - enzymes are produced
• Lactose present

  Genes switched on (transcription allowed) to make the enzymes Lac Z, Lac Y and Lac A to breakdown the lactose so it can use it as a food source
• Lactose absent

  Genes supressed, no transcription - energy saved
• Lac operon regulation

  1. A represor protein is continusously synthesised. It sits on a sequence of DNA juts infront of the lac operon, the operator site

  2. A small amount of sugar lactose is formed within the bacteria cell. This fits onto the repressor protein at another active site, causing the repressor protein to change its shape (a conformational shape change). It can no longer sit on the operator site. RNA polymerase can now reach its promoter site

  3. When glucose and lactose are present RNA polymerase can sit on the promotor site but is unstable and keeps falling off

  4. An activator protein is needed to stabilise RNA polymerase, but the activator protein only works when glucose is absent
• If glucose is present

  1. Regulatory gene produces regulatory protein

  2. Regulatory protein binds to operator

  3. Stops enzyme RNA polymerase transcribing structural genes lac z, y, and a

  4. No enzymes synthesised for digestion of lactose

  5. E.coli cell saves energy by not producing structural proteins
• If there is lactose present and no glucose

  1. Regulatory gene produces repressor protein

  2. Repressor protein binds to operator site

  3. Allolactose is an inducer, it binds to repressor protein

  4. Stops lac repressor protein blocking RNA polymerase on operator

  5. Enzyme RNA polymerase transcribes structural genes lac z, y, and a

  6. Genes code for proteins B-galactosidase and permease

  7. Lac will be synthesised for digestion of lactose, which can be used as energy source in absence of glucose
• Benefits of the Lac Operon

  • Bacteria E.coli only expresses structural genes coding for enzymes B-galactosidase and permease to metabolise lactose when present and glucose is absent
  • Saves energy for bacteria by not making proteins that aren't required and only making them when necessary