The lac operon

The lac operon
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  • The Lac Operon
  • What is the lac operon?
  • Promotes RNA polymerase binding
  • The lac operon
  • CAP
  • CAP site
  • RNA polymerase
  • Promoter
  • Repressor
  • Operator
  • lacZ      lacY      lacA
  • Blocks RNA polymerase
  • The Lac Operon is a operon which is important for the transport and metabolism of lactose in E.coli. It has three different genes in bacteria that produce the necessary enzymes to get energy from lactose.
  • When lactose is not present, the lac repressor binds tightly to the operator preventing the RNA polymerase transcripting.
  • CAP site
  • CAP site
  • RNA polymerase
  • RNA polymerase
  • Promoter
  • Promoter
  • Operator
  • Operator
  • Repressor
  • lacZ      lacY      lacA
  • lacZ      lacY      lacA
  • Repressor
  • Often, glucose is the sugar of choice for bacteria. When the glucose runs low, the lac operon allows an organism to survive on lactose. The three main genes that the lac operon contains is lacZ, lacY, and lacA. Each of these genes produce different enzymes needed to produce the lactose.
  • cAMP CAP
  • CAP
  • when glucose is low, cAMP is produced which attaches to CAP enabling it to bind to DNA. CAP also helps RNA polymerase bind to the promoter. This creates a high level of transcription.
  • CAP site
  • CAP site
  • RNA polymerase
  • RNA polymerase
  • Promoter
  • Promoter
  • Operator
  • Operator
  • High transcription
  • lacZ      lacY      lacA
  • Low transcription
  • lacZ      lacY      lacA
  • LacY is the enzyme which transports the lactose through the cell membrane. LacZ is the enzyme that separates lactose into glucose and galactose. LacA is important as it produces a molecule which helps metabolize galactose. There are mechanisms in place that ensure the lac operon is not expressed when not needed to be. When lactose is not available it cant be expressed by the lac operon, this is caused by another gene, lacI.
  • High level transcription
  • Low level transcription
  • No transcription
  • No transcription
  • Level of transcription
  • -
  • +
  • Glucose
  • +
  • -      +     +      -
  • -
  • +
  • Lactose
  • -
  •  +
  • -
  • -
  • Cap binds
  • +
  • -
  • +
  • Repressor binds
  • E.coli will use lactose if it is the only sugar available to them, for bacteria to use lactose the lac operon genes must be expressed and should only be expressed when lactose is available and glucose is not. The levels of lactose and glucose are detected by two things; the lac repressor and the catabolite activator protein (CAP). The lac repressor prevents transcription by RNA polymerase by binding itself to the operator.
  • With lactose, allolactose binds to the lac repressor and releases the operator. RNA polymerase is now able to transcribe the operon.
  • Allolactose
  • When lactose is available, the lac repressor isn't able to bind itself to the RNA polymerase, so transcription is made possible. RNA polymerase struggles to bind with the lac repressor and requires help from the CAP. The Cap binds to a part of the DNA and helps polymerase attach to the promoter. CAP is not always available therefore another molecule called cyclic AMP (cAMP) comes into placed when glucose levels are low, acting as a 'hunger signal'. cAMP binds to CAP and makes it able to bind DNA.
  • When glucose levels are high, no cAMP is produced resulting in the DNA not being able to bind. This produces a low level of transcription.
  • For the lac operon to switch on glucose must not be available and lactose must be available, this allows the activator to bind and the repressor to release, making transcription possible. Below is a summary of different variations that effect the lac operon. Glucose present, lactose absent: No transcription. Glucose present, lactose present: low-level transcription. Glucose absent, lactose absent: No transcription. Glucose absent, lactose present: high level transcription.
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