Operons and Gene Regulation

Coordinately Regulated Genes

  • Both eukaryotes and prokaryotes have genes that are coordinately regulated.

Operons

  • Operons are closely linked genes that produce a single messenger RNA (mRNA) molecule during transcription.
  • Operons are under the control of the same regulatory sequence.
  • An operon includes:
    • The genes to be transcribed.
    • The regulatory sequence.
    • The operator.

Operator

  • An operator is a sequence that either inhibits or promotes transcription by binding with regulatory proteins.
  • In prokaryotes, groups of genes called operons are transcribed in a single messenger RNA.

Structural Proteins

  • Structural proteins with related functions are typically encoded together in the genome.
  • These related genes that are transcribed together are controlled by a single regulatory sequence.
  • Regulatory genes can control the expression of all the genes at the same time.

Lac Operon

  • The lac operon is an example of an inducible operon.
  • It is considered inducible because it is usually turned off.
  • When the regulatory protein is bound to the operator, RNA polymerase cannot bind to the regulatory sequence.
  • The regulatory protein bound to the operator inhibits the transcription of the genes that are part of the lac operon.

Inducers

  • Inducers are molecules that can bind to the regulatory protein and cause it to change shape.
  • When the inducer binds to the regulatory protein, the protein changes shape, causing it to release from the operator. Allolactose functions as the inducer.
  • This process causes the regulatory protein to release from the operator, freeing up RNA polymerase to transcribe the operon's genes.
  • RNA polymerase can now bind to the regulatory sequence, and the genes located close to each other can be transcribed into a single messenger RNA, allowing those proteins to be made.

Role of Glucose

  • The amount of glucose in the cell also helps to regulate gene expression in the lac operon.
  • When glucose is low, other transcription factors bind to the regulatory sequence to further promote transcription.
  • cAMP and CAP bind to the regulatory sequence and promote transcription of the genes when glucose levels are low.
  • When glucose levels are high, these transcription factors (cAMP and CAP) are not present.

Allolactose

  • Allolactose, a derivative of lactose, acts as an inducer in the lac operon.