Control mechanisms

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Control mechanisms

• Lac operon
• Trp operon

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• cells need to control when transcription and
  translation occur, if there was no control
  proteins would be produced without end
• some genes must be transcribed all the time to
  meet the needs of the cell, these are called
  housekeeping genes
• Transcription factors turn genes on when required
• This control over production is called Gene
  regulation

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4 levels of gene regulation
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4 levels of gene regulation
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4 levels of gene regulation
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The lac operon

• lactose a disaccharide found in milk, composed
  of glucose and galactose
• E. coli found in mammalian stomach lining use
  this for energy
• To use this they must split the disaccharide by
  using B (beta) galactosidase

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The lac operon

• To determine when it would be useful for
  B-galactosidase to be produced a negative control
  system is used. If lactose is not present the
  production of the enzyme is blocked
• The gene for B-galatosidase is part of an operon

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The lac operon

• Operon a cluster of genes under the control of
  one promoter and one operator in prokaryotic
  cells acts as a simple regulatory loop
• Operator regulatory sequences of DNA to which a
  repressor protein binds
• Remember, we are talking about prokaryotic
  operons, not eukaryotic ones

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The lac operon genes

• The lac operon (3 genes lacZ, lacY, lacA) codes
  for proteins for the metabolism of lactose
• lacZ encodes the enzyme Beta galactosidase
• lacY encodes beta galactosidase permease (an
  enzyme that causes lactose to permeate the cell
  membrane)
• lacA encodes transacetylase (unknown function)

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• lacl protein is a repressor protein that binds to
  the operator and prevents polymerase from
  transcribing
• lactose serves as a signal molecule/inducer, it
  removes lacl from the operator and allows the
  polymerase to transcribe the genes

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Video

• Check out the video labeled lacoperon in mp4
  format

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The trp operon

• tryptophan is an AA used by E. coli for protein
  production
• it will absorb tryptophan if it available, if
  unavailable it will manufacture it
• if tryptophan is present it will repress the
  tryptophan manufacturing process

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The trp operon

• trp operon cluster of genes in a prokaryotic
  cell under the control of one promoter and one
  operator the genes govern the synthesis of the
  necessary enzymes required to synthesize the
  amino acid tryptophan.
• in the lac operon lactose induces gene
  transcription, in the trp operon tryptophan
  inhibts the gene transcription

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The trp operon

• trp operon consists of 5 genes
• the 5 genes code for 5 polypeptides that make 3
  enzymes needed to sythesize tryptophan
• since tryptophan lis needed to inactivate the trp
  operon it is called a corepressor