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Gene regulation in Prokaryotes


Gene regulation in Prokaryotes Bacteria were models for working out the basic mechanisms, but eukaryotes are different. Some genes are constitutive, others go from ... – PowerPoint PPT presentation

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Title: Gene regulation in Prokaryotes

Gene regulation in Prokaryotes
  • Bacteria were models for working out the basic
    mechanisms, but eukaryotes are different.
  • Some genes are constitutive, others go from
    extremely low expression (off) to high
    expression when turned on.
  • Many genes are coordinately regulated.
  • Operon consecutive genes regulated, transcribed
    together polycistronic mRNA.
  • Regulon genes scattered, but regulated together.

Rationale for Operon
  • Many metabolic pathways require several enzymes
    working together.
  • In bacteria, transcription of a group of genes is
    turned on simultaneously, a single mRNA is made,
    so all the enzymes needed can be produced at once.

Proteins change shape
When a small molecule binds to the protein, it
changes shape. If this is a DNA-binding protein,
the new shape may cause it to attach better to
the DNA, or fall off the DNA.
Definitions concerning operon regulation
  • Control can be Positive or Negative
  • Positive control means a protein binds to the DNA
    which increases transcription.
  • Negative control means a protein binds to the DNA
    which decreases transcription.
  • Induction
  • Process in which genes normally off get turned
  • Usually associated with catabolic genes.
  • Repression
  • Genes normally on get turned off.
  • Usually associated with anabolic genes.

Structure of an Operon
  1. Structural genes actual genes being regulated.
  2. Promoter region site for RNA polymerase to bind,
    begin transcription.
  3. Operator region site where regulatory protein
  4. Regulatory protein gene need not be in the same
    area as the operon. Protein binds to DNA.
  • Look up Animations showing the effects of the
    lactose repressor on the lac operon.
  • As with translation, details will vary. For
    example, the lactose repressor protein is a
    tetramer. How many sites depict it this way?
  • Be wary of oversimplification.

The Lactose Operon
  • The model system for prokaryotic gene regulation,
    worked out by Jacob and Monod, France, 1960.
  • The setting E. coli has the genes for using
    lactose (milk sugar), but seldom sees it. Genes
    are OFF.
  • Repressor protein (product of lac I gene) is
    bound to the operator, preventing transcription
    by RNA polymerase.

Green repressor protein Purple RNA polymerase
Lactose operon-2
  • When lactose does appear, E. coli wants to use
    it. Lactose binds to repressor, causing shape
    change repressor falls off DNA, allows
    unhindered transcription by RNA polymerase.
    Translation of mRNA results in enzymes needed to
    use lactose.

Lactose operon definitions
  • Control is Negative
  • When repressor protein is bound to the DNA,
    transcription is shut off.
  • This operon is inducible
  • Lactose is normally not available as a carbon
    source genes are shut off
  • In bacteria, many similar operons exist for using
    other organic molecules.
  • Proteins for transporting the sugar, breaking it
    down are produced.

Repressible operons
  • Operon codes for enzymes that make a needed amino
    acid (for example) genes are on.
  • Repressor protein is NOT attached to DNA
  • Transcription of genes for enzymes needed to make
    amino acid is occurring.
  • The change amino acid is now available in the
    culture medium. Enzymes normally needed for
    making it are no longer needed.
  • Amino acid, now abundant in cell, binds to
    repressor protein which changes shape, causing it
    to BIND to operator region of DNA. Transcription
    is stopped.
  • This is also Negative regulation (protein DNA

Repression picture
Transcription by RNA polymerase prevented.
Regulation can be fine tuned
The more of the amino acid present in the cell,
the more repressor-amino acid complex is formed
the more likely that transcription will be
Positive regulation
  • Binding of a regulatory protein to the DNA
    increases (turns on) transcription.
  • More common in eukaryotes.
  • Prokaryotic example the CAP-cAMP system
  • Catabolite-activating Protein
  • cAMP ATP derivative, acts as signal molecule
  • When CAP binds to cAMP, creates a complex that
    binds to DNA, turning ON transcription.
  • Whether there is enough cAMP in the cell to
    combine with CAP depends on glucose conc.

Positive regulation-2
  • Glucose is preferred nutrient source
  • Other sugars (lactose, etc.) are not.
  • Glucose inhibits activity of adenylate cyclase,
    the enzyme that makes cAMP from ATP.
  • When glucose is high, cAMP is low, less cAMP is
    available to bind to CAP.
  • CAP is free, doesnt bind to DNA, genes not on.
  • When glucose is low, cAMP is high
  • Lots of cAMP, so CAP-cAMP forms, genes on.
  • Works in conjunction with induction.

Cartoon of Positive Regulation
Attenuation fine tuning repression
  • Attenuation occurs in prokaryotic repressible
    operons. Happens when transcription is on.
  • Regulation at the level of translation
  • Several things important
  • Depends on base-pairing between complementary
    sequences of mRNA
  • Requires simultaneous transcription/translation
  • Involves delays in progression of ribosomes on

Mechanism of attenuation- tryp operon
Mech. of attenuation -2