Transposable Elements

• Families of mobile genetic elements in prokaryotes

  Integron integrase

  ISCR elements

  IS transposase

  MOB relaxases

  Tn3 transposase

  Tn7 transposase
• integron integrase


  intl1

  intl3

  intl4
• iscr elements


  is cr1

  is cr2

  is cr4
• is transposase 


  is 21

  is 3

  is 5
• tn3 transposases


  tn21

  tn3

  tn501
• tn7 transposase


  tn7
• larger than is elements and contain genes that are not necessary for transposition


  tn3 transposase
• crucial role in conjugation

  mob relaxases
• copy and paste


  retroelements
• cut and paste


  dna transposons
• 2 TE in eukaryotes

  TE

  retrotransposons
• transposons in maize

  insertion

  deletion

  translocation
• uses of transposons


  cloning vehicles

  vectors for transferring genes

  drug resistance genes

  mutagenesis

  insertional inactivation
• application of mobile elements

  gene expression

  protein analysis

  genetic engineering

  gene therapy
• negative effects of transposable elements

  disabling the gene

  chromosome duplication

  disease
• first discovered transposable elements in corn in the 1940

  barbara mcclintock
• characteristics of transposable elements

  code enzymes

  recombination and replication

  cannot replicate apart from the host chromosome
• additional characteristics


  enzymatic coding for self-duplication and insertion

  transposition events

  disruption of target protein integriy

  activation of dormant genes

  dependency on host chromosome for replication

  non-homologous insertion
• types of TE in prokaryotes

  insertion sequences

  transposon

  bacteriophage mu
• first transposable elements identified as spontaneous insertion


  insertion sequence
• 800 to 1500 base pairs and do not code for proteins, carry genetic information for transposase


  insertion sequence
• mechanism of transposition in insertion sequences

  rolling circle replication

  formation of non-replicative circular structure

  homologous recombination
• nicking by a replication initiator protein on one parental strand to generate a primer for leading-strand initiation and a single priming site for lagging-strand synthesis

  rolling-circle replication
• role of insertion sequence

  genetic change of microbial population

  promote resistance
• similar to IS but carry additional genes

  transposons
• several thousands base pair long and have genes coding for one or more proteins

  transposons
• short direct repeats, target sequence

  transposons
• types of transposons


  composite transposon

  non-composite transposon
• flanked by two copies of an IS and central coding region with antibiotic resistant gene and NO marker gene

  composite transposon
• do not terminate with IS elements but contain terminal inverted repeats

  non-composite transposons
• three genes at central region of non-composite transposon

  lactamase for breaking amphicilin

  transposase for insertion

  resolvase for recombination
• longest transposon, numerous genes for viral head and tail formation

  bacteriophage mu
• giant mutator transposon


  bacteriophage mu
• terminal part of the mu phage, carries viral genome 


  head
• middle portion which acts as the joining element

  neck
• long, thick, and contractile because of the presence of crossbands

  tail
• size of phage mu

  37 kb
• mechanism of transposition


  transposase cleaves and catalysis at 5' or 3' of both ends of transposon

  duplication of 3 to 12 bases at insertion site

  after attachment of transposon, two replication forks form
• transposition


  movement of TE from one location to another

  staggered cuts in target DNA

  TE is joined to single stranded ends of target DNA

  DNA replication in single stranded gap
• Non replicative transposition


  nick donor dna

  double strand dna repair

  fill gaps, seal nicks