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Horizontal gene transfer from transgenic plants to bacteria

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Codon usage. Introns. Integration into existing regul. Units. Promotor acivity ... Codon usage. Protein sequence. Protein structure. Methods for detection ... – PowerPoint PPT presentation

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Title: Horizontal gene transfer from transgenic plants to bacteria


1
Horizontal gene transfer from transgenic plants
to bacteria
  • Se Genetic Modified Plants in
  • the Environment
  • Gertrude Wegl
  • 0007737 A441

2
1. Introduction
  • GMOs organism containing new combinations of
    genetic material, which can not arise naturally
  • subject of both ethical, political,
    environmental and health debates
  • Development of GMO important step
  • ? Gene technology expanded the
  • genetic variation to plant breeders

3
Field Trails
  • Since first field trails 1986- concerns over
    ecological impacts
  • Start of risk assessment of dissemination of
    transfered genes into environment

4
Field Trails in EU
5
Gene Transfer
  • vertical tranfer
  • an organism receives genetic material from its
    ancestor

6
Gene Transfer
  • Horizontal gene transfer
  • an organism transfers genetic material
  • to another cell that is not its offspring

7
2. Horizontal gene transfer
  • In plants no identified mechanism for gene
    transfer ? HGT from plants to bacteria approached
    within the known mechanism in bacteria
  • HGT of genetic information between bacteria
    demonstrated in vitro and in natural systems

8
Horizontal gene transfer
  • In bactria transfer of genes gained much interest
    since the first multi drug resistant bacteria
    (60-ties 70 ties)
  • Seems to be an important mechanism in evolution
    (base composition, MLST, Comparative genome
    sequencing)

9
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10
Horizontal gene transfer
  • Transgenes
  • Native genes slight modification
  • From same/ different species
  • Synthetic genes
  • Gene clusters encoding entire pathways
  • Difference
  • Functional combination of DNA
  • Broad host range promotors
  • Produce distinct selectable phenotyp
  • Increased homology to prokaryotes

11
Reasons to suspect HGT
  • artificial gene constructs structurally unstable
  • artificial constructs vectors designed to be
    invasive to foreign genomes
  • Mechanism enabling foreign genes to insert also
    enable to jump out
  • Viral promotors used in GMOs ? recombination hot
    spots
  • Increased DNA homology to prokaryotes

12
Horizontal gene transfer
  • Depends on
  • number of bacterial species capable of
  • transferring genes
  • host range regulating factors
  • nature and availability of transferred DNA
  • transfer efficiency and selective forces

13
2.1. Mechanism of HGT
  • exist 3 different mechanism
  • A) Transduction
  • B) Conjugation
  • C) Transferation

14
A) Transduction
  • transfer of bacterial genes from one bacterium
    to another mediated by bacterial viruses
  • One of the most important gene transfer mechanism
  • Shown to occur at high frequencies in water
  • Viruses that function in plants and bacteria are
    not yet identified

15
B) Conjugation
  • DNA transfer occurs from one bacteria cell to
    another to direct cell contact specialised
    conjugation apparatus
  • between closely related species , different
    genera and even Gram and Gram-, transferable
    genes reside on plasmids
  • Bifunctional plasmids interspecies transfer
    conjugative transposons

16
C) Transformation
  • Uptake and incorporation of naked DNA from the
    surrounding medium
  • Bacterium must be in state of competence
  • Heritable capability of natural transformation
    detected
  • Relatively common in bacteria, essential role in
    nature

17
Mechanism of HGT
18
2.2. Barriers of HGT
  • Main barriers to interdomain exchange
  • transfer barriers
  • establishment barriers
  • But also temporal and spatial barriers
  • selection/ evolutionary barriers

19
Availability of DNA
  • In case of HGT from plants - studies concentrate
    on natural transformation
  • free DNA must be present
  • from living cells or dead decaying cells
    (plant roots, leaves)
  • Stability of DNA very important
  • Physical and chemical condition of DNA important

20
Availability of DNA
21
Uptake of DNA in Bacteria
  • Influenced by 3 factors
  • Dependence on active transfer of DNA from the
    donor bacterium
  • Development of competence for the uptake
  • Rapid degradation of DNA by restriction enzymes

22
Uptake of DNA in Bacteria
  • Development of competence
  • state which bacteria require before
    successful uptake from DNA of the environment,
  • ability to take up naked DNA genetically
    encoded and regulate
  • Degradation of DNA by restriction enzymes
  • restriction of heteroloug DNA
  • - single stranded DNA may not be affected

23
Stabilization of DNA in Bacteria
  • Recirculating into self replicating plasmid
  • Stabilization by a plasmid
  • rescue like mechanism
  • Integration into the bacterial chromosome

24
Stabilization of DNA in Bacteria
  • Homologous
  • most efficient for ssDNA
  • integration of genes requires
  • sequence homology
  • degree of heterology ?
  • main factor determining
  • the barrier

25
Stabilization of DNA in Bacteria
  • illegimate
  • poorly undertood mechanism, does not require
    substantial DNA homology
  • site specific
  • mobile or mobilized elements
  • Stabilization of heterologous DNA - requires
    linkage to an ori

26
Uptake of DNA in Bacteria
  • Expression of heterologous DNA
  • failure to express genes taken up ? strong
    barrier
  • Codon usage
  • Introns
  • Integration into existing regul. Units
  • Promotor acivity

27
Uptake of DNA in Bacteria
  • Selection and environmental impact of bacterial
    transformants
  • Selection strength on the
  • outcome very important

28
2.3. Methods for detection
  • Comparison of DNA sequences
  • phylogenetic approach comparing
  • Nucleotide sequence
  • G C content
  • Codon usage
  • Protein sequence
  • Protein structure

29
Methods for detection
  • Screening of bacteria from environmental samples
  • f. ex. Sugar beet

30
Methods for detection
31
Methods for detection
  • Experimental studies in the laboratory
  • HGT from GMP to Bacteria only investigated
    experimentally (Acinetob.)
  • - mostly under optimized conditions
  • transfer frequencies 1,4108 5,4109

32
4. Conclusion
  • Experimental evidence demonstrating HGT of
    heterolouges genes from GMOs to naturally
    occuring bacteria currently not available
  • No single barrier but several factors limiting

33
Conclusion
  • probably frequencies of successful HGT from
    plants to bacteria very low
  • Selection of transfered genes will determine
    stability and impact

34
Conclusion
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