Horizontal gene transfer from transgenic plants to bacteria

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Horizontal gene transfer from transgenic plants
to bacteria

• Se Genetic Modified Plants in
• the Environment
• Gertrude Wegl
• 0007737 A441

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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

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Field Trails

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

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Field Trails in EU
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Gene Transfer

• vertical tranfer
• an organism receives genetic material from its
  ancestor

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Gene Transfer

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

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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

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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)

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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

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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

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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

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2.1. Mechanism of HGT

• exist 3 different mechanism
• A) Transduction
• B) Conjugation
• C) Transferation

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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

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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

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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

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Mechanism of HGT
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2.2. Barriers of HGT

• Main barriers to interdomain exchange
• transfer barriers
• establishment barriers
• But also temporal and spatial barriers
• selection/ evolutionary barriers

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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

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Availability of DNA
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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

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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

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Stabilization of DNA in Bacteria

• Recirculating into self replicating plasmid
• Stabilization by a plasmid
• rescue like mechanism
• Integration into the bacterial chromosome

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Stabilization of DNA in Bacteria

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

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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

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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

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Uptake of DNA in Bacteria

• Selection and environmental impact of bacterial
  transformants
• Selection strength on the
• outcome very important

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2.3. Methods for detection

• Comparison of DNA sequences
• phylogenetic approach comparing
• Nucleotide sequence
• G C content
• Codon usage
• Protein sequence
• Protein structure

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Methods for detection

• Screening of bacteria from environmental samples
• f. ex. Sugar beet

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Methods for detection
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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

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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

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Conclusion

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

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Conclusion