Horizontal Evolution

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Horizontal Evolution

• Conjugation
• Transformation
• Transposition
• Transduction

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R Streptococci (avirulent) S Streptococci
(virulent)
Griffiths Experiment
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Transformation

• Bacteria directly take up free DNA from the
  environment. (different from conjugation)
• Competence the ability of bacteria to take up
  DNA

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Natural Transformation

• Only some bacteria can take up free DNA
  naturally
• Genetic exchange
• DNA repair
• DNA as food

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Specificity of DNA Uptake

• Some bacteria only take up their own DNA
• (e.g. haemophilus)
• Avoids dangerous DNA?
• Repair their own damaged DNA?
• Some bacteria take up all DNA
• (e.g. Bacillus)
• As food?

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Control of Competence in Bacillus subtilius

• ComX a pheromone released from cells
• ComP a membrane-localized histidine kinase
• ComA activator
• A group of regulatory proteins

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Control of Competence in Bacillus subtilius

• Concentration of pheromones is proportional to
  cell density (quorum-sensing)
• ComQ is responsible for cleavage and modification
  of pre-ComX inside cells

10 AA
55 AA
ComX
Pre-ComX
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Transfer of Phosphate From ComP to ComA
ComP-His ATP ComX
ComP-His-P
ComP-His-P ComA-Asp
(Inactive)
ComP-His ComA-Asp-P
(Active)
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Schematic Representation of the Two Component
Systems
Signal input
Histidine kinase
H
P
P
Response regulator
D
Signal output
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Other Two Component Systems

• EnvZ/OmpR - osmoregulation in E. coli
• PhoR/PhoB - phosphate scavenging in E. coli
• NtrB/NtrC - nitrogen assimilation in a variety of
  bacteria
• DctB/DctD - dicarboxylate transport in Rhizobium
  leguminosarum
• VirA/VirG - virulence by Agrobacterium
  tumefaciens

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Other Two Component Systems

• Bacteria
• Archae
• Single-celled eukaryotic organisms
• Fungi
• Higher plants

So far, there are over 500 two-component systems
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Artificially Induced Competence

• Calcium solution treatment
• Make cell membrane temporarily permeable to
• small DNA molecules
• Electroporation
• Expose cells to DNA solution under high frequent
• electrical pulse

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Electroporation Device
DC pulse is thought both to disrupt temporarily
the membrane and to
electrophorese DNA into cells
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Applications of Transformation

• Introduce DNA into cells
• Plasmids
• Double strand DNA
• Single strand DNA
• Introduce RNA into cells
• RNAi

(RNA interference)