Bacterial plasmids

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Bacterial plasmids

• by
• E. Börje Lindström

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Definitions

• Replicon

- a DNA-molecule with its own replication

• Plasmid

• a small extra-chromosomal replicon (autonomous)
• DS, closed molecule
• 1 1000 bp

• Episome

• a plasmid that can be integrated into the
  chromosome
• e.g. the F-plasmid

- a movable genetic element

• Transposon

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Different plasmids
Grouped after their properties

• F-plasmid /factor

? 100 kb
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Different plasmids, cont.
- have genes for resistance against antibiotics
and/or heavy metals

• R- plasmid

Size in kb pairs
Plasmid R100
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Different plasmids, cont.

• Col plasmids

• produces colicins
• antibacterial

• Catabolic plasmids

• have properties to use odd carbon/ energy sources
• many Psuedomonas have such plasmids

• Cryptic plasmids

- no known property
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Plasmid genes
Grouping
1) Essential genes for keeping the plasmid
within the cell

• Replication

• uses the replication system of the host cell
• have its own initiation, elongation and
  termination
• occurs during the entire cell cycle

• a certain amount of copies present per cell
• controlled by the initiation frequency
• low (1-4) to high (10-100)

• Copy number

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Plasmid genes, cont.

• Partitioning

- only a problem for low and medium copy number

• Host specificity/range

- low to broad
2) Non-essential important for transfer

• Are spread horizontally (among bacteria)
• Important genes
• - pili-genes
• - oriT
• - tra/ mob genes

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Plasmid genes, cont.
3) Non-essential with surviving value

• Resistance against antibiotics
• Production of antibacterial substances
  (colicins)
• genes for pathogenesis/virulence
• genes to be able to use special energy/carbon
  sources, e.g. phenol