Bacterial plasmids

by

E. Börje Lindström

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

• Transposon: - a movable genetic element

Different plasmidsGrouped after their properties:

• F-plasmid /factor: 100 kb

Different plasmids, cont.• R- plasmid: - have genes for resistance

against antibiotics and/or heavy metals

Plasmid R100

Size in kb pairs

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

Plasmid genesGrouping

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

•Copy number: -a certain amount of copies present per cell

- controlled by the initiation frequency

- low (1-4) to high (10-100)

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

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