Plasmids

Plasmid

Plasmids are Extrachromosomal DNA, usually circular in shape. They Usually encode ancillary functions for in vitro growth. Plasmids are required for specific cellular functions like: produnction of virulence, formation of antibiotics resistancea and production of bacteriocins (colicins). For a plasmid to survive it must be a replicon I.e. self-replicating genetic unit. Its DNA must replicate every time host cell divides or it will be lost.

High copy plasmids are usually small and low copy plasmids can be large. Plasmid replication requires host cell functions and host cellular machinery. Copy number is regulated by initiation of plasmid replication. As plasmids interfere with each other’s replication they are incompatible when they cannot be stably maintained in the same cell.

A Plasmid replication requires host DNA replication machinery. Most wild plasmids carry required genes needed for transfer and copy number control. All self replication plasmids have a oriV which is origin of replication gene while some plasmids carry and oriT which origin of transfer gene.  These plasmids will also carry mob genes for mobilization purpose. There are 5 main “incompatibility” groups of plasmid replication.  But not all plasmids can live with each other.

F-plasmid has a large (100 kb) and a low copy (1-2 copies/cell). They are self transmissible and requires protein synthesis. repE gene encodes RepE protein. This RepE protein binds to origin of replication (oriS) and initiates DNA replication. RepE binds to the repE promoter and activates transcription. This RepE binds to the copA/incC locus binding copies of F together via RepE – inhibiting replication (coupling).

Several F plasmids have different functions. For example the plasmids ccdAB is reuired for inhibition of host cell division. incBCE has got incompatibiltity function. onV has a role in bidirectional replication.

Other scientific functions of plasmids include.

1.      Carry antibiotic resistance.

2.      Carry genes for metabolic activity.

3.      They have the ability to produce antibacterial proteins.

4.      They can contain genes for virulence factors for many bacterias.