Plasmid Biology

What is a plasmid?

A plasmid is a small cellular inclusion consisting of a ring of DNA that is not in a chromosome but is capable of autonomous replication.

Typically, plasmids have the following features:

• They are stably inherited in an extrachromosomal state, i.e. are separate from chromosomal DNA and capable of replicating independently
  
  

• Plasmids are usually double stranded and circular. However, some linear plasmid molecules have been identified in Streptomyces sp and Borrelia burgdorferi. In such cases, there is a need to prevent endonuclease digestion which is achieved by (i) repeated sequences ending in terminal DNA hairpin loop (Borrelia) or (ii) ends protected by covalent attachment of a protein (Streptomyces)
• Plasmid sizes vary from 1 to over 1,000 kilobase pairs (kbp), while their mass varies from 1x10⁶ daltons to > 200x10⁶ daltons
• Plasmids encode few proteins required for own replication. The genes for these proteins are situated close to ORI. Thus, only ORI and only a small region surrounding it are required for replication.

Plasmid Characteristics

Plasmids confer specific phenotypes to their host cells (eg. antibiotic resistance).

Conjugative Plasmids:
-tra genes present
-high molecular weight
-1 to 3 copies

Non-conjugative:

-tra genes absent

-low molecular weight
- multiple copies

Phenotypes conferred by Plasmids

Antibiotic resistance/production
Degradation of aromatic compounds
Hemolysin production
Sugar fermentation
Enterotoxin production
Heavy-metal resistance
Bacteriocin production
Plant tumour induction
H₂S production
Host controlled restriction and modification

Important Examples of Plasmids

• ColE1
• Restricted host range
• found only in enteric bacteria (E.coli, Salmonella, etc.)
• Copy number in ColE1 derived-plasmids is regulated by regulating the initiation of replication. This is achieved by either one of the two following mechanisms: 
• (i) by antisense RNA
• primer for DNA replication
• 555bp ribonucleotide molecule
• RNA II  --> forms RNA-DNA hybrid at the replication origin and can only act as a primer if cleaved by RNAseH to yield a free 3'--OH
• (ii) by binding of essential proteins to repeated sequences called “introns"

• RP4
• Promiscuous
• conjugative
• found in Gram –ve bacteria
•  RSF1010
• stable
• found in Gram +ve and -ve bacteria
• non-conjugative

Plasmids which do not have phenotypic traits ascribed to them as yet are referred to as “cryptic” plasmids.