• The host range of a plasmid is determined by its ori region. Plasmids whose ori region is derived from plasmid Col E1 have a restricted host range. They only replicate in enteric bacteria, such as E. coli, Salmonella, etc. Plasmids of the RP4 type will replicate in most gram negative bacteria, to which they are readily transmitted by conjugation. Plasmids like RSF1010 are not conjugative but can be transformed into a wide range of gram -ve bacteria. Plasmids isolated from Staphylococcus aureus have a broad host range and can replicate in many other gram-positive bacteria.

Fig 1-3.2 : Bacterial Genomic DNA with plasmid

 

Some of the phenotypes which the naturally occurring plasmids confer on their host cells:

•  Antibiotic resistance

•  Antibiotic production

•  Degradation of aromatic compounds

•  Haemolysin production

•  Sugar fermentation

•  Enterotoxin production

•  Heavy metal resistance

•  Bacteriocin production

•  Induction of plant tumors

•  Hydrogen sulphide production

 

Most plasmids exist as double-stranded circular DNA molecules. However, the inter-conversion of super coiled, relaxed covalently closed circular DNA and open circular DNA is possible. Not all plasmids exist as circular molecules. Linear plasmids have been found in a variety of bacteria, e.g. Streptomyces sp. and Borrelia burgdorferi.

However, few types of plasmids are also able to replicate by integrating into bacterial chromosomal DNA; these are known as integrative plasmids or episomes. They are found mainly in prokaryotes but some eukaryotes are also found to harbour them. In prokaryotes they are found in Escherichia coli , Pseudomonas species, Agrobacterium species etc. In eukaryotes they are mainly found in Saccharomyces cerevisiae.