Plasmids are extra-chromosomal genetic elements that replicate independently of the host chromosome. They are small, circular (some are linear), double-stranded DNA molecules (mostly) that exist in bacterial cells and in some eukaryotes. The sizes of plasmids range from roughly one to more than 1000 kilobase pairs.
A typical plasmid is a circular double stranded DNA molecule less than 1/20 the size of the chromosome.
The number of plasmids may vary from none to several per bacterial cell. Different plasmids are present in a cell in a particular number called the copy number. Some plasmids are present in the bacterial cell in only 1-3 copies, whereas others may present in as many as 100 copies. This feature is controlled by the genes present on the plasmid and by interactions between the host and the plasmid.
Individual bacterial cells may contain several different types of plasmids and in some cases more than 10 at a time. Plasmids are generally isolated from the bacterial cells in the supercoiled configuration. So far, thousands of different types of plasmids have been isolated. More than 300 different types of naturally occurring plasmids have been isolated from E.coli alone.
Though, plasmids are not considered as part of the cell’s genome, when a bacterial cell divides each daughter cells receives a copy of each plasmid. Plasmids can also be transferred from one bacterial cell to another by the process called conjugation. Plasmids that govern their own transfer by conjugation are called conjugative plasmids but not all plasmids are conjugative.
Some important components of plasmids are
- Origin of replication (Ori): A DNA sequence that allows bacteria to make more copies of the plasmid as they grow and divide.
- Antibiotic resistance gene: A specific gene that allows bacteria with the plasmid to grow in the presence of an antibiotic specific to the gene.
- Gene: A DNA sequence encoding a particular protein that a researcher has inserted into the plasmid to study.
- Promoter: A DNA sequence that allows the cell to produce the protein encoded by the gene.
- Restriction sites: DNA sequences that allow a researcher to cut and paste components of plasmids together
Types & Functions of Plasmids
As a single plasmid may carry many different genes, the classification of a plasmid in a single phenotypic category is difficult. Some of the notable types of plasmids and their functions are:
- Conjugative plasmids: Examples include F plasmid present in E.coli, conjugative P plasmid of Vibrio cholerae.
- Resistance plasmids (R plasmids): R plasmids confer resistance to antibiotics and various inhibitors of growth.
- It carries a variety of antibiotic resistance genes which encode proteins that either inactivate the antibiotic or affect its uptake into the cell. Example: Plasmid R100 carries resistance genes for sulfonamides, streptomycin, fusidic acid, chloramphenicol and tetracycline.
- R100 also carries several genes that confer resistance to mercury.
- Resistant strains can transfer resistance to sensitive strains via cell-to-cell contact. R100 can transfer itself between enteric bacteria of the genera Escherichia, Klebsiella, Proteus, Salmonella and Shigella but does not transfer to the nonenteric bacterium Pseudomonas.
- Plasmids that code for virulence characteristics & toxins
- Adherence/Colonization: Some of the plasmids code for the proteins that increase the ability of the organism to attach and colonize in specific sites within the host e.g. Colonization factor antigen (CFA) of E.coli.
- Toxin production: Toxin production in various pathogenic bacteria is found to be linked with the presence of plasmids. For example:
- Hemolysin (lyse RBCs) and Enterotoxin (induces extensive secretion of salt and water in the bowel) property of Enteropathogenic Escherichia coli (EPEC) are governed by plasmids.
- Production of coagulase, hemolysin, fibrinolysin and enterotoxin property of S.aurues is linked to the presence of the plasmid.
- Bacteriocins: Many bacteria produce peptides that inhibit or kill closely related species or even different strains of the same species. The gene responsible for this peptide and or its post-translational modification is coded in plasmids. E.g colicin of E.coli is coded by Col plasmids.
Although plasmids carry useful genes such as genes that confer “antibiotic resistance”, as mentioned above, they do not carry genes that are essential to the host under all conditions.
The presence of plasmids in a cell can also have other biological significance such as:
- Nodulation and symbiotic nitrogen fixation: Rhizobium
- Transfer genetic information for a biochemical pathway for the degradation of organic compounds such as octane, camphor, naphthalene, salicylate etc: Pseudomonas.
- Pigment production: Erwinia, Staphylococcus
- Lactose, sucrose, urea utilization, nitrogen fixation: Enteric bacteria
Plasmids can be constructed artificially (artificial plasmids are called vectors) and are used to introduce foreign DNA into another cell of interest. Plasmids play crucial roles in genetic engineering, molecular cloning and various areas of Biotechnology.
This plasmid rap video is awesome…
References and further reading
- Plasmids 101 (addgene blog)
- Madigan MT, Martinko JM. Brock Biology of Microorganisms. Pearson Educational International.