J. Lederberg invented the term plasmids. Plasmids are independent self-replicating DNA molecules that exist as separate, extrachromosomal genetic components in bacteria. They are also found in some eukaryotes, such as yeast.
Plasmids are replicons. Replicons are self-replicating nucleic acid molecules. Natural plasmids have an origin of replication (which determines the plasmid’s host range and copy quantity) and often include a survival gene, such as an antibiotic resistance gene.
In most cases, plasmids encode traits that are not necessary for the survival of bacteria and multiply separately from chromosomes. Plasmids typically carry at least one gene, and many of the genes carried by plasmids are helpful to their host species. They aren’t regarded as autonomous life even if they have different genes from their hosts. Plasmids are typically circular, negatively supercoiled, dsDNA molecules. However, linear plasmids have been reported in the genera Borrelia and Streptomyces.
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Types of plasmids
1. Conjugative plasmid
It is a plasmid that carries genes and aids the pairing of a particular donor and recipient. Conjugative plasmids contain genes for the development of pili, which help in the conjugation process. These plasmids are frequently big ones (>40 kbp).
2. Mobilized plasmid
This plasmid is not self-transmissible due to the absence of the tra gene but can be transferred by a self-transmissible plasmid. Mobilization is the process by which it is transmitted. The transmission is accomplished through the product of tra genes specified by a self-transmissible plasmid.
3. Self-transmissible plasmid
A self-transmissible plasmid encodes all the proteins required for transfer between donor and recipient. It is both mobilizable and conjugative. Nontransmissible plasmids lack the necessary gene for successful contact and DNA transfer. Most naturally occurring plasmids are either self-transmissible or mobilized.
4. Resistance plasmid ( R- plasmid)
The host cell becomes resistant to one or more antibiotics as a result of these Plasmids. Some R-plasmids have only a single resistance gene, whereas others contain several resistance genes. Many R-plasmids are mobilizable plasmids.
5. Promiscuous plasmid
These plasmids contain a transfer system that allows them to transfer DNA between species.
6. Col (Colicinogenic) plasmid
Bacteriocin, a protein produced by Col plasmids, inhibits the growth of sensitive strains that lack a Col plasmid. Proteins known as bacteriocins can interact with susceptible bacteria and prevent one or more vital functions from occurring, including DNA replication, transcription, translation, and energy metabolism. Colicins are the name given to the bacteriocins made by E. coli. There are a variety of colicins, each with a unique way of inhibiting sensitive cells denoted by a letter (for example, colicin B).
Some examples of plasmids and their functions are as follows:
Type of plasmid Gene function
- Resistance Antibiotic resistance
- Killer Synthesis of toxin that kills other bacteria.
- Virulence Pathogenicity
7. Degradative plasmid
Degradative plasmids assist the host bacterium in digesting chemicals that are not frequently found in nature, such as camphor, xylene, toluene, and salicylic acid.
8. Virulence plasmid
A virulence plasmid transforms a bacterium into a pathogen, or an agent of disease when it is present inside the bacteria.
Properties of plasmids
a. Plasmid replication
Typical plasmids consist of circular dsDNA and use two distinct processes to replicate their DNA.
- Most plasmids replicate like bacterial chromosomes. They have a replication origin where bidirectional replication starts after the DNA opens. The two replication forks go in opposite ways around the circular plasmid until they come together.
Bidirectional plasmid replication
Image source: https://www.sciencedirect.com/science/article/abs/pii/B9780128132883000239
The rolling circle replication mechanism is the other replication process. It begins by nicking the replication origin and unrolling one strand while using the other, still circular, strand as a template for DNA synthesis. Some plasmids, like E. coli’s F-plasmid, can move from one bacteria to another. Such plasmids have two distinct replication origins
b. Copy number
Copy number refers to the typical quantity of a given plasmid molecule in a bacterial cell. More specifically, it signifies the number of plasmid copies in a cell right after cell division. Low copy number or stringent plasmids (with 1 to 2 copies per cell) and high copy number or relaxed plasmids (with 10 to 100 copies per cell) are two different plasmids. Every type of plasmid has unique genes for controlling the beginning of replication and, consequently, the number of plasmid copies per cell.
Three main approaches are employed to govern the initiation of plasmid replication:
I. Regulation by counter-transcribed RNAs, often known as antisense RNA.
II. Regulation by replication protein binding to recurrent 18-22 bp regions known as iterons.
III. Regulation by ctRNA (counter-transcribed RNA) and a protein.
c. Plasmid Incompatibility
Plasmid incompatibility is the inability of two closely related plasmids to exist stably in the same cell. As a result, a cell having plasmids A and B will eventually produce offspring that only contains one of the two plasmids, not both. Two plasmids that cannot coexist in the same bacterial cell belong to the same incompatibility group, which means that two plasmids are incompatible if they belong to the same incompatibility group.
Plasmid incompatibility
Image source: https://www.sciencedirect.com/science/article/abs/pii/B9780128132883000239
d. Partitioning
After cell division, partitioning ensures that each daughter cell receives a copy of the plasmid through an active process. Random diffusion might be sufficient to deliver at least one copy of a plasmid to each daughter cell for plasmids with high copy numbers. However, random segregation of low copy-number plasmids would probably mean that one of the daughter cells wouldn’t acquire a plasmid after cell division. Eventually, the plasmid would be diluted from the population.
Function encoded by plasmids
Plasmids can encode hundreds of distinct proteins, depending on their size. However, plasmids rarely encode gene products required for growth, such as RNA polymerase, ribosomal subunits, or tricarboxylic acid cycle enzymes. Plasmid genes often only provide bacteria with a selective advantage under certain circumstances. Plasmid-encoded gene products include enzymes for the utilization of unusual carbon sources such as toluene, resistance to chemicals such as heavy metals and antibiotics, antibiotic production, and the production of toxins and proteins that allow the effective infection of higher organisms. Some traits encoded by plasmids are as follows:
Plasmid Traits
- Ti Tumor initiation in plants
- CoIE1 Bacteriocin which kills E.coli
- RK2 Resistance to ampicillin, tetracycline, and Kanamycin
Plasmids in eukaryotes
Plasmids are not confined to bacteria. Plasmids, for instance, are also present in eukaryotic microorganisms like yeast. The 2p circle is one yeast plasmid. Most Saccharomyces cerevisiae strains contain a circular, extrachromosomal element called the 2p circle, which is 6.3 kb in size. It is consistently kept at 50 to 100 copies per yeast cell’s haploid genome. The 2p circle has a nucleosome coating, just like the host chromosomes, and host replication enzymes start replication once every cell cycle.
Function of plasmids
- Plasmids facilitate the replication process.
- Plasmids can also enable bacteria to fix nitrogen.
- They help the organism to survive.
Applications of plasmids
- In genetic engineering, plasmids are used to amplify or make multiple copies of specific genes.
- A plasmid is a particular kind of vector used in molecular cloning.
- Short DNA segments can be cloned with the help of plasmids.
- Proteins, such as the one that codes for insulin, can be repeatedly produced using plasmids.
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References
- https://www.sciencedirect.com/science/article/abs/pii/B9780128132883000239.
- https://www.researchgate.net/publication/346261306_Plasmids_types_and_its_usage.
- http://mmcmodinagar.ac.in/econtent/biotech/plasmids.pdf.
- https://byjus.com/free-ias-prep/plasmid/.
- https://biologydictionary.net/plasmid/#conjugative-and-non-conjugative.
- https://asm.org/Articles/2023/January/Plasmids-and-the-Spread-of-Antibiotic-Resistance-G
