What is R-Factor?

R-Factor is also called as the resistance factors or resistance plasmids. They are a group of conjugative plasmids which promotes the bacterial host resistance to specific antibiotics and to some metal ions, including sulphonamide, streptomycin, tetracycline, arsenic, cadmium, mercury, etc.

What are Plasmids?

Plasmids are an extrachromosomal genetic element that is found in many bacterial strains. They are a small, circular piece of DNA molecules that replicate independently of the bacterial chromosomes. Plasmids are mainly found in bacteria, Achaea and other multicellular organisms.


Types of Plasmids

  • R plasmids: Also called the resistance plasmids and are responsible for resistance to the drugs.

  • Sex factor plasmids: The cell which possesses this plasmid are called as F+, or male, or the donor cell, while the cells which do not possess this plasmid is called as “F-” or the recipient cell.

  • Col- plasmids: It is responsible for the production of Bacteriocins.

  • Heavy metal ion resistant plasmids: It is responsible for resistance from heavy metal ions that the bacteria may get exposed in the environment.

  • Plasmids of catabolic activity: It is responsible for the degradation of highly complex compounds, such as hydrocarbons, etc.

  • Virulence plasmids: It is responsible for the production of virulence factors such as adhesive factors, toxin, etc.

Resistance Plasmids

The R-Factor also called as the resistance factors or resistance plasmids. They are self-replicating, small, circular DNA elements, that codes for antibiotic resistance. These were first discovered in Japan in the year 1959 when the strains of Shigella (a genus of Gram-negative, aerobic, non-spore-forming, non-motile, rod-shaped bacteria) were found to become resistant to several antibiotics used during a dysentery epidemic. The R factor by itself is non-transmissible.

Also, read about: Gram-Negative Bacteria

Structure of Resistance Plasmid

Structure of Resistance Plasmid

The structure of resistance plasmid can be generally described as a circular piece of DNA, its length ranging between 80 – 95 kb and constitutes the major portion of the R-RTF (Resistance Transfer Factor) molecules. This plasmid is largely homologous to the F factor and contains similar genes. It also contains a fin 0 gene that represses the function of transfer operon. The R factors vary in their size and in the content of genes for drug resistance. The R determinant is smaller than the RTF. Both the RTF and R determinant combine to form one unit and are separated from each other by one IS 1 element on either side. The IS 1 elements help in the exchange of R determinant between the different types of R-RTF units.

Functions of Resistance Plasmid

The functions of resistance plasmid are as follows:

  • The resistance plasmids play an important role in the transmission of MDR genes in bacteria.

  • They are also involved in the gene carrier for the autonomous replication, conjugation, and resistance to ampicillin.

  • The resistance plasmids contain genes that can build resistance against the antibiotics and help bacteria in producing Pilli.

Gene Amplification

When drugs with low concentration are used to inhibit the growth of the cells containing R-factor, the cells that survive show increased resistance to the drug. This happens due to the duplication of the resistance gene in the R determinant. Recombination occurs between the daughter strands during the replication of R-factor.

Horizontal Gene Transfer

Bacteria containing F-factors can construct a sex pilus which helps in the horizontal gene transfer between the donor bacterium and the recipient bacterium. Antibiotic resistance can easily spread among a bacterial population since many R-factors contain F-plasmids. R-factors can also be taken up by the DNA through transformation or transduction The gene called the Resistance Transfer Factor (RTF) is present in them.

Stay tuned with BYJU’S Biology to learn more in detail about R-Factor, plasmid, gene, antibiotic resistance, or any other related topics.

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