Meaning & Structure of Resistance Plasmid

Meaning of Resistance Plasmid:

Resistance factors or resistance plasmids are a group of conjugative plasmids which confer to their bacterial host resistance to specific antibiotics, such as, chloramphenicol, sulphonamide, streptomycin, tetracycline etc., and to several metal ions, e.g., Hg, As, Cd etc. These factors cannot get inserted into the bacterial chromosome.

They are self-replicating, small, circular DNA elements; they were first discovered in Japan in 1959 when strains of Shigella were found to become resistant to several antibiotics used during a dysentery epidemic. The R factor by itself is non-transmissible.

There is another factor known as “resistance transfer factor”(RTF) which carries gene for resistance to ampicillin only. When an R factor is linked with the RTF, the combined “R-RTF” behaves as an F factor, so that it can be transferred to other bacterial strains through conjugation.

Structure of Resistance Plasmid:

The resistance transfer factor (RTF) is about 80 kb in length and carries genes for autonomous replication, conjugation and resistance to ampicillin. The RTF is largely homologous to F factor and contains similar genes.

However, it also contains a fin 0 gene that represses the function of transfer operon (tra) while the F factor does not contain the fin 0 gene. The R factors (R determinant) vary in 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; they are separated from each other by one IS 1 element on either side (Fig. 18.7).

The IS 1 elements promote the exchange of R determinant between different types of R-RTF units. Both RTF and R factors dissociate in some cases especially when transferred from E. coli to Proteus. After dissociation, they may segregate into daughter calls or they may recombine in the cell. Certain R plasmids (not all) from E. coli specify restriction endonucleases, such as Eco RI and Eco RII.