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.
Map of an RFactor

Related Posts:

  • Transformation of bacterial genome Historically, the discovery of transformation in bacteria preceded the other two modes of gene transfer. The experiments conducted by Frederick Griffith in 1928 indicated for the first time that a gene-controlled characte… Read More
  • F-Plasmids and Sexduction Just as F-plasmids can be integrated into the chromosome, so they can also be occasionally excised from the Hfr-chromosome to produce a free plasmid. Sometimes, the excision does not occur in a precise manner by exact rever… Read More
  • Restricted Transduction of Bacterial genome  Restricted transduction is mediated by certain temperate bacteriophages, like the λ-phage (lambda phage) of E. coli, which enters into a lysogenic relationship with the host bacterium. This type of phages normally doe… Read More
  • The F-Plasmid F-Plasmid: The F-plasmid of E. coli is a self-transmissible, low copy-number extra-chromosomal genetic element mediating its own transfer. The transfer process requires products of a good number of genes (about 40) which a… Read More
  • Features of interrupted mating between Hfr x F– cells of E. coli The important features of interrupted mating between Hfr x F– cells of E. coli are briefly mentioned below: (a) Transfer of Hfr chromosome to P cell begins at a particular point on chromosome determined by the site of… Read More

0 Comments:

Powered by Blogger.

Visitors

198453
Print Friendly Version of this pagePrint Get a PDF version of this webpagePDF


 download University Notes apps for android

Popular Posts

Flag Counter