The R plasmids are the multi-drug resistant plasmids. R-factors encode resistance to several antibiotics. These are transferred rapidly during conjugation of the cells. They have been found to carry many different kinds of genes in bacteria. R factors pick up genes conferring resistance to different antibiotics.
The drug resistant genes reside on mobile genetic element called a transposon. They are of two types namely, composite and simple transposons.
Composite transposons contains a variety of genes that reside between two nearly identical IS (insertion sequence) elements that are oriented in opposite direction and form inverted repeat sequence. Transpose encoded by one of these identical IS elements are necessary to catalyze the movement of the entire transposon. The IS elements that make up composite transposons are not capable of transposing on their own without the rest of the transposon because of mutations in their inward repeats.
Simple transposons consist of bacterial genes flanked by inverted repeat sequences. These sequences are very short. A transposon can jump from a plasmid to a bacterial chromosome or from one plasmid to another plasmid.
In this manner, multiple drug resistant plasmids are generated.