Plasmid-Derived Cloning Vectors

Plasmids, circular molecules of DNA that rephcate autonomously in a bacterial host cell, are widely distributed among prokaryotes They vary in size from a few to several hundred ktlobase pairs Although generally not essential to cellular survival, plasmids can confer a selecttve advantage on then host by encoding phenotypes, such as resistance to antibiotics, heavy-metal resistance, production of toxms or other virulence factors, bacteriocm production, or the catabolism of aromatic compounds Naturally occurring plasmids often have characteristics that make them ideally suited for development as clomng vectors Particularly important in this respect has been the exploitation of the many and varied anttbiotic-resistance mechanisms encoded by plasmids 1 ,2