A Brief History of Plasmids

species. One of the more fascinating developments in plasmid biology was the discovery of linear plasmids in the 1980s. A remarkable feature of the Ti plasmids of Agrobacterium tumefaciens is the presence of two DNA transfer systems. A definitive demonstration that plasmids consisted of duplex DNA came from interspecies conjugal transfer of plasmids followed by separation of plasmid DNA from chromosomal DNA by equilibrium buoyant density centrifugation. The formation of channels for DNA movement and the actual steps involved in DNA transport offer many opportunities for the discovery of proteins with novel activities and for establishing fundamentally new concepts of macromolecular interactions between DNA and specific proteins, membranes, and the peptidoglycan matrix.