Plasmids: Histories of a Concept

Abstract

The plasmid concept is rooted in the notion of particulate determinants of inheritance and the chromosome theory of heredity, but some biologists saw genes as determinants of the way an organism developed from a fertilized ovum into a mature adult; some of these determinants seem to be passed on through cytoplasmic transfer. In a 1952 review, J. Lederberg proposed that all “extrachromosomal hereditary determinants” be designated “plasmids.” In 1958, Jacob and Wollman suggested that genetic elements which were optionally associated with the chromosomes, such as the F-factor, the colicinogenic factor, and bacteriophage lambda, be termed “episomes.” Allan Campbell (Adv Genetics 11:101–145, 1962) proposed a beautifully simple solution to the problem of how episomes could be associated with the chromosome when he suggested the recombinational interaction of one circular molecule with another. The key to the modern concept of the plasmid was the confirmation that DNA molecules can, and often do, exist as circular structures. Many observations (mainly on yeast and protozoans) suggested that nonchromosomal heredity exists in eucaryotes as well, and eventually, cytochemical, electron microscopic, and biochemical evidence established the existence of cytoplasmic genes in eucaryotes. By the end of the 1960s, both the genetic and physical understanding of plasmids and cytoplasmic heredity had reached a level of detail to allow exploitation of these genetic elements as tools to manipulate and study cell genetics by various techniques of lateral gene transfer.