Extrachromosomal Inheritance: Yeast Linear Killer Plasmids as a Tool in Genetic Engineering

In principle, every stably replicated DNA species can serve as a vector for cloning foreign DNA in yeast or any other organism, in which such DNA is found. In strains of Saccharomyces cerevisiae a high copy number, autonomously replicating, extrachromosomal DNA element, the 2 µm DNA, was found to exist (Sinclair et al. 1967). This covalently closed circular DNA element resembles in many respects bacterial plasmids. Since it is present in approximately 50 copies per cell it is relatively stably maintained. It was used to construct vectors for cloning and expression of foreign genes in Saccharomyces cerevisiae (for review see Hollenberg 1982). The occurrence of circular plasmids is not restricted to baker’s yeast, because similar elements were also detected in Zygosaccharomyces (Tohe and Utatsu 1985) and in Kluyveromyces drosophilarum (Chen et al. 1986). The latter plasmid, pKD1, is 1.6 µm in size and was also transferred and stably maintained in other yeast species of the genus, i.e., Kluyveromyces lactis (Bianchi et al. 1987).