Yeasts naturally harbour various plasmids. Notable among them are 2 μm plasmids—small circular plasmids often used for genetic engineering of yeast—and linear pGKL plasmids from Kluyveromyces lactis, that are responsible for killer phenotypes.
Other types of plasmids are often related to yeast cloning vectors that include:
* Yeast integrative plasmid (YIp), yeast vectors that rely on integration into the host chromosome for survival and replication, and are usually used when studying the functionality of a solo gene or when the gene is toxic. Also connected with the gene URA3, that codes an enzyme related to the biosynthesis of pyrimidine nucleotides (T, C);
* Yeast Replicative Plasmid (YRp), which transport a sequence of chromosomal DNA that includes an origin of replication. These plasmids are less stable, as they can be lost during budding.