The chromosomes of yeast contain many genomic segments at which DNA replication is ordinarily initiated. On average, one is found every 40 kb in the Saccharomyces cerevisiae genome. These regions can act as the origin of replication for a circular plasmid. When such a segment is linked to a nonreplicative DNA fragment, it can confer on that fragment the ability to replicate autonomously as a plasmid in a yeast cell in the S-phase of the cell cycle. This construction is called an autonomously replicating fragment (ARS).
An ARS-containing DNA fragment can transform yeast cells at much higher frequencies than a fragment without an ARS, because it can replicate autonomously without undergoing a rare homologous recombination before its genes can be expressed. In contrast to the latter transformants, however, those formed with an ARS-containing DNA are extremely unstable and lose the transforming DNA even under selective conditions. For unknown reasons, they tend to remain associated with the mother cell without being evenly distributed to the daughter cell upon division.
ARS have been found in Saccharomyces cerevisiae and but also in other yeasts, such as Candida maltosa, Hansenula polymorpha, Kluyveromyces lactis, and Schizosaccharomyces pombe. They are not restricted to yeasts and have also been described in other lower eukaryotes, such as Aspergillus nidulans and Entamoeba histolytica, and even in at or near potential replicative origins from Drosophila, Chinese hamster, and humans.
