Yeast plasmid requires a cis-acting locus and two plasmid proteins for its stable maintenance.

Unstable ars1 (autonomously replicating sequence)-containing plasmids can be stabilized by connecting particular DNA segments of yeast 2 mu-plasmid. A cis-acting locus for the plasmid's stability (the STB locus) was found in the large unique region and separated from the replication origin of 2 mu-plasmid. Several direct repeats found in this region were essential for its function. Two proteins encoded by the plasmid (B and C) were required as trans-acting factors. The average copy number of the plasmid was not affected whether the plasmid carried the stability system or not, suggesting that the system was not directly associated with the replication function but was involved in partitioning at cell division.