Copy number and partition of the Saccharomyces cerevisiae 2 micron plasmid controlled by transcription regulators.

The 2 micron plasmid of Saccharomyces cerevisiae is maintained by the action of plasmid-encoded gene products that control copy number and promote equipartition of plasmid copies at cell division. We show that the REP1 and REP2 plasmid-encoded gene products are master regulators that act in concert to autoregulate the level of their own transcripts and to regulate transcript levels of the FLP gene that promotes plasmid copy amplification. REP1 and REP2 are also shown to repress transcription at REP3, the cis-acting site essential for plasmid equipartitioning. We propose a model in which REP3 acts by dislodging transcription apparatuses that otherwise cause plasmid molecules to adhere to the mother nucleus and segregate asymmetrically. On the basis of their ability to generate specific chromatin structures, we also propose that the REP1 and REP2 gene products interact with different specific sequences found iterated in the 2 micron plasmid.