The yeast plasmid 2mu circle encodes components required for its high copy propagation.

The yeast plasmid 2mu and certain hybrid plasmids constructed from it are maintained stably and at high copy number in yeast cells. By examining various mutant hybrid 2mu plasmids, we show that these properties require the integrity of four plasmid loci. Two of these, designated REP1 and REP2, are active in trans and correspond to two open coding regions of 2mu. The other two loci are active only in cis and correspond to the origin of replication and to a region, designated REP3, located several hundred bp away from the origin and consisting of direct repeats of a 62 bp sequence. We propose that the REP loci constitute a copy control system that overrides normal cellular restriction on plasmid replication and amplifies the plasmid when copy number is low.