The 2-micron plasmid is a relatively small (6318 bp) nuclear multicopy extrachromosomal element found in most common strains of Saccharomyces cerevisiae. Its presence confers no obvious advantage to its host, nor does it appear to impose any disadvantage at its steady-state copy number of 40-60 molecules (2). The plasmid contains 4 protein-coding loci (FLP1, REP1, REP2, RAF1) and 4 cis-acting loci (an origin of replication, a partitioning locus called STB, and 2 Flp Recombination Targets or FRTs) (3).

Raf1p (REP Antagonizing Factor or Recombinase Activating Factor), the product of the "D ORF", is an anti-repressor that positively regulates the plasmid amplification process by relieving repression of the FLP1 site-specific recombinase by the Rep1-Rep2p bipartite transcription regulator (1). RAF1 is itself repressed by the Rep1p-Rep2p complex, but requires a higher level of repressor activity for complete inhibition than does FLP1 (1).

The 2-micron plasmid propagates itself with chromosome-like stability through the combined action of a plasmid amplification system and a plasmid partitioning system. The amplification system compensates for any copy number decreases caused by missegregation events, and consists of Flp1p along with a pair of 599-bp FRT sites present in the plasmid genome in a head-to-head orientation (4). The partitioning system ensures roughly equal distribution of replicated plasmids to daughter cells by overcoming the normal segregation bias that favors the mother cell over daughters in plasmid retention (4). This system comprises Rep1p, Rep2p, and the partitioning locus STB which consists of two subloci, one containing approximately six iterations of a consensus 65-bp repeat, and another which is involved in maintaining the active configuration of STB and contains the termination site for two transcripts directed toward the plasmid replication origin (4). Multiple copies of the 2-micron plasmid exist as a tight-knit cluster within the nucleus that stays together throughout the cell cycle (5), and this plasmid cluster is the segregation entity, effectively reducing copy number to one (5, 6).

Last updated: 2006-08-17