Selfishness in Moderation: Evolutionary Success of the Yeast Plasmid

The yeast plasmid 2-μm circle is an extrachromosomal selfish DNA element whose genetic endowments are devoted to its stable, high copy propagation. The mean steady state plasmid copy number of approximately 60 per cell appears to be evolutionarily optimized at its permissible maximum value. A plasmid-encoded negative regulatory mechanism prevents a rise in copy number that might imperil normal host metabolism and thus indirectly reduce plasmid fitness. The plasmid utilizes the host replication machinery for its own duplication. A plasmid-encoded partitioning system mediates even distribution of the replicated molecules to daughter cells, apparently by feeding into the chromosome segregation pathway. The plasmid also harbors an amplification system as a potential safeguard against a fall in copy number due to an occasional missegregation event. The 2-μm circle provides a model for how moderation of selfishness can ensure the successful persistence of an extrachromosomal element without compromising the fitness of its host.