Stable persistence of the yeast plasmid by hitchhiking on chromosomes during vegetative and germ-line divisions of host cells.

The chromosome-like stability of the Saccharomyces cerevisiae plasmid 2 micron circle likely stems from its ability to tether to chromosomes and segregate by a hitchhiking mechanism. The plasmid partitioning system, responsible for chromosome-coupled segregation, is comprised of 2 plasmid coded proteins Rep1 and Rep2 and a partitioning locus STB. The evidence for the hitchhiking model for mitotic plasmid segregation, although compelling, is almost entirely circumstantial. Direct tests for plasmid-chromosome association are hampered by the limited resolving power of current cell biological tools for analyzing yeast chromosomes. Recent investigations, exploiting the improved resolution of yeast meiotic chromosomes, have revealed the plasmid's propensity to be present at or near chromosome tips. This localization is consistent with the rapid plasmid movements during meiosis I prophase, closely resembling telomere dynamics driven by a meiosis-specific nuclear envelope motor. Current evidence is consistent with the plasmid utilizing the motor as a platform for gaining access to telomeres. Episomes of viruses of the papilloma family and the gammaherpes subfamily persist in latently infected cells by tethering to chromosomes. Selfish genetic elements from fungi to mammals appear to have, by convergent evolution, arrived at the common strategy of chromosome association as a means for stable propagation.