Stable Maintenance of an Extrachromosomal Selfish DNA Element

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DESCRIPTION (provided by applicant): The yeast plasmid 2 micron circle provides a simple model for a 'benign parasite genome'. The genetic organization of the plasmid has been evolutionarily optimized for its high-copy propagation without compromising the fitness of the host cells. The central molecular component responsible for plasmid persistence is a stability system that ensures equal distribution of replicated plasmid molecules to daughter cells. Two plasmid encoded proteins (Replp and Rep2p), together with a cis-acting DNA locus (called STB), constitute this stability system. In addition, the plasmid has evolved an amplification system as a safety device. It comes into play only when a rare missegregation event causes a drop in copy number. Amplification is mediated by the Flp recombinase (Flp ='Flip' for flipping or inverting DNA), whose activity converts a single replication initiation event into a multiple copying mechanism, thus quickly restoring copy number to steady state levels. We recently discovered several unsuspected features of the plasmid segregation mechanism. First, the yeast cohesin complex, required for faithful segregation of sister chromosomes, is recruited to the STB locus in a Replp and Rep2p dependent manner. Second, in contrast to cohesin recruitment at chromosome arms that at the plasmid is absolutely dependent on the integrity of the mitotic spindle. The timing as well as the life-time of plasmid cohesin-association during the cell cycle is critical. Equal plasmid segregation fails if the plasmid does not acquire cohesin concomitant with DNA replication or if the anaphase disassembly of cohesin is blocked. In this proposal, we describe experiments that attempt to shed light on (1) the mechanisms by which the Rep1 and Rep2 proteins help the plasmid gain access to the chromosome segregation pathway, (2) the role of the mitotic spindle in promoting plasmid-cohesin association, (3) the influence of chromatin architecture and remodeling on plasmid partitioning and (4) the potential molecular connection between plasmid segregation and repeated chromosomal DNA segregation. Some of the principles gleaned from this study will have implications in global symbiotic or commensalist relationships among host-parasite genomes.