Selfishness in moderation for self-propagation : the yeast plasmid purloins the host mitotic apparatus for its segregation

The 2 micron circle of Saccharomyces cerevisiae is a high-copy, ‘selfish’ extrachromosomal DNA element that resides in the nucleus and propagates itself stably in the cell population. The plasmid segregates as a single cluster with the assistance of an active partitioning machinery, consisting of just two plasmid encoded proteins Rep1p and Rep2p and a cis-acting element STB. The work presented in this thesis reveals the molecular strategies by which the plasmid channels the chromosome segregation machinery towards its own partitioning. The yeast cohesin complex, a multiprotein molecular glue that keeps duplicated sister chromatids together until they are ready to be separated, plays an important role in plasmid segregation. During the cell cycle, the partitioning proteins mediate the recruitment of cohesin to STB. The timely association of cohesin with the plasmid during S phase as well as the timely dissociation of cohesin from it during anaphase are essential for equal partitioning of the plasmid to daughter cells. The plasmid exploits the host mitotic machinery in a second and quite unexpected manner. The mitotic spindle specifies the precise nuclear localization of the plasmid cluster, facilitates its compact organization, and is essential for the enlistment of the cohesin complex. When the spindle is restored from an initially depolymerized state of the microtubules, the cohesin complex can be assembled at STB, but is unable to support equal segregation of the plasmid. This finding underscores the importance of cell cycle timing in the functional association between cohesin and the plasmid. We propose that the cohesin complex plays analogous roles in chromosome and plasmid segregation: to pair and unpair sister chromatids in the former case, and sister clusters in the latter. In one plausible model, cohesin mediated pairing occurs between two clusters containing roughly equal numbers of replicated plasmid molecules. In the second, cohesin pairs each molecule in one cluster with its sister molecule in the second cluster, concomitant with DNA replication. One might look upon the segregation entity consisting of the sixty or so plasmid copies as “the plasmosome”, a non-essential yeast chromosome, in the manner in which binary partitioning units are segregated one to one.