Yeast plasmid segregation: investigation of host/plasmid interactions

The 2µm circle is a high copy-number plasmid stably maintained in the nucleus of most strains of the bakers yeast, Saccharomyces cerevisiae. The 2µm circle is a "selfish" DNA, borrowing host machinery for its replication and segregation to ensure it is seldom lost when the host cell divides, while conferring no advantage to the host. We have identified sumoylation (modification of target proteins with a small ubiquitin-related protein called SUMO) as a host function that regulates plasmid segregation. Biochemical, molecular genetics, and cell biology approaches will be used to elucidate the link between sumoylation and plasmid maintenance and to identify and investigate other host proteins borrowed by the plasmid that enable its inheritance. Fluorescence microscopy will be used to investigate the effect of absence of host proteins identified in our studies on equal partitioning of the multiple plasmid copies at cell division and on plasmid localization within the nucleus. Our studies will also help identify the cellular process to which the identified host proteins contribute and reveal new functions for others. This project will determine how the 2µm circle interacts with other components of the nucleus to ensure its efficient segregation. Understanding how sumoylation and other host processes are exploited by the yeast plasmid may reveal how other genetic elements, such as viruses are maintained and help determine the essential role sumoylation plays in faithful segregation of chromosomes during cell division.