Yeast autonomously replicating sequence (ARS) plasmids exhibit an unusual segregation pattern during mitosis. While the nucleus divides equally into mother and daughter cells, all copies of the ARS plasmid will often remain in the mother cell. A screen was designed to isolate mutations that suppress this segregation bias.A plasmid with a weakARS (wARS) that displayed an extremely high segregation bias was constructed. When cells were grown under selection for the wARS plasmid, the resulting colonies grew slowly and had abnormal morphology. A spontaneous recessive mutation that restored normal colony morphology was identified. This mutation suppressed plasmid segregation bias, as indicated by the increased stability of the wARS plasmid in the mutant cells even though the plasmid was present at a lower copy number. An ARS1 plasmid was also more stable in mutant cells than in wild-type cells. The wild-type allele for this mutant gene was cloned and identified as POL8 (CDC2). This gene encodes DNA polymerase 8, which is essential for DNA replication. These results indicate that DNA polymerase 8 plays some role in causing the segregation bias of ARS plasmids.The autonomously replicating sequence (ARS) element is defined by its effect on a plasmid transformed into the yeast Saccharomyces cerevisiae. The ARS element converts an integrating plasmid, a plasmid that must integrate into the chromosome in order to be stably inherited, into an autonomously replicating plasmid, a plasmid that can persist in the cell as an extrachromosomal element (28). ARS elements have always been suspected to be origins of replication, and there is now direct evidence that replication initiates at some ARS elements when they are present either on a plasmid or in a chromosome (reviewed in references 6 and 18).One property ofARS elements that is difficult to explain in view of their role as origins of replication is the maternal segregation bias of ARS plasmids. During mitosis, it is common for all of the circular ARS plasmids, often over 30 copies per cell, to remain in the mother cell. As a result,ARS plasmids are lost from one of the resulting cells at the rate of about 40% per division (17). This segregation bias may be even stronger in some weak ARS plasmids, which are extremely unstable yet present in higher copy number per cell containing the plasmid than are standard ARS plasmids (4). This segregation bias is suppressed by the addition of a CEN element, containing the centromere sequence, to the ARS plasmid. The stability of the CEN-ARS plasmid increases dramatically as a result of the segregation machinery of the mitotic spindle, even though the copy number falls to one (reviewed in reference 18).Murray and Szostak (17) have discussed possible explanations for the segregation bias of ARS plasmids. Segregation bias is not simply a result of unequal partitioning of nuclear volume between mother and daughter cell. When viewed with both light and electron microscopes, the nucleus divides approximately equally between mother and daughter....
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