The 2 micron plasmid purloins the yeast cohesin complex

Abstract

The yeast 2 micron plasmid achieves high fidelity segregation by coupling its partitioning pathway to that of the chromosomes. Mutations affecting distinct steps of chromosome segregation cause the plasmid to missegregate in tandem with the chromosomes. In the absence of the plasmid stability system, consisting of the Rep1 and Rep2 proteins and the STB DNA, plasmid and chromosome segregations are uncoupled. The Rep proteins, acting in concert, recruit the yeast cohesin complex to the STB locus. The periodicity of cohesin association and dissociation is nearly identical for the plasmid and the chromosomes. The timely disassembly of cohesin is a prerequisite for plasmid segregation. Cohesin-mediated pairing and unpairing likely provides a counting mechanism for evenly partitioning plasmids either in association with or independently of the chromosomes.

Faithful segregation of the multicopy yeast plasmid through cohesin-mediated recognition of sisters

“…Neither model requires cohesin for plasmid clustering per se, which is likely mediated through the Rep proteins in conjunction with associated host factors. Consistent with this notion, the clustered plasmid organization is not altered in G 1 -arrested cells even though they lack cohesin at STB (7,15).…”

“…Consistent with a potential role for the RSC2 complex in remodeling STB chromatin, inactivation of the complex blocks cohesin assembly on the plasmid (5,11). The timely recruitment of cohesin at STB and its timely disassembly are critical events in 2-m-plasmid segregation (7).…”

“…To critically verify the notion that pairing of replicated plasmid copies is cohesin mediated, we exploited previously established features of plasmid-cohesin association. Cohesin recruitment at STB is absolutely dependent on Rep1 and Rep2 proteins (7,11). The STB chromatin harbors the histone H3 variant Cse4p (4), and inactivation of Cse4p by a temperaturesensitive mutation blocks STB-cohesin association.…”

“…Based on the role of cohesin in chromosome segregation, one might imagine that cohesin assembly pairs duplicated plasmid clusters and cohesin cleavage triggers their disjunction (7). However, there is no direct evidence for this ''pairing-unpairing'' mode of plasmid segregation.…”

“…The maturation of Cse4p-containing STB chromatin into its functional state appears to be mediated through the RSC2 chromatin remodeling complex (4-6). The yeast cohesin complex, required for one-to-one segregation of sister chromatids, is assembled at STB in a Rep1p-and Rep2p-assisted manner during early S phase, and this association lasts until anaphase (7,8). Like chromosome-cohesin association, plasmid-cohesin association also requires the loading factors Scc2 and Scc4 (ref.…”