Abstract #39

A novel role for the centromere specific histone 3 variant at a non-centromeric locus: Assembly of the yeast plasmid partitioning complex at an impostor centromere. Makkuni Jayaram, Sujata Hajra, Ghosh Santanu, Hong Cui. Molecular Genetics & Microbiol, Univ Texas, Austin, Austin, TX.
   The CENP-A homolog Cse4p, required for kinetochore assembly and faithful chromosome segregation, has been thought to be the exquisite hallmark of centromeric chromatin in S. cerevisiae. However, this notion is no longer valid. Cse4p is also resident at the partitioning locus STB of the 2 micron plasmid, an extrachromosomal nuclear resident with nearly the same stability as the chromosomes. This finding is completely unexpected, as STB and CEN have no similarities in DNA organization. The multi-copy plasmid segregates as a single clustered unit (analogous to chromosomes), with the assistance of two plasmid coded proteins Rep1 and Rep2 that functionally interact with STB. The yeast cohesin complex, assembled at STB in a Rep1-Rep2 dependent step, pairs duplicated sister plasmid clusters as they do sister chromatids; and cohesin disassembly during anaphase is a prerequisite for the segregation of the plasmid clusters as it is for the segregation of chromosomes. The association between Cse4p and plasmid is strictly restricted to the STB chromatin, and absolutely requires the Rep proteins. Cse4p-STB association is initiated at G1/S during each cell cycle, and lasts through late telophase until the disassembly of the spindle. The kinetochore mutation ndc10-1 excludes Cse4p from centromeres but does not dislodge it from STB. Lack of functional Cse4p negates the characteristic association of the plasmid with chromosome spreads, disrupts remodeling of the STB chromatin by the RSC2 complex, interferes with Rep2p binding and cohesin assembly at STB, and leads to plasmid missegregation. The poaching of a specific histone variant by the plasmid to mark its partitioning locus with a ‘centromere tag’ reveals yet another one of the molecular trickeries it performs for achieving nearly chromosome-like fidelity in segregation.

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