Sister chromatid separation in budding yeast

Cell survival depends on the accurate transmission of a cell's genetic material to its daughters. Coordinating chromosome behavior with the cell cycle machinery ensures that the products of cell division are two viable and genetically identical progeny. Chromosomes replicate to produce two sister chromatids that are held together by topological and protein-mediated linkages. At anaphase, the protein and topological connections between sisters are quickly resolved to allow the separation and segregation of sister chromatids away from each other to opposite poles of the mitotic spindle.;We undertook two genetic screens to identify proteins required for sister chromatid cohesion and separation. We visually screened a temperature sensitive collection of mutants that contained a GFP marked chromosome for defects in sister chromatid cohesion in metaphase (p&barbelow;recocious d&barbelow;issociation of s&barbelow;isters, pds) and sister chromatid separation at anaphase (l&barbelow;oss o&barbelow;f c&barbelow;ohesion, loc). We identified fifteen PDS complementation groups and nine LOC complementation groups.;The budding yeast YCS4 gene encodes a conserved regulatory subunit of the condensin complex. We isolated an allele of this gene in the LOC screen. The phenotype of the ycs4-1 mutant is similar to topoisomerase II mutants and distinct from the esp1-1 mutant: the topological resolution of sister chromatids is compromised in ycs4-1 despite normal removal of cohesins from mitotic chromosomes. However, the ycs4-1 mutant exhibits additional defects in chromosome behavior aside from those observed during sister chromatid separation; inter-repeat recombination in the rDNA array is elevated and transcriptional silencing is defective at the permissive temperature. Taken together, our data suggest that there are mitotic as well as non-mitotic chromosomal abnormalities associated with loss of condensin function in budding yeast.