Supplemental Experimental Procedures Media, Yeast Strains, and Molecular Genetics

Media were as described [S1]. Complete YES medium was used unless otherwise stated. Synthetic medium is PMG. The cdc10-129 and cdc25-22 alleles were used to arrest cells in the G1 and G2 phases of the cell cycle, respectively, and 20 mM hydroxyurea in YES medium was used to arrest cells in early S phase. Complete arrest was monitored by measuring cell density and the decrease of the septation index over time (from w11 % in cycling cells to <2 % in arrested cells). Genetic Screen for Cohesion Mutants A strain in which Swi6 synthesis is driven by an attenuated version of the thiamine-repressible nmt1 promoter was constructed [S2]. Since Swi6 is nonessential, such an nmt-swi6 strain grows indis-criminately in the absence or presence of thiamine. The strain was mutagenized with ethyl methyl sulfonate, and mutants that were vi-able in the absence of thiamine but dead in the presence of thiamin