Chromosome condensation factor brn1p is required for chromatid separation in mitosis

This work describes BRN1, the budding yeast homologue of Drosophila Barren and Xenopus condensin subunit XCAP-H. The Drosophila protein is required fo r proper chromosome segregation in mitosis, and Xenopus protein functions i n mitotic chromosome condensation. Mutant brn1 cells show a defect in mitot ic chromosome condensation and sister chromatid separation and segregation in anaphase. Chromatid cohesion before anaphase is properly maintained in t he mutants. Some brn1 mutant cells apparently arrest in S-phase, pointing t o a possible function for Brn1p at this stage of the cell cycle. Brn1p is a nuclear protein with a nonuniform distribution pattern, and its level is u p-regulated at mitosis. Temperature-sensitive mutations of BRN1 can be supp ressed by overexpression of a novel gene YCG1, which is homologous to anoth er Xenopus condensin subunit, XCAP-G. Overexpression of SMC2, a gene necess ary for chromosome condensation, and a homologue of the XCAP-E condensin, d oes not suppress brn1, pointing to functional specialization of components of the condensin complex.