MITOTIC CHROMOSOME CONDENSATION IN THE RDNA REQUIRES TRF4 AND DNA TOPOISOMERASE-I IN SACCHAROMYCES-CEREVISIAE

DNA topoisomerase I (topo I) is known to participate in the process of DNA replication, but is not essential in Saccharomyces cerevisiae. Th e TRF4 gene is also nonessential and was identified in a screen for mu tations that are inviable in combination with a top1 null mutation. He re we report the surprising finding that a top1 trf4-ts double mutant is defective in the mitotic events of chromosome condensation, spindle elongation, and nuclear segregation, but not in DNA replication. Dire ct examination of rDNA-containing mitotic chromosomes demonstrates tha t a top1 trf4-ts mutant fails both to establish and to maintain chromo some condensation in the rDNA at mitosis. We show that the Trf4p assoc iates physically with both Smc1p and Smc2p, the S. cerevisiae homologs of Xenopus proteins that are required for mitotic chromosome condensa tion in vitro. The defect in the top1 trf4-ts mutant is sensed by the MAD1-dependent spindle assembly checkpoint but not by the RAD9-depende nt DNA damage checkpoint, further supporting the notion that chromosom e structure influences spindle assembly. These data indicate that TOP1 (encoding topo I) and TRF4 participate in overlapping or dependent st eps in mitotic chromosome condensation and serve to define a previousl y unrecognized biological function of topo I.