Castaño I B, Brzoska P M, Sadoff B U, Chen H, Christman M F
Department of Radiation Oncology, University of California, San Francisco 94143, USA.
Genes Dev. 1996 Oct 15;10(20):2564-76. doi: 10.1101/gad.10.20.2564.
DNA topoisomerase I (topo I) is known to participate in the process of DNA replication, but is not essential in Saccharomyces cerevisiae. The TRF4 gene is also nonessential and was identified in a screen for mutations that are inviable in combination with a top1 null mutation. Here 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. Direct examination of rDNA-containing mitotic chromosomes demonstrates that a top1 trf4-ts mutant fails both to establish and to maintain chromosome condensation in the rDNA at mitosis. We show that the Trf4p associates physically with both Smclp and Smc2p, the S. cerevisiae homologs of Xenopus proteins that are required for mitotic chromosome condensation in vitro. The defect in the top1 trf4-ts mutant is sensed by the MAD1-dependent spindle assembly checkpoint but not by the RAD9-dependent DNA damage checkpoint, further supporting the notion that chromosome structure influences spindle assembly. These data indicate that TOP1 (encoding topo I) and TRF4 participate in overlapping or dependent steps in mitotic chromosome condensation and serve to define a previously unrecognized biological function of topo I.
已知DNA拓扑异构酶I（拓扑异构酶I）参与DNA复制过程，但在酿酒酵母中并非必需。TRF4基因也不是必需的，它是在筛选与top1缺失突变结合时无法存活的突变时被鉴定出来的。在这里，我们报告了一个惊人的发现，即top1 trf4-ts双突变体在染色体凝聚、纺锤体伸长和核分离的有丝分裂事件中存在缺陷，但在DNA复制中没有缺陷。对含有rDNA的有丝分裂染色体的直接检查表明，top1 trf4-ts突变体在有丝分裂时既不能在rDNA中建立也不能维持染色体凝聚。我们表明，Trf4p与Smclp和Smc2p在物理上相关联，Smclp和Smc2p是非洲爪蟾蛋白质的酿酒酵母同源物，在体外有丝分裂染色体凝聚中是必需的。top1 trf4-ts突变体中的缺陷由MAD1依赖的纺锤体组装检查点检测到，但不由RAD9依赖的DNA损伤检查点检测到，这进一步支持了染色体结构影响纺锤体组装的观点。这些数据表明，TOP1（编码拓扑异构酶I）和TRF4在有丝分裂染色体凝聚中参与重叠或依赖的步骤，并有助于定义拓扑异构酶I以前未被认识的生物学功能。
