THE SPATIAL REGULATION OF CONDENSIN ACTIVITY IN CHROMOSOME CONDENSATION

Condensin mediates chromosome condensation, which is essential for proper chromosome segregation during mitosis. Prior to anaphase of budding yeast, the ribosomal DNA (RDN) condenses to a thin loop that is distinct from the rest of the chromosomes. We provide evidence that the establishment and maintenance of this RDN condensation require the regulation of condensin by Cdc5p (polo) kinase. We show that Cdc5p is recruited to the site of condensin binding in the RDN by cohesin, a complex related to condensin. Cdc5p and cohesin prevent condensin from misfolding the RDN into an irreversibly decondensed state. From these and other observations, we propose that the spatial regulation of Cdc5p by cohesin modulates condensin activity to ensure proper RDN folding into a thin loop. This mechanism may be evolutionarily conserved, promoting the thinly condensed constrictions that occur at centromeres and RDN of mitotic chromosomes in plants and animals.凝缩蛋白（condensin）介导染色体凝缩过程，这对于有丝分裂过程中染色体的正常分离至关重要。在出芽酵母的有丝分裂后期到来之前，核糖体DNA（ribosomal DNA, RDN）会凝缩为一种与其余染色体显著不同的纤细环状结构。本研究提供实验证据表明，该RDN凝缩状态的建立与维持，需要Cdc5p（polo激酶）对凝缩蛋白进行调控。我们证实，黏连蛋白（cohesin）——一种与凝缩蛋白相关的复合物——可将Cdc5p招募至RDN区域内的凝缩蛋白结合位点。Cdc5p与黏连蛋白能够阻止凝缩蛋白将RDN错误折叠为不可逆解凝的状态。基于上述发现与其他相关实验结果，我们提出如下假说：黏连蛋白对Cdc5p的空间调控可调节凝缩蛋白的活性，以此确保RDN能够正确折叠为纤细环状结构。该调控机制可能在进化上具有保守性，能够促进动植物有丝分裂染色体的着丝粒（centromeres）与RDN区域形成纤细凝缩的缢缩结构。