The ribosomal DNA metaphase loop of Saccharomyces cerevisiae gets condensed upon heat stress in a Cdc14-independent TORC1-dependent manner

Chromosome morphology in Saccharomyces cerevisiae is only visible at the microscopic level in the ribosomal DNA array (rDNA). The rDNA has been thus used as a model to characterize condensation and segregation of sister chromatids in mitosis. It has been established that the metaphase structure ("loop") depends, among others, on the condensin complex; whereas its segregation also depends on that complex, the Polo-like kinase Cdc5 and the cell cycle master phosphatase Cdc14. In addition, Cdc14 also drives rDNA hypercondensation in telophase. Remarkably, since all these components are essential for cell survival, their role on rDNA condensation and segregation was established by temperature-sensitive (ts) alleles. Here, we show that the heat stress (HS) used to inactivate ts alleles (25 ºC to 37 ºC shift) causes rDNA loop condensation in metaphase-arrested wild type cells, a result that can also be mimicked by other stresses that inhibit the TORC1 pathway. Because this condensation might challenge previous findings with ts alleles, we have repeated classical experiments of rDNA condensation and segregation, yet using instead auxin-driven degradation alleles (aid alleles). We have undertaken the protein degradation at lower temperatures (25 ºC) and concluded that the classical roles for condensin, Cdc5, Cdc14 and Cdc15 still prevailed. Thus, condensin degradation disrupts rDNA higher organization, Cdc14 and Cdc5 degradation precludes rDNA segregation and Cdc15 degradation still allows rDNA hypercompaction in telophase. Finally, we provide direct genetic evidence that this HS-mediated rDNA condensation is dependent on TORC1 but, unlike the one observed in anaphase, is independent of Cdc14.