Abstract
Condensin mediates chromosome condensation, which is essential for proper chromosome segregation during mitosis. We show that in budding yeast, polo kinase Cdc5p stimulates condensin to promote condensation of the ribosomal DNA (RDN). We show that cohesin, a complex related to condensin, recruits Cdc5p to the RDN. This spatial control of condensin activity prevents misfolding of the RDN into an irreversibly decondensed state. This regulation is also necessary to promote condensation of the RDN into a thin loop, distinct from the rest of the chromosomes. This thin condensed structure is reminiscent of constrictions associated with the pericentric and ribosomal loci of mitotic chromosomes in animals and plants. Therefore, we propose that the localized stimulation of condensin by cohesin and polo kinase described in the yeast RDN could be an evolutionarily conserved mechanism to generate a distinct type of chromosome folding at specific loci.