A role of topoisomerase II in linking DNA replication to chromosome condensation

The condensin complex and topoisomerase 11 (topo 11) have different biochemical activities in vitro, and both are required for mitotic chromosome condensation. We have used Xenopus egg extracts to investigate the functional interplay between condensin and topo 11 in chromosome condensation. When unreplicated chromatin is directly converted into chromosomes with single chromatids, the two proteins must function together, although they are independently targeted to chromosomes. In contrast, the requirement for topo 11 is temporarily separable from that of condensin when chromosome assembly is induced after DNA replication. This experimental setting allows us to find that, in the absence of conclensin, topo 11 becomes enriched in an axial structure within uncondensed chromatin. Subsequent addition of conclensin converts this structure- into mitotic chromosomes in an ATP hydrolysis-dependent manner. Strikingly, preventing DNA replication by the addition of geminin or aphidicolin disturbs the formation of topo II-containing axes and alters the binding property of topo 11 with chromatin. Our results suggest that topo 11 plays an important role in an early stage of chromosome condensation, and that this function of topo 11 is tightly coupled with prior DNA replication.