ICRF-193, an inhibitor of topoisomerase II, demonstrates that DNA replication in sperm nuclei reconstituted in Xenopus egg extracts does not require chromatin decondensation.

A bis(2,6-dioxopiperazine) derivative, ICRF-193, is a specific inhibitor of topoisomerase II without cleavable complex-stabilizing activity. In Xenopus egg extract containing ICRF-193, demembranated sperm head chromatins were inhibited from decondensation. However, nuclear envelope-lamina assembled on the inhibited chromatins. The nuclear envelope-lamina continued to expand even after loss of contact with the chromatin surface. On the other hand, semiconservative DNA replication was initiated as soon as the lamina was assembled onto the surface of condensed chromatin, though the initiation was retarded and its extent was reduced, compared with that in noninhibited chromatins. Thus, it is concluded that topoisomerase II activity is not required for the formation of active DNA replication clusters and the extension of nuclear envelope-lamina on the chromatin, while the nuclear envelope-mediated decondensation of sperm chromatins is dependent on topoisomerase II activity.