Inhibitors of topoisomerases I and II arrest DNA replication, but do not prevent nucleosome assembly in vivo.

Specific inhibitors of eukaryotic DNA topoisomerases I and II (camptothecin and VM-26, respectively) were used to examine the involvement of topoisomerases in DNA replication and chromatin assembly in vivo. When used singly, either camptothecin or VM-26 inhibited DNA synthesis in HeLa cells by more than 80%; when used simultaneously, the inhibitors effectively stopped replication, demonstrating that at least one class of topoisomerase must be active for fork propagation in vivo. To study nucleosome assembly during topoisomerase inhibition, three experimental strategies were employed: (1) pulse-chase experiments; (2) analyses of chromatin synthesized during residual replication in the presence of either camptothecin or VM-26; and (3) the assembly of previously replicated, unassembled DNA, generated in the presence of protein synthesis inhibitors. Using sensitivity to micrococcal nuclease and the maturation of non-nucleosomal replication intermediates as criteria, neither camptothecin nor VM-26, alone or in concert, inhibited nucleosome assembly under any experimental protocol tested. These data provide evidence that, although topoisomerase activity is essential for DNA replication, neither continuous fork propagation nor topoisomerase activity is required for chromatin assembly on new DNA.