Topoisomerase II inactivation prevents the completion of DNA replication in budding yeast.

Type II topoisomerases are essential for resolving topologically entwined double-stranded DNA. Although anti-topoisomerase 2 (Top2) drugs are clinically important antibiotics and chemotherapies, to our knowledge, the mechanisms of cell killing by Top2 depletion and inactivation have never been directly compared. We show that depletion of Top2 protein from budding yeast cells prevents DNA decatenation during S phase. Cells complete DNA replication and enter the ensuing mitosis on schedule, suffering extensive chromosome missegregation. Cytokinesis through incompletely segregated chromosomes causes lethal DNA damage. By contrast, expression of catalytically inactive Top2 causes a stable G2 arrest requiring an intact DNA damage checkpoint. Checkpoint activation correlates with an inability to complete DNA replication, resulting in hypercatenated, gapped daughter DNA molecules. Thus, Top2 depletion and inactivation kill cells by different mechanisms, which has implications for understanding the nature of the catenation checkpoint, how DNA replication terminates, how anti-Top2 drugs work, and how new drugs might be designed.