Both and Isoforms of Mammalian DNA Topoisomerase II Associate with Chromosomes in Mitosis

Two isoforms of DNA topoisomerase II, and , coded by separate genes, are expressed in actively cycling vertebrate cells. Some previous studies have suggested that only topoisomerase II remains associated with chromosomes at mitosis. Here, the distributions of topoisomerase II and in mitosis were studied by subcellular fractionation and by immunolocalization. Both isoforms of topoisomerase II were found to remain associated with mitotic chromatin. Topoisomerase II was distributed along chromosome arms throughout mitosis and was highly concentrated at centromeres until mid-anaphase, particularly in some cell types. Topoisomerase II showed weak concentration at centromeres in early mitosis in some cell types and was distributed along chromosome arms at every stage of mitosis through telophase. These studies suggest that in most cells both the major topoisomerase II isoforms may play roles in chromatin remodeling during M phase.