During both interphase and mitosis, DNA topoisomerase II interacts with DNA as well as RNA through the protein's C-terminal domain

DNA topoisomerase II (topo II) is thought to be a nuclear enzyme; during in terphase most was insoluble and could be recovered in the pellet after cent rifugation of cell homogenates at 10,000 g (P-10). Upon entry into mitosis, the majority of topo II did not associate,vith condensed chromosomes but w as apparently solubilized and redistributed throughout the cell. Although t wo nonchromosomal subfractions of mitotic topo II were defined by centrifug ation at 130,000 g, the vast majority (greater than or equal to 90%) was re covered in the pellet (P-130). In vivo nucleic acid interactions with topo II were monitored by a recently developed approach of UV-photo-crosslinking , immunoprecipitation and P-32-labeling. P-10 (interphase) topo II was larg ely associated with DNA. P-130 (mitotic non-chromosomal) topo II was primar ily associated with RNA. These nucleic acid interactions with both interpha se and mitotic topo II occurred through the catalytically inert and as yet, poorly understood C-terminal domain of the protein. P-10 topo II was highl y active enzymatically. Activity, measured by the ability of topo II to dec atenate kDNA minicircles, was reduced by treatment with phosphatase. In con trast, P-130 topo II was relatively inactive but activity could be increase d by phosphatase treatment. In vivo, P-130 topo II was more heavily phospho rylated than P-10 topo II; in both, only the C-terminal domain of topo II w as detectably modified. Our observations suggest that cell cycle-dependent changes in the distribution, nucleic acid interactions and enzymatic activi ty of topo II are regulated, at least in part, by phosphorylation/dephospho rylation.