ORGANIZATION OF DNA TOPOISOMERASE-II ISOTYPES DURING THE CELL-CYCLE OF HUMAN-LYMPHOCYTES AND HELA-CELLS

We have monitored the organization of DNA topoisomerase II (Topo II) i n relation to chromatin disaggregation during mitogen stimulation of l ymphocytes and to the mitotic chromosome condensation cycle by immunof luorescence microscopy with isozyme-specific antibodies. Labelling for both Topo II alpha and Topo II beta was diffusely nucleoplasmic and n on-nucleolar in resting lymphocytes and the pattern changed little dur ing stimulation. Topo II alpha labelling intensity increased in parall el with the extent of cell stimulation, but a fraction of fully stimul ated cells was labelled very brightly. Topo II beta labelling intensit y was also greater in stimulated cells, but all partially and fully st imulated cells were labelled at the same, higher, intensity. In additi on, anti-Topo II beta detected a few small spots within nucleoli of st imulated cells that coincided with regions containing fibrillarin. In lymphocytes and HeLa, chromosome association of Topo II alpha began in prophase and lasted throughout mitosis. In contrast, Topo II beta sta yed nucleoplasmic in prophase, was diffusely cytoplasmic during mitosi s, and was first detected post-mitotically in nuclei with decondensing chromosomes and a reformed nuclear envelope. The results are consiste nt with a role for Topo II alpha, but not for Topo II beta, in mitotic chromosome condensation, and indicate that the isotypes may play inde pendent roles in the reorganization of chromatin structure during lymp hocyte mitogenic activation.