We have monitored the organization of DNA topoisomerase II (Topo II) in relation to chromatin disaggregation during mitogen stimulation of lymphocytes and to the mitotic chromosome condensation cycle by immunofluorescence microscopy with isozyme-specific antibodies. Labelling for both Topo II alpha and Topo II beta was diffusely nucleoplasmic and non-nucleolar in resting lymphocytes and the pattern changed little during stimulation. Topo II alpha labelling intensity increased in parallel with the extent of cell stimulation, but a fraction of fully stimulated cells was labelled very brightly. Topo II beta labelling intensity was also greater in stimulated cells, but all partially and fully stimulated cells were labelled at the same, higher, intensity. In addition, anti-Topo II beta detected a few small spots within nucleoli of stimulated 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 stayed nucleoplasmic in prophase, was diffusely cytoplasmic during mitosis, and was first detected post-mitotically in nuclei with decondensing chromosomes and a reformed nuclear envelope. The results are consistent with a role for Topo II alpha, but not for Topo II beta, in mitotic chromosome condensation, and indicate that the isotypes may play independent roles in the reorganization of chromatin structure during lymphocyte mitogenic activation.

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