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A more recent version of this article appeared on May 1, 2004
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Submitted on August 5, 2003
Revised on December 31, 2003
Accepted on January 14, 2004
: Targeting to subchromosomal sites of activity during interphase and mitosis
1 Institute of Histology, Faculty of Medicine, University of Lisbon, 1649-028 Lisbon, Portugal
2 Institute of Histology, Faculty of Medicine, University of Lisbon, 1649-028 Lisbon, Portugal, Department of Morphology and Function, CIISA, Faculty of Veterinary Medicine, Lisbon, Portugal
* Corresponding author. E-mail address: hjoao{at}fm.ul.pt.
Mammalian topoisomerase II
(topo II) plays a vital role in the removal of topological complexities left on DNA during S phase. Here, we developed a new assay to selectively identify sites of catalytic activity of topo II with subcellular resolution. We show that topo II activity concentrates at replicating heterochromatin in late S in a replication-dependent manner, and at centric heterochromatin during G2 and M phases. Inhibitor studies indicate that this cell cycle-dependent concentration over heterochromatin is sensitive to chromatin structure. We further show that catalytically active topo II concentrates along the longitudinal axis of mitotic chromosomes. Finally, we found that catalytically inert forms of the enzyme localize predominantly to splicing speckles in a dynamic manner, and that this pool is differentially sensitive to changes in the activities of topo II itself and RNA polymerase II. Together, our data implicate several previously unsuspected activities in the partitioning of the enzyme between sites of activity and putative depots.
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