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Vol. 15, Issue 5, 2388-2400, May 2004
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: Targeting to Subchromosomal Sites of Activity during Interphase and Mitosis





* Institute of Histology, Faculty of Medicine, University of Lisbon, 1649028 Lisbon, Portugal;
Department of Morphology and Function, CIISA, Faculty of Veterinary Medicine, 1300-477 Lisbon, Portugal
Submitted August 5, 2003;
Revised December 31, 2003;
Accepted January 14, 2004
Monitoring Editor: Pamela Silver
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.
Abbreviations used: BrdU, bromodeoxyuridine; DRB, 5,6-dichloro-1-
-d-ribofuranosylbenzimidazole; FRAP, fluorescence recovery after photobleaching; GFP, green fluorescent protein; EGFP, enhanced green fluorescent protein; HDAC, histone deacetylase; NURD, nucleosome remodeling and deacetylation complex; PABP2, poly(A)-binding protein II; TSA, trichostatin A.
These authors contributed equally to this work.
Corresponding author. E-mail address: hjoao{at}fm.ul.pt.
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