Nucleic Acids Research, Vol 26, Issue 9 2042-2049, Copyright © 1998 by Oxford University Press
V Borde and M Duguet
The nearly perfect synchrony of nuclear division in a plasmodium of
Physarum polycephalum provides a powerful system to analyze topoisomerase
II cleavage sites in the course of the cell cycle. The histone H4 locus,
whose schedule of replication and transcription is precisely known, was
chosen for this analysis. Drug-induced topoisomerase II sites are clustered
downstream of the histone H4 gene and appear highly dependent on cell cycle
stage. They were only detected in mitosis and at the very beginning of S
phase, precisely at the time of replication of the histone H4 region. The
sites, which were absent in G2 phase, reappeared at the next mitosis.
Remarkably, DNase I hypersensitive sites occurred in nearly the same
location, but their schedule was totally different: they were absent in
mitosis and present in G2. This schedule follows H4 transcription, which
peaks in mid-S phase and in the second part of G2 phase and is off during
mitosis. These results suggest that topoisomerase II may not be involved in
transcription, but plays a role in remodeling chromatin structure, both
during chromosome condensation in prophase/metaphase to allow their
decatenation and during chromosome decondensation after metaphase to allow
replication fork passage throughout the region.
ARTICLES
DNA topoisomerase II sites in the histone H4 gene during the highly synchronous cell cycle of Physarum polycephalum [published erratum appears in Nucleic Acids Res 1998 Oct 15;26(20):following 4789]
Laboratoire d'Enzymologie des Acides Nucleiques, Institut de Genetique et Microbiologie, URA 2225 CNRS, Bat. 400, Universite de Paris Sud, Centre d'Orsay, 91405 Orsay Cedex, France.
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