Cell Growth & Differentiation, Vol 1, Issue 1 39-45, Copyright © 1990 by American Association of Cancer Research
Assembly of transcriptionally active chromatin in vitro: a possible role for topoisomerase II
RQ To and EB Kmiec
Department of Biochemistry and Biophysics, University of California, Davis 95616.
Some models of in vitro chromatin assembly suggest a biphasic molecular
mechanism. The first phase, nucleosome formation, is comprised of the
formation of histone-DNA complexes which mature into a canonical nucleosome
structure. The second phase represents the process by which these
nucleosomes become properly spaced with a regular periodicity on the DNA.
In this report, we examine the role of DNA topoisomerases in the latter
phase of chromatin assembly. To study this process, we use a Xenopus laevis
cell-free extract, which assembles quantitative amounts of chromatin on
circular DNA templates, and the type II topoisomerase-specific antitumor
drugs VM-26 and endrofloxicin. Our results suggest that nucleosome
formation is unaffected by the presence of VM-26 or endrofloxicin. However,
periodic spacing of nucleosomes is inhibited significantly by these drugs.
In the absence of proper chromatin assembly, circular DNA molecules are
processed into nucleoprotein complexes which are transcribed poorly. Taken
together, these results indicate that the antitumor drugs VM-26 and
endrofloxicin influence gene expression indirectly by blocking the periodic
spacing of nucleosomes.