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Molecular and Cellular Biology, January 2000, p. 594-603, Vol. 20, No. 2
Department of Microbiology, Duke University
Medical Center, Durham, North Carolina 27710
Received 23 August 1999/Returned for modification 21 September
1999/Accepted 20 October 1999
Many antitumor and antibacterial drugs inhibit DNA topoisomerases
by trapping covalent enzyme-DNA cleavage complexes. Formation of
cleavage complexes is important for cytotoxicity, but evidence suggests
that cleavage complexes themselves are not sufficient to cause cell
death. Rather, active cellular processes such as transcription and/or
replication are probably necessary to transform cleavage complexes into
cytotoxic lesions. Using defined plasmid substrates and two-dimensional
agarose gel analysis, we examined the collision of an active
replication fork with an antitumor drug-trapped cleavage complex.
Discrete DNA molecules accumulated on the simple Y arc, with branch
points very close to the topoisomerase cleavage site. Accumulation of
the Y-form DNA required the presence of a topoisomerase cleavage site,
the antitumor drug, the type II topoisomerase, and a T4 replication
origin on the plasmid. Furthermore, all three arms of the Y-form DNA
were replicated, arguing strongly that these are trapped replication
intermediates. The Y-form DNA appeared even in the absence of two
important phage recombination proteins, implying that Y-form DNA is the
result of replication rather than recombination. This is the first
direct evidence that a drug-induced topoisomerase cleavage complex
blocks the replication fork in vivo. Surprisingly, these blocked
replication forks do not contain DNA breaks at the topoisomerase
cleavage site, implying that the replication complex was inactivated
(at least temporarily) and that topoisomerase resealed the drug-induced DNA breaks. The replication fork may behave similarly at other types of
DNA lesions, and thus cleavage complexes could represent a useful
(site-specific) model for chemical- and radiation-induced DNA damage.
0270-7306/0/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
An Antitumor Drug-Induced Topoisomerase Cleavage
Complex Blocks a Bacteriophage T4 Replication Fork In Vivo
*
Corresponding author. Mailing address: Department of
Microbiology, Box 3020, Duke University Medical Center, Durham, NC
27710. Phone: (919) 684-6466. Fax: (919) 681-8911. E-mail:
kenneth.kreuzer{at}duke.edu.
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