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J. Biol. Chem., Vol. 275, Issue 35, 27386-27392, September 1, 2000
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From the DNA Repair Group, Health and Environment Unit, Laval
University Medical Research Center, CHUQ, Faculty of Medicine,
Laval University, 2705 Boulevard Laurier, Sainte-Foy, Quebec G1V
4G2, Canada
To investigate the mechanism of double strand DNA
break formation in mammalian cells, an in vitro assay was
established using closed circular DNA containing two uracils on
opposite DNA strands (18 and 30 base pairs apart) and extracts prepared
from human cells. In this assay, formation of double strand breaks was
detected by the conversion of circular DNA to linear DNA. Approximately 4-fold more double strand DNA breaks were produced by extracts from
cells deficient in DNA ligase I (46BR) relative to those produced by
extracts from control cells (MRC5, derived from a clinically normal
individual). In parallel with the amount of double strand DNA breaks,
extracts from 46BR cells produced longer repair patches (up to 24 bases
in length) than those from MRC5 cells (typically <5 bases long). When
purified DNA ligase I was added to 46BR extracts to complement the DNA
ligase deficiency, only a negligible difference was found between the
amount of doublestrand DNA breaks or the repair patch size
generated in the assay relative to MRC5 extracts. Together, our data
demonstrate that double strand DNA breaks are produced through
formation of DNA repair patches. We refer to this process of double
strand break formation as the "DNA repair patch-mediated pathway."
Supported by a scholarship from the Medical Research Council of
Canada. To whom correspondence should be addressed. Tel.: 418-656-4141 (ext. 7340); Fax: 418-654-2159; E-mail:
Masahiko.Sato@crchul.ulaval.ca.
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