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Molecular and Cellular Biology, August 2001, p. 4889-4899, Vol. 21, No. 15
0270-7306/01/$04.00+0 DOI: 10.1128/MCB.21.15.4889-4899.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Identification of rad27 Mutations That Confer
Differential Defects in Mutation Avoidance, Repeat Tract Instability,
and Flap Cleavage
Yali
Xie,1,
Yuan
Liu,2
Juan Lucas
Argueso,1
Leigh A.
Henricksen,2
Hui-I
Kao,2
Robert A.
Bambara,2 and
Eric
Alani1,*
Department of Molecular Biology and Genetics, Cornell
University, Ithaca, New York 14853,1 and
Department of Biochemistry and Biophysics, University of
Rochester School of Medicine and Dentistry, Rochester, New York
146422
Received 26 February 2001/Returned for modification 10 April
2001/Accepted 7 May 2001
In eukaryotes, the nuclease activity of Rad27p (Fen1p) is thought
to play a critical role in lagging-strand DNA replication by removing
ribonucleotides present at the 5' ends of Okazaki fragments. Genetic
analysis of Saccharomyces cerevisiae also has identified
a role for Rad27p in mutation avoidance. rad27
mutants display both a repeat tract instability phenotype and a high
rate of forward mutations to canavanine resistance that result
primarily from duplications of DNA sequences that are flanked by direct repeats. These observations suggested that Rad27p activities in DNA
replication and repair could be altered by mutagenesis and specifically
assayed. To test this idea, we analyzed two rad27 alleles, rad27-G67S and rad27-G240D, that
were identified in a screen for mutants that displayed repeat tract
instability and mutator phenotypes. In chromosome stability assays,
rad27-G67S strains displayed a higher frequency of
repeat tract instabilities relative to CAN1 duplication
events; in contrast, the rad27-G240D strains displayed
the opposite phenotype. In biochemical assays, rad27-G67Sp
displayed a weak exonuclease activity but significant single- and
double-flap endonuclease activities. In contrast, rad27-G240Dp
displayed a significant double-flap endonuclease activity but was
devoid of exonuclease activity and showed only a weak single-flap
endonuclease activity. Based on these observations, we hypothesize that
the rad27-G67S mutant phenotypes resulted largely from
specific defects in nuclease function that are important for degrading
bubble intermediates, which can lead to DNA slippage events. The
rad27-G240D mutant phenotypes were more difficult to
reconcile to a specific biochemical defect, suggesting a structural role for Rad27p in DNA replication and repair. Since the mutants provide the means to relate nuclease functions in vitro to genetic characteristics in vivo, they are valuable tools for further analyses of the diverse biological roles of Rad27p.
*
Corresponding author. Mailing address: Department of
Molecular Biology and Genetics, Cornell University, 459 Biotechnology Building, Ithaca, NY 14853-2703. Phone: (607) 254-4811. Fax: (607) 255-6249. E-mail: eea3{at}cornell.edu.

Present address: Department of Microbiology and Molecular Genetics,
University of California, Los Angeles, CA
90095.
Molecular and Cellular Biology, August 2001, p. 4889-4899, Vol. 21, No. 15
0270-7306/01/$04.00+0 DOI: 10.1128/MCB.21.15.4889-4899.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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