title: Saccharomyces cerevisiae flap endonuclease 1 uses flap equilibration to maintain triplet repeat stability.
abstract: Flap endonuclease 1 (FEN1) is a central component of Okazaki fragment maturation in eukaryotes. Genetic analysis of Saccharomyces cerevisiae FEN1 (RAD27) also reveals its important role in preventing trinucleotide repeat (TNR) expansion. In humans such expansion is associated with neurodegenerative diseases. In vitro, FEN1 can inhibit TNR expansion by employing its endonuclease activity to compete with DNA ligase I. Here we employed two yeast FEN1 nuclease mutants, rad27-G67S and rad27-G240D, to further define the mechanism by which FEN1 prevents TNR expansion. Using a yeast artificial chromosome system that can detect both TNR instability and fragility, we demonstrate that the G240D but not the G67S mutation increases both the expansion and fragility of a CTG tract in vivo. In vitro, the G240D nuclease is proficient in cleaving a fixed nonrepeat double flap; however, it exhibits severely impaired cleavage of both nonrepeat and CTG-containing equilibrating flaps. In contrast, wild-type FEN1 and the G67S mutant exhibit more efficient cleavage on an equilibrating flap than on a fixed CTG flap. The degree of TNR expansion and the amount of chromosome fragility observed in the mutant strains correlate with the severity of defective flap cleavage in vitro. We present a model to explain how flap equilibration and the unique tracking mechanism of FEN1 can collaborate to remove TNR flaps and prevent repeat expansion.
authors.affiliation: Department of Biochemistry and Biophysics, University of Rochester School of Medicine and Dentistry, Rochester, New York 14642,USA.
authors.forename: Yuan
authors.initials: Y
authors.lastname: Liu
authors.forename: Haihua
authors.initials: H
authors.lastname: Zhang
authors.forename: Janaki
authors.initials: J
authors.lastname: Veeraraghavan
authors.forename: Robert A
authors.initials: RA
authors.lastname: Bambara
authors.forename: Catherine H
authors.initials: CH
authors.lastname: Freudenreich
mesh_terms: D001483:Base Sequence; D043171:Chromosomal Instability; D004247:DNA / Q000235:genetics; D004249:DNA Damage; D004260:DNA Repair; D004261:DNA Replication; D045585:Flap Endonucleases / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008969:Molecular Sequence Data; D009154:Mutation; D009690:Nucleic Acid Conformation; D012441:Saccharomyces cerevisiae / Q000235:genetics* / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism*; D018911:Trinucleotide Repeats
chemical_list: C022262:Okazaki fragments; D029701:Saccharomyces cerevisiae Proteins; D004247:DNA; D045585:Flap Endonucleases