Yeast Pif1 accelerates annealing of complementary DNA strands

ABSTRACT: Pif1 is a helicase involved in the maintenance of nuclear and mitochondrial genomes in eukaryotes. Here we report a new activity of Saccharomyces cerevisiae Pif1, annealing of complementary DNA strands. We identified preferred substrates for annealing as those that generate a duplex product with a single-stranded overhang relative to a blunt end duplex. Importantly, we show that Pif1 can anneal DNA in the presence of ATP and Mg2+. Pif1-mediated annealing also occurs in the presence of single-stranded DNA binding proteins. Additionally, we show that partial duplex substrates with 3′-single-stranded overhangs such as those generated during double-strand break repair can be annealed by Pif1. Helicases are defined as a class of proteins that movedirectionally on nucleic acids (NA) separating the strands of duplex NA using the energy of NTP hydrolysis (reviewed in refs 1−4). However, an increasing number of helicases have been shown to anneal or rewind complementary strands of oligonucleotides in the presence or absence of ATP.5−11 Annealing helicases are proposed to use their strand annealing activity for stabilization of stalled replication forks, double