The Pif1 helicase is actively inhibited during meiotic recombination to prevent excess length of gene conversions

Meiotic recombination ensures proper chromosome segregation to form viable gametes and results in gene conversion events between homologs. Conversion tracts are shorter in meiosis than in mitotically dividing cells. This results at least in part from the binding of a complex, formed of the Mer3 helicase and the MutL heterodimer, to meiotic recombination intermediates. The molecular actors inhibited by this complex are elusive. The Pif1 DNA helicase is known to stimulate DNA polymerase delta (Pol δ) -mediated DNA synthesis from D-loops in vitro, allowing long synthesis required for break-induced replication. We show that Pif1 is recruited genome wide to meiotic double-strand break (DSB) sites. We further show that Pif1, through its interaction with PCNA, is required for the long gene conversions observed in the absence of MutL recruitment to recombination sites. in vivo, Mer3 interacts with the PCNA clamp loader RFC, and in vitro, Mer3-MutL ensemble inhibits Pif1-stimulated D-loop extension by Pol δ and RFC-PCNA. Mechanistically, our results suggest that Mer3-MutL may compete with Pif1 for binding to RFC-PCNA. Taken together, our data show that Pif1’s activity to promote meiotic recombination-related DNA synthesis is restrained by the Mer3-MutL ensemble to prevent long gene conversion events and possibly associated mutagenesis. 4 samples total: 2 independent replicates of Pif1-Myc ChIP at 5 h in pCUP1-IME1 synchronized meiosis and 2 independent replicates of Pif1-Myc ChIP in a spo11Y135F strain at 5 h in pCUP1-IME1-synchronized meiosis