The COMPASS subunit Spp1 protects nascent DNA at stalled replication fork barrier by limiting DNA availability to nucleases

Abstract Homologous recombination factors play a crucial role in protecting nascent DNA during replication but the role of chromatin in this process is largely unknown. Here, we used the bacterial Tus/Ter barrier known to induce a site-specific replication fork stalling in S. cerevisiae. We report that the COMPASS subunit Spp1 is recruited behind the stalled replication fork independently of its interaction with Set1. Spp1 chromatin recruitment depends on the interaction of the Spp1 PHD domain with H3K4me3 parental histones deposited behind the stalled fork. Its recruitment prevents accumulation of ssDNA at the stalled fork by restricting the access of Exo1. We further show that deleting SPP1 accelerates fork progression and increase mutation rate upstream the barrier consistent with ssDNA accumulation. Finally, we report that Spp1 protects nascent DNA at challenged replication forks. We propose that Spp1 limits remodeling of the fork that ultimately limits nascent DNA availability to nucleases.