RESEARCH ARTICLE Ku Stabilizes Replication Forks in the Absence

DNA replication errors are a major source of genome instability in all organisms. In the fis-sion yeast Schizosaccharomyces pombe, the DNA damage response protein Brc1 binds phospho-histone H2A (γH2A)-marked chromatin during S-phase, but how Brc1 protects ge-nome integrity remains unclear. Here we report that the non-homologous end-joining (NHEJ) protein Ku becomes critical for survival of replication stress in brc1Δ cells. Ku’s pro-tective activity in brc1Δ cells does not involve its canonical NHEJ function or its roles in pro-tecting telomeres or shielding DNA ends from Exo1 exonuclease. In brc1Δ pku80Δ cells, nuclear foci of Rad52 homologous recombination (HR) protein increase and Mus81-Eme1 Holliday junction resolvase becomes critical, indicating increased replication fork instability. Ku’s localization at a ribosomal DNA replication fork barrier associated with frequent repli-some-transcriptosome collisions increases in brc1Δ cells and increased collisions correlate with an enhanced requirement for Brc1. These data indicate that Ku stabilizes replication forks in the absence of Brc1