Ku Stabilizes replication forks in the absence of Brc1

Abstract

DNA replication errors are a major source of genome instability in all organisms. In the fission yeast Schizosaccharomyces pombe, the DNA damage response protein Brc1 binds phospho-histone H2A (γH2A)-marked chromatin during S-phase, but how Brc1 protects genome 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 protective activity in brc1Δ cells does not involve its canonical NHEJ function or its roles in protecting 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 replisome-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.