Chromatin assembly and DNA replication are temporally coupled, and DNA replication in the absence of histone synthesis causes inviability. Here we demonstrate that chromatin assembly factor Asf1 also affects DNA replication. In budding yeast cells lacking Asf1, the amounts of several DNA replication proteins, including replication factor C (RFC), proliferating cell nuclear antigen (PCNA), and DNA polymerase (Pol ), are reduced at stalled replication forks. In contrast, DNA polymerase ␣ (Pol ␣) accumulates to higher than normal levels at stalled forks in asf1⌬ cells. Using purified, recombinant proteins, we demonstrate that RFC directly binds Asf1 and can recruit Asf1 to DNA molecules in vitro. We conclude that histone chaperone protein Asf1 maintains a subset of replication elongation factors at stalled replication forks and directly interacts with the replication machinery.[Keywords: Chromatin assembly; histone deposition; DNA replication; genome stability] Supplemental material is available at http://www.genesdev.org. In eukaryotes, DNA is assembled into a nucleoprotein complex called chromatin. The fundamental repeating unit of chromatin is the nucleosome, which consists of 146 bp of DNA wrapped around an octamer of core histone proteins, comprised of two H2A/H2B dimers flanking an inner (H3/H4) 2 tetramer. In vivo, histone deposition can occur by DNA replication-coupled or replication-independent mechanisms (for review, see Franco and Kaufman 2004). In either case, histone deposition is mediated by specialized assembly proteins. The best characterized replication-coupled chromatin assembly factor is chromatin assembly factor-1 (CAF-1), an evolutionarily conserved protein complex that binds histone H3 and H4 and delivers them to replicating DNA through an interaction with proliferating cell nuclear antigen (PCNA), the DNA polymerase processivity protein (for review, see Asf1, another evolutionarily conserved histone chaperone, functions during both replication-coupled and replication-independent chromatin assembly. Asf1 bound to histones H3/H4 stimulates histone deposition by CAF-1 in vitro (Tyler et al. 1999;Sharp et al. 2001), and Asf1 physically interacts with the p60/Cac2 subunit of CAF-1 (Tyler et al. 2001;Krawitz et al. 2002;Mello et al. 2002). In yeast, Asf1 and the Hir proteins directly interact and function together to promote heterochromatic gene silencing (Kaufman et al. 1998;Sharp et al. 2001;Sutton et al. 2001;Krawitz et al. 2002). Consistent with a role in multiple deposition pathways, Asf1 copurifies with both CAF-1 and Hir protein complexes in human cell extracts (Tagami et al. 2004).Asf1 also contributes to genome stability during S phase in a manner distinct from both CAF-1 and the Hir proteins. Unlike yeast cells lacking CAF-1 or Hir proteins, cells lacking Asf1 are sensitive to the DNA synthesis inhibitor hydroxyurea (HU) and the DNA topoisomerase I inhibitor camptothecin (CPT), and display synthetic genetic interactions with DNA synthesis genes including the initiation/elongation factor C...