Double strand breaks repaired by homologous recombination (HR) are first resected to form single stranded DNA which binds replication protein A (RPA). RPA attracts mediators which load the Rad51 filament to promote strand invasion, the defining feature of HR. How the resection machinery navigates nucleosome-packaged DNA is poorly understood. Using Schizosaccharomyces pombe we report that a conserved DDB1-CUL4-associated factor (DCAF), Wdr70, is recruited to DSBs as part of the Cullin4-DDB1 ubiquitin ligase (CRL4Wdr70) and stimulates distal H2B lysine 119 monoubiquitination(uH2B). Wdr70 deletion, or uH2B loss, results in increased loading of the checkpoint adaptor and resection inhibitor Crb253BP1, decreased Exo1 association and delayed resection. Wdr70 is dispensable for resection upon Crb253BP1 loss, or when the Set9 methyltransferase that creates docking sites for Crb2 is deleted. Finally we establish that this histone regulatory cascade similarly controls DSB resection in human cells.

The correct levels of deoxyribonucleotide triphosphates and their relative abundance are important to maintain genomic integrity. Ribonucleotide reductase (RNR) regulation is complex and multifaceted. RNR is regulated allosterically by two nucleotide-binding sites, by transcriptional control, and by small inhibitory proteins that associate with the R1 catalytic subunit. In addition, the subcellular localization of the R2 subunit is regulated through the cell cycle and in response to DNA damage. We show that the fission yeast small RNR inhibitor Spd1 is intrinsically disordered and regulates R2 nuclear import, as predicted by its relationship to Saccharomyces cerevisiae Dif1. We demonstrate that Spd1 can interact with both R1 and R2, and show that the major restraint of RNR in vivo by Spd1 is unrelated to R2 subcellular localization. Finally, we identify a new behavior for RNR complexes that potentially provides yet another mechanism to regulate dNTP synthesis via modulation of RNR complex architecture.

Schizosaccharomyces pombe Crb2 is a checkpoint mediator required for the cellular response to DNA damage. Like human 53BP1 and Saccharomyces cerevisiae Rad9 it contains Tudor(2) and BRCT(2) domains. Crb2-Tudor(2) domain interacts with methylated H4K20 and is required for recruitment to DNA dsDNA breaks. The BRCT(2) domain is required for dimerization, but its precise role in DNA damage repair and checkpoint signaling is unclear. The crystal structure of the Crb2-BRCT(2) domain, alone and in complex with a phosphorylated H2A.1 peptide, reveals the structural basis for dimerization and direct interaction with gamma-H2A.1 in ionizing radiation-induced foci (IRIF). Mutational analysis in vitro confirms the functional role of key residues and allows the generation of mutants in which dimerization and phosphopeptide binding are separately disrupted. Phenotypic analysis of these in vivo reveals distinct roles in the DNA damage response. Dimerization mutants are genotoxin sensitive and defective in checkpoint signaling, Chk1 phosphorylation, and Crb2 IRIF formation, while phosphopeptide-binding mutants are only slightly sensitive to IR, have extended checkpoint delays, phosphorylate Chk1, and form Crb2 IRIF. However, disrupting phosphopeptide binding slows formation of ssDNA-binding protein (Rpa1/Rad11) foci and reduces levels of Rad22(Rad52) recombination foci, indicating a DNA repair defect.

Schizosaccharomyces pombe Crb2 is a checkpoint mediator required for the cellular response to DNA damage. Like human 53BP1 and Saccharomyces cerevisiae Rad9 it contains Tudor(2) and BRCT(2) domains. Crb2-Tudor(2) domain interacts with methylated H4K20 and is required for recruitment to DNA dsDNA breaks. The BRCT(2) domain is required for dimerization, but its precise role in DNA damage repair and checkpoint signaling is unclear. The crystal structure of the Crb2-BRCT(2) domain, alone and in complex with a phosphorylated H2A.1 peptide, reveals the structural basis for dimerization and direct interaction with gamma-H2A.1 in ionizing radiation-induced foci (IRIF). Mutational analysis in vitro confirms the functional role of key residues and allows the generation of mutants in which dimerization and phosphopeptide binding are separately disrupted. Phenotypic analysis of these in vivo reveals distinct roles in the DNA damage response. Dimerization mutants are genotoxin sensitive and defective in checkpoint signaling, Chk1 phosphorylation, and Crb2 IRIF formation, while phosphopeptide-binding mutants are only slightly sensitive to IR, have extended checkpoint delays, phosphorylate Chk1, and form Crb2 IRIF. However, disrupting phosphopeptide binding slows formation of ssDNA-binding protein (Rpa1/Rad11) foci and reduces levels of Rad22(Rad52) recombination foci, indicating a DNA repair defect.