Molecular Characterization of the Role of the Schizosaccharomyces pombe nip1+/ctp1+ Gene in DNA Double-Strand Break Repair in Association with the Mre11-Rad50-Nbs1 Complex

TheSchizosaccharomyces pombe nip1 + /ctp1 + gene was previously identified as anslr (s yntheticallyl ethal withrad2 ) mutant. Epistasis analysis indicated that Nip1/Ctp1 functions in Rhp51-dependent recombinational repair, together with the Rad32 (spMre11)-Rad50-Nbs1 complex, which plays important roles in the early steps of DNA double-strand break repair. Nip1/Ctp1 was phosphorylated in asynchronous, exponentially growing cells and further phosphorylated in response to bleomycin treatment. Overproduction of Nip1/Ctp1 suppressed the DNA repair defect of annbs1 -s10 mutant, which carries a mutation in the FHA phosphopeptide-binding domain of Nbs1, but not of annbs1 null mutant. Meiotic DNA double-strand breaks accumulated in thenip1 /ctp1 mutant. The DNA repair phenotypes and epistasis relationships ofnip1 /ctp1 are very similar to those of theSaccharomyces cerevisiae sae2 /com1 mutant, suggesting that Nip1/Ctp1 is a functional homologue of Sae2/Com1, although the sequence similarity between the proteins is limited to the C-terminal region containing the RHR motif. We found that the RxxL and CxxC motifs are conserved inSchizosaccharomyces species and in vertebrate CtIP, originally identified as a cofactor of the transcriptional corepressor CtBP. However, these two motifs are not found in other fungi, includingSaccharomyces andAspergillus species. We propose that Nip1/Ctp1 is a functional counterpart of Sae2/Com1 and CtIP.