Nej1p, a cell type-specific regulator of nonhomologous end joining in yeast

Mutant yeast strains lacking the silencing proteins Sir2p, Sir3p, or Sir4p have a defect in a DNA double-strand break (DSB) repair pathway, called nonhomologous end joining (NHEJ) [1, 2]. Mutations in sir genes also lead to the simultaneous expression of a and α mating type information, thus generating a nonmating haploid cell type [3] with many properties shared with a/α diploids. We addressed whether cell type or Sir proteins per se regulate NHEJ by investigating the role of a novel haploid-specific gene in NHEJ. This gene, NEJ1, was required for efficient NHEJ, and transcription of NEJ1 was completely repressed in a/α diploid [4] and sir haploid strains. The NEJ1 promoter contained a consensus binding site for the a1/α2 repressor, explaining the cell type-specific expression. Expression of Nej1p from a constitutive promoter in a/α diploid and sir mutant strains completely rescued the defect in NHEJ, thus showing that Sir proteins per se were dispensable for NHEJ. Nej1p and Lif1P[5], the yeast XRCC4 homolog, interacted in two independent assays, and Nej1p localized to the nucleus, suggesting that Nej1p may have a direct role in NHEJ.