It has been well documented that Tel1 positively regulates telomere-end resection by promoting Mre11-Rad50-Xrs2 (MRX) activity, while Rif2 negatively regulates telomere-end resection by inhibiting MRX activity. At uncapped telomeres, whether Tel1 or Rif2 plays any role remains largely unknown. In this work, we examined the roles of Tel1 and Rif2 at uncapped telomeres in yku70Δ and/or cdc13-1 mutant cells cultured at non-permissive temperature. We found that deletion of TEL1 exacerbates the temperature sensitivity of both yku70Δ and cdc13-1 cells. Further epistasis analysis indicated that MRX and Tel1 function in the same pathway in telomere protection. Consistently, TEL1 deletion increases accumulation of Exo1-dependent telomeric single-stranded DNA (ssDNA) at uncapped telomeres, which stimulates checkpoint-dependent cell cycle arrest. Moreover, TEL1 deletion in yku70Δ cells facilitates Rad51-dependent Y′ recombination. In contrast, RIF2 deletion in yku70Δ cells decreases the accumulation of telomeric ssDNA after 8 h of incubation at the non-permissive temperature of 37 °C and suppresses the temperature sensitivity of yku70Δ cells, likely due to the increase of Mre11 association at telomeres. Collectively, our findings indicate that Tel1 and Rif2 regulate telomere protection at uncapped telomeres via their roles in balancing MRX activity in telomere resection.

已有大量文献报道，Tel1通过促进Mre11-Rad50-Xrs2（MRX）活性来正向调节端粒末端切除，而Rif2通过抑制MRX活性来负调控端粒末端切除。在未封端的端粒中，Tel1或Rif2是否起任何作用仍然未知。在这项工作中，我们研究了在非许可温度下培养的yku70Δ和/或cdc13-1突变细胞中，无盖端粒中Tel1和Rif2的作用。我们发现，TEL1的缺失加剧了yku70Δ和cdc13-1 细胞的温度敏感性。进一步的上位性分析表明，MRX和Tel1在端粒保护中以相同的途径起作用。一致地，TEL1缺失增加了未封端的端粒上Exo1依赖的端粒单链DNA（ssDNA）的积累，从而刺激了关卡依赖的细胞周期停滞。此外，yku70Δ细胞中的TEL1缺失促进了Rad51依赖的Y'重组。与此相反，RIF2缺失yku70 Δ细胞减少端粒ssDNA的在37℃的非许可温度下培养8小时后的积累和抑制的温度敏感性yku70 Δ细胞，可能是由于在端粒MRE11关联的增加。总体而言，我们的研究结果表明，Tel1和Rif2通过平衡端粒切除中的MRX活性来调节未封端的端粒的端粒保护。