| Summary: | Telomeres cap the end of linear chromosomes and are vital for the genomic stability of eukaryotic cells. The function of telomeres depends on the interaction between the telomeric DNA and specialised telomere-bound proteins. In this study, I show that the budding yeast Rif1 (Rap1-interacting factor) exhibits unique telomere capping properties, which are distinct from its known functions. Deletion of RIF1 enhances the capping defect of the cdc13-1 mutant, whereas overexpression of RIF1 rescues the thermosensitivity of cdc13- 1 cells, and lowers the single-stranded DNA levels at damaged subtelomeres. Interestingly, Rif1 is recruited to internal damaged loci upon telomere uncapping, where it inhibits the binding of checkpoint proteins to these regions. The recruitment of Rif1 appears to be Rap1- independent as demonstrated by a mutant strain in which the Rif1-Rap1 interacting sequence was removed. In addition, Rif1 was also found essential for survival of yeast cells lacking telomeres through a checkpoint adaptation process. Furthermore, an unexpected telomere-independent role of Rif1 was discovered at an induced double strand break. Rif1 is recruited to the break site when overexpressed, where it promotes DNA repair via the nonhomologous end joining pathway. My results suggest that the budding yeast Rif1 is involved in end protection of telomeres and checkpoint adaptation. Budding yeast Rif1 may interact with the Cdc13, Stn1, and Ten1 (CST) complex and together cap the chromosome ends; scRif1 and CST may represent the functional equivalent of the vertebrate shelterin complex in the budding yeast telomeres. My study highlights the conserved function of Rif1, and implies that Rif1 may have a significant role in genomic stability and carcinogenesis in humans.
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