Telomeres are indispensable elements of eukaryotic chromosomes important for full replication of linear genomes and for chromosome end capping. These functions are essential for genome stability and long term cell survival. The key feature of telomeres is their capability to differentiate native chromosome ends from being recognized as deleterious DNA breaks. Telomeres protection is thought to require single-stranded DNA protrusions (G-overhangs) that are either bound by specific protein complexes or form secondary DNA structures such as t-loops. We discovered that only ~50% of plant telomeres possess G-overhangs; the remaining telomeres are either blunt-ended or nearly blunt-ended. Such telomeres cannot form t-loops and their protection must be mediated by a novel mechanism. We discovered that integrity of blunt-ended telomeres relies on Ku, an evolutionary conserved heterodimer that is required for DNA repair by non-homologous end-joining. Here we propose to study processes that contribute to the formation and protection of blunt-ended telomeres. My first aim is to determine why at telomeres Ku supports end protection, but does not promote chromosome-end-top end fusions. The key approach is to generate Ku complexes with modified DNA interaction properties and test their function in vivo. In the second aim I want to use genetic and biochemical approaches to identify and characterize plant telomere binding proteins and determine their role in processing and protection of blunt-ended telomeres. And finally, I will exploit natural variation in Arabidopsis telomere structure to identify gene(s) involved in telomere processing and maturation. Blunt-ended telomeres are not oddity of plants; they are expected to temporarily arise on telomeres replicated by leading strand mechanism in all eukaryotes. Thus, knowledge generated in the proposed research will shed a light on poorly characterized processes contributing to the formation of telomere cap.

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