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Novel Players in Telomere Maintenance and Beyond
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Telomeres are specialized nucleoprotein caps at the end of linear chromosomes, critical for genome stability. A major function of telomeres is to distinguish chromosome ends from ends of double strand breaks. A second function is to counteract incomplete end-replication via telomerase extension. POT1 (Protection of Telomere 1) is a highly conserved telomere protein known for its essential role in chromosome end-protection and end-replication. Arabidopsis thaliana encodes three POT1 paralogs, POT1a, POT1b, and POT1c. AtPOT1a promotes telomerase processivity and therefore is required for telomere length homeostasis. The functions of AtPOT1b and AtPOT1c are less understood. In this dissertation, I characterized the function of POT1b at telomeres. In contrast to POT1a, I found that POT1b is dispensable for telomere length maintenance and serves as a negative regulator of telomerase. In addition, I tested the hypothesis that TER2/POT1b works in concert with Ku to stabilize the blunt-ended telomeres. Further characterization of POT1b using biochemical and genetic approaches revealed several unexpected features. First, unlike POT1a, which is primarily localized to the nucleus, POT1b accumulates in the cytoplasm, where its binding partner TER2 also resides. This observation suggests a potential regulatory pathway for TER2 RNP via subcellular trafficking. In addition, I found that early development of POT1b mutants is significantly delayed, indicating that POT1b has a novel role in plant development. Together, these studies provide insights into the role of AtPOT1b in telomere biology and expand our understanding of POT1 protein function and evolution. In addition to these studies of POT1 proteins, I examined the role of chromosome remodeler DDM1 (Deficient in DNA Methylation 1) in telomere length maintenance. I showed that plants deficient in DDM1 suffer from abrupt telomere shortening in the sixth generation of the deficiency due to deletional recombination. This telomere rapid deletion (TRD) coincides with increased transposon activation and increased DNA damage sensitivity at the root apical meristem, suggesting that TRD may serve as a mechanism to stimulate programmed cell death, thereby eliminating stem cells with massive DNA damage. These studies open a new avenue for telomere function in promoting genome integrity.
Xie, Xiaoyuan (2017). Novel Players in Telomere Maintenance and Beyond. Doctoral dissertation, Texas A & M University. Available electronically from