| Abstract: | Telomeres consist of simple G-rich DNA repeats located at the end of chromosomes that serve two distinct roles. One role is to solve the "end protection problem" by physically capping chromosome ends. This capping is conveyed by different Telomere Associated Proteins (TAPs), highlighting the shelterin complex, which protects the genetic material from end-to-end chromosome fusion, nucleolytic attack and activation of a DNA damage response. A second role is to solve the "end replication problem", in which the DNA replication machinery is not able to fully replicate the lagging strand, leaving a shorter chromosome after every cell division. In germline cells and stem cells, this problem is overcome by telomerase-mediated addition of (TTTAGGG)n in plants and (TTAGGG)n in mammals. An additional emerging role for telomeres is sensing and protecting the genome from oxidative stress. In this dissertation I demonstrate that chromosome ends accumulate more oxidative stress lesions than chromosome bodies. Moreover, certain TAPS and telomerase have roles in the oxidative stress response, functions that may occur either inside and outside the nucleus. I also provide evidence that telomeres act as barriers to Reactive Oxygen Species (ROS) through their association with particular TAPs. Of special importance in telomere biology is the TAP, Protection of Telomeres 1(POT1) protein, the most conserved of all the members of the shelterin telomere protection complex. Human POT1 (hPOT1) is involved in chromosome end protection and in the recruitment of telomerase to telomeres. The flowering plant Arabidopsis thaliana is unusual as it encodes two highly divergent POT1 paralogs: AtPOT1a and AtPOT1b. Using a series of genetic, biochemical and cell biological assays, I provide evidence that AtPOT1b has a novel role in protecting the genome, particularly telomeres, from oxidation arising from oxidative damage. This was observed in increased genome abnormalities and telomere length changes upon application of oxidative and genotoxic stress. Thus, AtPOT1b serves as a link between oxidative stress response and telomere biology. A second major focus of my dissertation research was investigation of the impact of space flight on plant telomeres. Telomeres are a marker of survivability and proliferation in all eukaryotes. Hence, it is important to understand how space travel affects genome integrity and physiology of plants, since these organisms will be integral for all space exploration projects as a source of food, carbon dioxide removal and oxygen generation. I discovered that plants, in contrast to human astronauts, have a highly robust mechanism of telomere length maintenance, resulting in negligible telomere length fluctuations during spaceflight and in simulated microgravity conditions on the ground. Thus, plants may be exceptionally well equipped to survive the stresses imposed by interstellar colonization. The electronic version of this dissertation is accessible from https://hdl.handle.net/1969.1/198483 |
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