Date of Award

2013

Degree Type

Dissertation

Department

Biological, Geological and Environmental Sciences

First Advisor

Boerner, Valentin

Subject Headings

Parasite antigens -- Variation, Trypanosoma brucei

Abstract

Trypanosoma brucei is a protozoan parasite that causes sleeping sickness in humans and nagana in cattle. When inside the mammalian host, T. brucei cells stay in extracellular spaces and regularly switch their surface antigen, Variant Surface Glycoprotein (VSG), to escape the host's immune responses. To ensure the effectiveness of VSG switching, T. brucei expresses a single type of VSG at any time exclusively from one of 20 identical VSG expression sites located next to the telomere. Monoallelic expression of VSG and VSG switching are important for T. brucei's pathogenesis. Our major goal is to understand the mechanisms of antigenic variation regulation. Binding of the Origin Recognition Complex (ORC) to replication origins is essential for initiation of DNA replication, but ORC has non-essential functions outside of DNA replication including gene silencing and telomere maintenance. An Orc1/Cdc6 homolog has been identified in T. brucei, but its function in gene silencing has not been investigated. In this study, we show that depletion of TbORC1 resulted in derepression of ES-linked silent VSGs in both mammalian-infectious bloodstream and insect procyclic forms. In addition, TbORC1 associates with telomere repeats but independently of known T. brucei telomere proteins, TbRAP1 and TbTRF. We conclude that TbORC1 is required to control telomere linked VSG gene silencing (Mol. Microbiol. 87(1): 196-210). Telomeres are nucleoprotein complexes located at ends of linear chromosomes and are essential for maintaining genome integrity. We have identified T. brucei TRF as a duplex telomere DNA binding factor. Nevertheless, the function of TbTRF in antigenic variation has not been studied. In this study, we solve the NMR structure of the TbTRF MYB domain and we determined point mutations that weaken or abolish the DNA binding activity of TbTRF in vitro. We successfully targeted these point mutations in vivo and studied the role of TbTRF DNA binding mutants in antigenic variation. Finally, we found that the modified

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