The process of antigenic variation in African trypanosomes allows the survival of the parasite by constantly switching the variant surface glycoprotein (VSG) expressed in their surface. There are believed to be several hundred copies of these silent VSG genes in the parasite's genome and they are expressed differentially. The majority of these genes are not capable of being transcribed in situ and must therefore be expressed from specialised transcriptional units known as bloodstream expression sites (BESs). Only one such site is active at any one time, ensuring that a single VSG is expressed in the trypanosome's surface coat. Switching the expressed VSG involves replacing the VSG in the active BES, or activating a new BES in conjunction with silencing the previously active. Differential expression of variant surface glycoprotein (VSG) genes, has a strong association with telomeres. All BESs are telomeric and differential activation involves recombination into the telomeric environment or silencing/activation of subtelomeric promoters. A number of pathogen contingency gene systems associated with immune evasion involve telomeric loci, which has prompted speculation that chromosome ends provide conditions conducive for the operation of rapid gene switching mechanisms. Ku is a protein associated with yeast telomeres that is directly involved in DNA recombination and gene silencing. The main aim of this thesis was to test the hypothesis that Ku in trypanosomes is centrally involved in differential VSG expression. In order to compare trypanosome Ku homologues with those from other organisms, it was necessary to compile homology alignments with other Ku homologues using Clustal W analysis. Subsequent experiments looked at the fate of exogenously introduced restriction enzyme target sites after transient transformation with cassettes encoding the restriction enzyme. A final analysis looked for the presence of NHEJ in homologous recombination- deficient trypanosomes. Disrupting this element of DNA repair would hopefully lead to other forms of repair becoming detectable, and even up-regulated. Rad51, in yeast a member of the Rad52 epistasis group (integral in yeast homologous recombination), had previously been demonstrated to be involved in DNA repair in trypanosomes (McCulloch & Barry, 1999). rad51 mutants were electroporated with cassettes containing noncompatible ends that would prevent their integration into the endogenous genome via conventional homologous recombination. This cassette also contained promoter DNA sequence to allow selection in the event of integration into non-transcribed regions of the genome. Study of the junctions encompassing the integration sites of the cassette allowed investigation into how the cassettes were integrated, and revealed to us the extent of the sequence homology required to catalyse integration. The method of repair detection observed indicated that classical homologous recombination is not the only pathway utilised by African trypanosomes to metabolise DNA double-strand breaks.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:248211 |
| Date | January 2002 |
| Creators | Conway, Colin |
| Publisher | University of Glasgow |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://theses.gla.ac.uk/30956/ |
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