Metacyclic VSG Gene Activation in Trypanosoma brucei rhodesiense

Matthews, Keith Roland (1990) Metacyclic VSG Gene Activation in Trypanosoma brucei rhodesiense. PhD thesis, University of Glasgow.

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African trypanosomes are able to evade the immune response of their mammalian host by their ability to change periodically their surface glycoprotein coat, in a process known as antigenic variation. This surface coat is first displayed in the parasites' metacyclic stage, which exists in the salivary glands of the parasites' insect vector, the tsetse fly. Previous analyses of the features of antigenic variation in the metacyclic stage have revealed that the system appears very different from the more complex and better characterized system employed in parasite populations established in the mammalian bloodstream. The work presented here has attempted to elucidate the reasons for these differences. Firstly, the mechanism for the activation of the expression of the metacyclic variant surface glycoprotein (VSG) coat has been studied. This has been done by infecting tsetse flies with trypanosomes expressing metacyclic VSG genes as they are believed to be activated in the tsetse fly salivary gland. An analysis of the resulting metacyclic population after cyclical transmission has revealed that there is apparently no heritable genomic change associated with activation of these genes. This is distinct from what is found for bloodstream VSG genes, the activation signals for which are preserved through the tsetse fly, and suggests that the metacyclic VSG repertoire is reset with transmission. Secondly, the structure of a metacyclic VSG gene expression locus has been investigated. This has involved the cloning, by the progressive isolation of overlapping restriction fragments, of 16Kb of the ILTat 1.61 gene environment. Analysis of the clones has revealed that the locus has just a very short upstream restriction site barren region, which contrasts with the long barren region associated with bloodstream VSG expression sites. Evidence for the presence of a region which cross hybridizes with one of the several families of expression site associated genes found in all bloodstream VSG expression sites has also been found. An analysis of nascent transcripts from trypanosomes expressing the ILTat 1.61 gene as it is believed to be expressed in the tsetse fly, has revealed that the transcription unit is very short with respect to other examined VSG expression sites. Finally, the reactivation of a gene encoding the metacyclic variant antigen type ILTat 1.22 in established bloodstream parasites has been investigated. This reactivation has been found always to involve the generation of a duplicate gene copy by conversion of one of a number of distinct bloodstream VSG expression sites. The upstream limit of the gene conversion process which results in the generation of this duplicate gene has been found always to occur within a region containing just 1.5 copies of the transposition associated 70bp repeat motif, of which upstream barren regions on bloodstream expression sites are composed. The sequence of the ILTat 1.22 gene and transposed segment has been determined. Neither is unusual with respect to the environment of bloodstream VSG genes. The determined characteristics of metacyclic VSG gene activation and expression site structure have provided potential explanations for the unusual features of surface coat expression in the tsetse fly.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Genetics, Parasitology
Date of Award: 1990
Depositing User: Enlighten Team
Unique ID: glathesis:1990-78101
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:40
Last Modified: 30 Jan 2020 15:40

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