Denny, Paul William; (1997) The apicomplexan plastid DNA: An evolutionary and molecular study. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The discovery and characterisation in this laboratory of a 35 kilobase plastid genome from the malaria parasite, Plasmodium, has led to intense speculation concerning its origins and function. This thesis describes how sequence data were garnered from malaria's distant apicomplexan cousins, the coccidians and piroplasms, and used to make an evolutionary analysis of their corresponding plastid DNAs. From the sequence data produced it appears highly likely that the plastid DNA was gained by an ancient progenitor of the Apicomplexa, and singularly reorganised upon the adoption of a parasitic lifestyle. Previously, the identification in this laboratory of a Plasmodium plastid encoded open reading frame with homologues in rhodoplast genomes led to the suggestion that the apicomplexan organelle has a red algal origin. However, phylogenetic analysis of the tuf gene, which encodes the ubiquitous plastid and prokaryotic elongation factor Tu, provides support for the hypothesis that the organelle was derived through endosymbiosis of a green alga. The highly conserved nature of the extrachromosomal DNA found across the range of parasites studied here suggests that the organelle within which the genome is housed performs a vital cellular role. However, the results presented provide no indication of its particular function. The sequence data generated also led to the speculation that certain plastid encoded products may provide targets for novel anti-apicomplexan chemotherapeutics. Finally, it is inferred from the sequence of the plastid DNA of the coccidians, Toxoplasma gondii and Eimeria tenella, that opal stop codons are occasionally used as tryptophan codons. Such a system would be the first identified in a plastid organelle.

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