Segregation of the plastid and its genome in apicomplexan parasites
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The phylum Apicomplexa contains a large group of protozoan parasites responsible for numerous important human and livestock diseases. Significant challenges remain in the antimicrobial drug treatment of these diseases. The discovery of a remnant chloroplast, the apicoplast, now presents several parasite specific pathways that can be exploited as specific drug targets to help overcome these challenges. Genomic, genetic and pharmacological data shows that the apicoplast is essential for the parasite development and pathogenesis validating it as an important drug target. My work focusses on elucidating the mechanisms used by apicomplexan parasites to faithfully replicate and segregate this important organelle. The chloroplast division machinery in plants and algae depends on several proteins of cyanobacerial origin, specifically the homologs of bacterial tubulin ftsZ, which are highly conserved. The failure to find any clear homologs of these proteins in sequenced Apicomplexa genomes raises the question, how the plastid in Apicomplexa is divided in the absence of the conserved division machinery? Based on our cell biological studies we hypothesize that in sharp contrast to plants the plastid in Apicomplexa is segregated using genuinely eukaryotic apparatus Ð the spindle poles. We have tested our hypothesis using a set of highly compatible cell biological, comparative genomic and genetic experiments and developed a mechanistic model of plastid division in Apicomplexan parasites. Our current model elucidates that apicoplast is faithfully segregated into daughter cells by attaching to the spindle poles during the cell division. We show that this mechanism is conserved in apicomplexan parasites employing divergent cell division modes. We further demonstrate that plastid fission concurs with daughter cell formation and that a constrictive ring at the posterior end of forming daughter cells is responsible for plastid fission and nuclear division. To study how the apicoplast genome is replicated and segregated we have used genomic analysis to identify a bacterial histone-like protein and a PolA DNA polymerase that localize to the apicoplast. We have used these reagents as markers for the position of the apicoplast genome and replication machinery and tested various mechanistic models of plastid genome segregation.