The phylum Apicomplexa contains a large group of protozoan parasites responsible for numerous important human and livestock diseases. Several of these organisms (Toxoplasma, Cryptosporidium & Cyclospora) are also listed as type B pathogens of potential biodefense concern. Significant challenges remain in the antimicrobial drug treatment for 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 some of these challenges. Genomie, genetic and pharmacological data show that the apicoplast is essential for development and pathogenesis for Plasmodium and Toxoplasma validating it as a target and demonstrating the importance of the organelle for the biology of the organism. <P>This proposal is focused on unraveling the mechanisms used by the parasite to faithfully replicate and segregate this important organelle and its genome. The chloroplast division machinery in plants and algae depends overwhelmingly on genes of cyanobacterial origin, with the bacterial tubulin homolog ftsZ being the most prominent. Our genomic analysis has not identified any clear homologs of these genes in Apicomplexa.<P> How is the apicoplast divided in the absence of the conserved machinery? Based on our cell biological studies we hypothesize that in sharp contrast to plants the plastid in Apicomplexa is segregated using a genuinely eukaryotic mechanism - association with the centrosomes of the mitotic spindle.<P> This proposal develops a number of mechanistic models to explain how the plastid is faithfully segregated into daughter cells, how fission occurs and is timed within the parasite cell cycle, and how the replication and maintenance of the organellar genome is tied into this process. To test these hypotheses we have assembled a set of highly compatible cell biological, comparative genomic and genetic experiments. Genomic and genetic screens will permit us to further refine our hypotheses, and will help us to populate them with additional molecular players beyond the genes and proteins already in hand.