PLASTIDS IN APICOMPLEXAN PARASITES

The discovery in malarial and toxoplasmodial parasites of genes normal ly occurring in the photosynthetic organelle of plants and algae has p rompted speculation that these so-called protozoans might harbour a ve stigial plastid. The plastid-like parasite genes occur on an extrachro mosomal, maternally inherited, 35 kb DNA circle with an architecture r eminiscent of plastid genomes. The 35kb genome is distinct from the 6- 7 kb linear mitochondrial genome. Localization of the 35kb genome with in the parasite cells by high resolution in situ hybridization has ide ntified a multi membrane-bound organelle in which the plastid-like gen ome resides. Since phylogenetic trees incorporating genes from the 35k b genome group them within the plastid radiation, we believe that the parasite organelle is a reduced plastid, that is probably no longer ph otosynthetic. Combined molecular and ultrastructural evidence indicate plastids to be widespread among apicomplexan parasites. The origin an d role of the plastid in obligate intracellular parasites is completel y unknown. The potential utility of the plastid as a parasite-specific target for therapeutic agents is examined.