NUCLEAR-ENCODED PROTEINS TARGET TO THE PLASTID IN TOXOPLASMA-GONDII AND PLASMODIUM-FALCIPARUM

A vestigial, nonphotosynthetic plastid has been identified recently in protozoan parasites of the phylum Apicomplexa. The apicomplexan plast id, or ''apicoplast,'' is indispensable, but the complete sequence of both the Plasmodium falciparum and Toxoplasma gondii apicoplast genome s has offered no clue as to what essential metabolic function(s) this organelle might perform in parasites. To investigate possible function s of the apicoplast, we sought to identify nuclear-encoded genes whose products are targeted to the apicoplast in Plasmodium and Toxoplasma. We describe here nuclear genes encoding ribosomal proteins S9 and L28 and the fatty acid biosynthetic enzymes acyl carrier protein (ACP), b eta-ketoacyl-ACP synthase III (FabH), and beta-hydroxyacyl-ACP dehydra tase (FabZ). These genes show high similarity to plastid homologues, a nd immunolocalization of S9 and ACP verifies that the proteins accumul ate in the plastid. All the putatively apicoplast-targeted proteins be ar N-terminal presequences consistent with plastid targeting, and the ACP presequence is shown to be sufficient to target a recombinant gree n fluorescent protein reporter to the apicoplast in transgenic T. gond ii, Localization of ACP, and very probably FabH and FabZ, in the apico plast implicates fatty acid biosynthesis as a likely function of the a picoplast. Moreover, inhibition of P. falciparum growth by thiolactomy cin, an inhibitor of FabH, indicates a vital role for apicoplast fatty acid biosynthesis. Because the fatty acid biosynthesis genes identifi ed here are of a plastid/bacterial type, and distinct from those of th e equivalent pathway in animals, fatty acid biosynthesis is potentiall y an excellent target for therapeutics directed against malaria, toxop lasmosis, and other apicomplexan-mediated diseases.