Apicomplexan parasites possess distinct nuclear-encoded, but apicoplast-localized, plant-type ferredoxin-NADP+ reductase and ferredoxin.

In searching for nuclear-encoded, apicoplast-localized proteins we have cloned ferredoxin-NADP(+) reductase from Toxoplasma gondii and a [2Fe-2S] ferredoxin from Plasmodium falciparum. This chloroplast-localized redox system has been extensively studied in photosynthetic organisms and is responsible for the electron transfer from photosystem I to NADP+. Besides this light-dependent reaction in nonphotosynthetic plastids (e.g. from roots), electrons can also flow in the reverse direction, from NADPH to ferredoxin, which then serves as an important reductant for various plastid-localized enzymes. These plastids possess related, but distinct, ferredoxin-NADP+ reductase and ferredoxin isoforms for this purpose. We provide phylogenetic evidence that the T. gondii reductase is similar to such nonphotosynthetic isoforms. Both the P. falciparum [2Fe-2S] ferredoxin and the T. gondii ferredoxin-NADP+ reductase possess an N-terminal bipartite transit peptide domain typical for apicoplast-localized proteins. The recombinant proteins were obtained in active form, and antibodies raised against the reductase recognized two bands on Western blots of T. gondii tachyzoite lysates, indicative of the unprocessed and native form, respectively. We propose that the role of this redox system is to provide reduced ferredoxin, which might then be used for fatty acid desaturation or other biosynthetic processes yet to be defined. Thus, the interaction of these two proteins offers an attractive target for drug intervention.