INTERACTION OF THIOSTREPTON WITH AN RNA FRAGMENT DERIVED FROM THE PLASTID-ENCODED RIBOSOMAL-RNA OF THE MALARIA PARASITE

Although eukaryotes are not generally sensitive to thiostrepton, growt h of the human malaria parasite Plasmodium falciparum is severely inhi bited by the drug. The proposed target in P. falciparum is the ribosom e of the plastid-like organelle (35 kb circular genome) of unknown fun ction. Positive identification of the drug target would confirm that t he organelle is essential for blood-stage development of Plasmodium an d help clarify the plastid's biological role. The action of thiostrept on as an antibiotic relates to its affinity for a conserved domain of eubacterial rRNA. Its effect on organelles is unknown. Because a numbe r of different point mutations within the Escherichia coli domain abro gates thiostrepton binding, extensive sequence differences between eub acterial and plastid domains brings into question the site of drug act ion. We have examined temperature-dependent hyperchromicity profiles o f synthetic RNAs corresponding to domains in the plastid and cytoplasm ic RNAs of P. falciparum. Thiostrepton induces a tertiary structure in the plastid-like fragment similar to that seen in eubacterial rRNA, e ven though the two share only about 60% sequence identity. A single po int mutation in the plastid-like fragment removes thiostrepton-depende nt tertiary structure formation. Thus, the plastid and eubacterial RNA s share a stabilized tertiary structure induced by the drug. This dire ct indicator of drug sensitivity in eubacteria suggests that the plast id-encoded ribosome is similarly sensitive to thiostrepton and that th e plastid is the site of drug action. Correlation of thiostrepton-sens itive and -resistant phenotypes with physical parameters suggests thio strepton resistance as a selectable marker for plastid transformation.