Abstract
Although eukaryotes are not generally sensitive to thiostrepton, growth of the human malaria parasite Plasmodium falciparum is severely inhibited by the drug. The proposed target in P. falciparum is the ribosome of the plastid-like organelle (35 kb circular genome) of unknown function. Positive identification of the drug target would confirm that the organelle is essential for blood-stage development of Plasmodium and help clarify the plastid's biological role. The action of thiostrepton as an antibiotic relates to its affinity for a conserved domain of eubacterial rRNA. Its effect on organelles is unknown. Because a number of different point mutations within the Escherichia coli domain abrogates thiostrepton binding, extensive sequence differences between eubacterial and plastid domains brings into question the site of drug action. We have examined temperature-dependent hyperchromicity profiles of synthetic RNAs corresponding to domains in the plastid and cytoplasmic RNAs of P. falciparum. Thiostrepton induces a tertiary structure in the plastid-like fragment similar to that seen in eubacterial rRNA, even though the two share only about 60% sequence identity. A single point mutation in the plastid-like fragment removes thiostrepton-dependent tertiary structure formation. Thus, the plastid and eubacterial RNAs share a stabilized tertiary structure induced by the drug. This direct indicator of drug sensitivity in eubacteria suggests that the plastid-encoded ribosome is similarly sensitive to thiostrepton and that the plastid is the site of drug action. Correlation of thiostrepton-sensitive and -resistant phenotypes with physical parameters suggests thiostrepton resistance as a selectable marker for plastid transformation.
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