Binding of the thiazolyl peptide antibiotic thiostrepton to the GTPase domain of 23S rRNA involves a few crucial nucleotides, notably A1067 (E. coli). Small RNA transcripts were prepared corresponding to the GTPase domain of the plastid 23S rRNA and the two forms of cytosolic 28S rRNAs found in the human malaria parasite Plasmodium falciparum, as well as the plastid form of rRNA of the AIDS-related pathogen Toxoplasma gondii. Binding affinities of the wild type and mutated RNA sequences were as predicted; the malarial plastid sequence had by far the highest affinity, whereas that from toxoplasma did not bind thiostrepton.