THE D-ARM OF TRNA(TYR) IS NECESSARY AND SUFFICIENT FOR IMPORT INTO LEISHMANIA-MITOCHONDRIA IN-VITRO

Transfer RNAs are selectively imported from the cytoplasm into mitocho ndria of kinetoplastid protozoa such as Leishmania. The specific struc tural features of tRNA which determine selectivity are largely unknown . Using an in organello system from Leishmania, the import signals on tRNA(Tyr) and on a synthetic transcript which binds to the same recept or, were studied by deletion and reconstruction analyses, In both case s, short oligoribonucleotides (minihelices) containing the sequence UG GYAGAG were imported with high efficiency in the presence of ATP, This motif is present in the D arm of tRNA(Tyr), as well as in the majorit y of imported Leishmania tRNAs, Deletion of the D arm, or a point muta tion in the conserved motif, reduces importability, The import signal coincides with the binding site for the mitochondrial receptor TAB, tR NA(Gln), which is not imported, forms non-productive, TAB-independent complexes with the mitochondrial surface, However, the observation tha t the imported:bound ratio of the D arm minihelix is higher than that of the entire molecule suggests that the post-binding translocation st ep is constrained in terms of size or structural flexibility. Kinetic studies of minihelix import indicate stepwise insertion of the molecul e into import channels.