Necessary and sufficient factors for the import of transfer RNA into the kinetoplast mitochondrion

The mechanism of active transport of transfer RNA ( tRNA) across membranes is largely unknown. Factors mediating the import of tRNA into the kinetoplast mitochondrion of the protozoon Leishmania tropica are organized into a multiprotein RNA import complex ( RIC) at the inner membrane. Here, we present the complete characterization of the identities and functions of the subunits of this complex. The complex contains three mitochondrionand eight nuclear- encoded subunits; six of the latter are necessary and sufficient for import. Antisense- mediated knockdown of essential subunits resulted in the depletion of mitochondrial tRNAs and inhibition of organellar translation. Functional complexes were reconstituted with recombinant subunits expressed in Escherichia coli. Several essential RIC subunits are identical to specific subunits of respiratory complexes. These findings provide new information on the evolution of tRNA import and the foundation for detailed structural and mechanistic studies.