The MRPP1/MRPP2 complex is a tRNA-maturation platform in human mitochondria

Mitochondrial polycistronic transcripts are extensively processed to give rise to functional mRNAs, rRNAs and tRNAs; starting with the release of tRNA elements through 5′-processing by RNase P (MRPP1/2/3-complex) and 3′-processing by RNase Z (ELAC2). Here, we show using in vitro experiments that MRPP1/2 is not only a component of the mitochondrial RNase P but that it retains the tRNA product from the 5′-processing step and significantly enhances the efficiency of ELAC2-catalyzed 3′-processing for 17 of the 22 tRNAs encoded in the human mitochondrial genome. Furthermore, MRPP1/2 retains the tRNA product after ELAC2 processing and presents the nascent tRNA to the mitochondrial CCA-adding enzyme. Thus, in addition to being an essential component of the RNase P reaction, MRPP1/2 serves as a processing platform for several down-stream tRNA maturation steps in human mitochondria. These findings are of fundamental importance for our molecular understanding of disease-related mutations in MRPP1/2, ELAC2 and mitochondrial tRNA genes.