Hierarchical RNA processing is rate limiting for mitochondrial ribosome assembly

In animals the organization of the compact mitochondrial genome and lack of introns have necessitated a unique mechanism for RNA processing. To date the regulation of mitochondrial RNA processing and its importance for ribosome biogenesis and energy metabolism are not clear. To understand the in vivo role of the endoribonuclease component of the RNase P complex, MRPP3, we created conditional knockout mice. Here we show that MRPP3 is essential for life, and heart and skeletal muscle-specific knockout leads to a cardiomyopathy early in life, indicating that it is the only RNase P enzyme in mitochondria. We show that RNA processing is required for the biogenesis of the respiratory chain and mitochondrial function. Transcriptome-wide parallel analyses of RNA ends (PARE) and RNA-Seq enabled us to identify the in vivo cleavage sites of RNase P. Cleavage of the 5' tRNA ends precedes 3' end processing in vivo and is required for the correct biogenesis of the mitochondrial ribosomal subunits and mitoribosomal proteins that are differentially stabilized or degraded in the absence of mature rRNAs. Finally we identify that large mitoribosomal proteins can form a subcomplex on a precursor mt-RNA containing the 16S rRNA indicating that mitoribosomal biogenesis proceeds co-transcriptionally. Taken together our data show that RNA processing links transcription to translation via assembly of the mitoribosome. Overall design: Total RNA was isolated from heart tissue from 11 week old control (Mrpp3loxP/loxP) and Mrpp3 knockout mice (Mrpp3loxP/loxP, +/Ckmm), TruSeq libraries produced in triplicate, sequenced and analysed for differential expression. Mitochondrial RNA was isolated from heart tissue from 11 week old control (Mrpp3loxP/loxP) and Mrpp3 knockout mice (Mrpp3loxP/loxP, +/Ckmm), PARE libraries produced in triplicate and sequenced for analysis of mitochondrial RNA processing.