We have previously described a mitochondrial activity that removes 5′ leaders from yeast mitochondrial precursor tRNAs and suggested that it is a mitochondrial RNase P. Here we demonstrate that the cleavage reaction results in a 5′ phosphate on the tRNA product and thus the activity is analogous to that of other RNase Ps. A mitochondrial gene called the tRNA synthesis locus encodes an A+U rich RNA required for this activity in vivo. Two regions of this RNA display sequence similarity to conserved sequences in bacterial RNase P RNAs. This sequence similarity coupled with the analogous activities of the enzymes has led us to conclude that the RNAs are homologous and that the tRNA synthesis locus does code for the mitochondrial RNase P RNA subunit. The smallest and most abundant transcript of the tRNA synthesis locus is 490 nucleotides long. However, during purification of the holoenzyme, RNA is degraded and pieces of the original RNA are sufficient to support RNase P activity in vitro.
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