The RNA molecule essential for the catalytic activity of RNase MRP, an enzymatically active ribonucleoprotein with two distinct roles in eukaryotes. In mitochondria it plays a direct role in the initiation of mitochondrial DNA replication. In the nucleus it is involved in precursor rRNA processing, where it cleaves the internal transcribed spacer 1 between 18S and 5.8S rRNAs.