Characterization of human MRP/Th RNA and its nuclear gene: full length MRP/Th RNA is an active endoribonuclease when assembled as an RNP

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Abstract
Vertebrate cells contain a site-specific endoribonuclease (RNase MRP) that cleaves mitochondrial RNA transcribed from the origin of leading-strand mitochondrial DNA replication. This report presents the characterization of the human enzyme and its essential RNA component. Human RNase MRP is a ribonucleoprotein with a nucleus- encoded RNA of 265 nucleotides. As expected, the single-copy RNA coding region is homologous (84%) to the corresponding mouse gene; surprisingly, at least 700 nucleotides of the immediate 5′-flanking region are conserved. The 265-nucleotide MRP RNA and an MAP RNA cleavage product representing the 3′-terminai 108 nucieotldes exist in nuclear and mitochondrial RNA isolates; the larger MRP RNA is present in greatest abundance in the nucleus. The putative processing site within the 265-nucleotide MRP RNA is oflset from that of mouse MRP RNA, but in each case cleavage is precise and occurs at the sequence ANCCCGC. Oligonucleotide-mediated inhibition experiments reveal that both the 5′ and 3′ portions of the MAP RNA are involved in cleavage by RNase MRP; this implies that full length MRP RNA complexed with proteins is an active species in vertebrate cells.