Identification of the 3'-ends of the two mouse mitochondrial ribosomal RNAs. The 3'-end of 16 S ribosomal RNA contains nucleotides encoded by the gene for transfer RNALeuUUR.

The 3'-termini of the mitochondrial 12 S and 16 S ribosomal RNAs from mouse L cells have been definitively characterized by mobility-shift RNA sequencing, RNase digestion followed by fingerprinting using two-dimensional homochromatography, and precise mapping of RNA-DNA duplexes using nuclease S1. The results have been correlated with the known DNA sequence of the rRNA region. The vast majority of the 12 S rRNA consists of a family of transcripts whose last template-encoded nucleotide corresponds to a position immediately adjacent to the 5' end of the tRNAVal gene in the DNA sequence. These transcripts are oligoadenylated at their 3'-ends with from 1 to about 5 adenylate residues that are not encoded in the DNA sequence. A minor proportion of the 12 S rRNA ends one nucleotide before the 12 S/tRNAVal gene boundary and is also oligoadenylated. In contrast, the 3'-termini of 16 S rRNA have considerably greater heterogeneity, with the genomic location of the last template-encoded nucleotide varying from the nucleotide immediately adjacent to the 5'-end of the tRNALeuUUR gene to any position up to 7 nucleotides downstream within the tRNALeuUUR gene sequence. These various 16 S rRNA transcripts are oligoadenylated to a somewhat greater degree than the 12 S rRNA. The extent of the 16 S rRNA 3' heterogeneity, as compared to the 12 S rRNA, suggests that the 16 S rRNA 3'-termini may be generated by a mechanism involving termination of transcription rather than by processing of a primary transcript. The data are similar to those reported for human mitochondrial rRNA 3'-termini, and support a general role for adenylation of 3'-termini in the termination or processing of mammalian mitochondrial RNAs.