Precise localization and nucleotide sequence of the two mouse mitochondrial rRNA genes and three immediately adjacent novel tRNA genes

The complete DNA sequence of the ribosomal RNA region of mouse L cell mitochondrial DNA has been determined. Genes for the small (12S) and large (16S) rRNAs have been precisely located by direct sequencing of the termini of the two mature rRNAs. A comparison of the lengths (956 and 1582 nucleotides) and terminal sequences of the mature rRNAs with the DNA coding sequences indicates that mouse mt rRNAs are not spliced. Computer analysis of the complete DNA sequence has identified three potential transfer RNA genes. A gene for phenylalanine tRNA is located immediately adjacent to the 5′ end of the 12S rRNA gene, a valine tRNA gene occupies the entire region between the 12S and 16S rRNA genes and a leucine tRNA gene is located immediately adjacent to the 3′ end of the 16S gene. Hybridization of 32P-labeled, tRNA-sized mtRNA to selected DNA restriction endonuclease fragments from the rRNA region confirms the existence of small, abundant mtRNAs transcribed from these DNA sequences. All three tRNA genes and both rRNA genes are transcribed from the heavy strand of mtDNA. The mt rRNA sequences exhibit notable homologies to other rRNAs and, in particular, to those of E. coli. Within the 3′ terminal 50 nucleotides, the mouse mt 12S rRNA contains a potential 10 bp hairpin structure and a sequence of 15 consecutive nucleotides common to the RNA of the small ribosomal subunit in all systems, but does not contain the mRNA binding site (ACCUCC) found in E. coli and corn chloroplast rRNAs. The mt tRNA genes do not have the 3′ terminal CCA sequence encoded in the DNA, nor do they contain any intervening sequences. Two of the three tRNAs would lack many features which are known to be strictly conserved in all other nonorganelle tRNAs which have been sequenced. The fact that all the genes in this region are directly contiguous with at most one intervening nucleotide suggests that the entire region is transcribed into a polycistronic precursor RNA which is processed by endonucleolytic cleavages. The organization of the genes of the rRNA operon of mouse mtDNA, when compared to the organization of rRNA and tRNA genes in bacterial or eucaryotic nuclear genomes, provides evidence for the endosymbiotic hypothesis of the biogenesis of mammalian mitochondria.