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The present study was undertaken to further clarify the unique features of the structure and function of the mitochondrial ribosomal RNAs (rRNAs) of animal cells. 1) The entire mitochondrial genome of the bullfrog, Rana catesbeiana, was cloned into a plasmid vector pBR322 at the unique Bam HI site, and the nucleotide sequences of the small (12S) and large (16S) rRNA genes and their flanking ND1 and two tRNA genes were determined. The organization and the directions of the transcription of these genes were exactly the same as those of the mitochondrial genomes of the five animal species so far known. 2) The Rana catesbeiana 12S and 16S rRNA genes consisted of 937 and 1,585 bases, respectively. The nucleotide sequence homologies of each gene to their counterparts in other animals were in the range of 66 to 75%. These values are much lower than the 95% observed among nuclear-encoded cytoplasmic rRNAs of the five animals, indicating that mutations occur faster in the mitochondrial than in the cytoplasmic rRNAs. As to the base composition, the GC contents of the 12S and 16S mitochondrial rRNAs of Rana catesbeiana were 43 and 40%, respectively, which are also characteristically low as compared with the values of 55% in the E. coli and 60% in the cytoplasmic rRNAs. 3) The putative secondary structures of the mitochondrial rRNAs of Rana catesbeiana were drawn by the comparative analytical method, based on the proposed models for E. coli and some cytoplasmic rRNAs. In spite of the considerable differences in size and base compositions found in the rRNAs from different species and different organelles, the secondary structures of the mitochondrial rRNAs were highly conserved, implying that these structures are essential for the functioning of rRNAs. 4) The ND1 gene of Rana catesbeiana was encoded by 961 bases and, as deduced from the nucleotide sequence, the ND1 product consisted of 320 amino acids with a molecular weight of 35,714. The nucleotide and amino acid sequence homologies of the ND1 between Rana and human and between Rana and Xenopus laevis were 67 and 66%, and 70 and 74%, respectively, indicating that the ND1 sequence is conserved to similar extents as those of other mitochondrial-encoded proteins. 5) The nucleotide sequences of tRNA-Val and tRNA-Leu (UUR), adjacent to the rRNAs and NDI genes, and the clover leaf structures of these tRNAs were also determined, which showed characteristic features seen in the mitochondrial tRNA genes of the higher animals.

The primary and secondary structures of the ribosomal RNAs of Rana catesbeiana mitochondrion.